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Beautiful sunshine on earth (HD1080p) MrBangthamai

ADVEXON TV BBC Electricity is the physical flow of electrons, referred to as an electrical current. Electricity is an energy carrier that efficiently delivers the energy found in primary sources to end users, who in turn convert it into energy services.


Order and Disorder: What is Energy? (Jim Al-Khalili) Reel Truth Science Documentaries Professor Jim Al-Khalili discovers the intriguing story of how we discovered the rules that drive the universe. Energy is vital to us all, but what exactly is energy? In attempting to answer this question Jim investigates a strange set of laws that link together everything from engines to humans to stars. It turns out that energy, so critical to daily existence, actually helps us make sense of the entire universe. Professor Jim Al-Khalili investigates the important concepts of energy and information.

China’s Mega Projects: Energy
Energy is driving the rapid development of China’s economy. Challenges have to be overcome to protect the environment. Check out this episode of the documentary series, China’s Mega Projects
Top 10 Energy Sources of the Future
The Daily Conversation
These are ten most promising alternative energy sources of tomorrow. It’s a really exciting time to be alive. We have a front row seat to the only known transformation of a world powered by dirty fossil fuels, to a planet that gets its energy from renewable, clean sources. It’s happening just once, right now.
Sustainable City | Fully Charged
We spent an amazing day at the Sustainable City, a housing development in Dubai with 3,500 people already living there and it’s still not quite finished. This truly is a remarkable achievement, a stark lesson to building contractors the world over. It’s not more expensive to build and it’s hugely cheaper and more efficient to live in. Inspiring sustainable city planning! Spread the word if you know any builders! More Info:
Smart Cities – Building for the Cities of Tomorrow
More people than ever are living in cities, while only 10 percent of the worlds population did so at the start of the 20th century. The number has grown to 50 percent today. By the end of this century, humans will be a predominantly urban species. As our cities grow, so do the challenges. Space is becoming scarce and social tensions, traffic, noise and pollution are all on the rise. At the same time cities offer many opportunities such as more efficient use of resources, energy and infrastructure. After remaining a relatively small specialty field for decades, sustainable building today hold the technical key for the future. Subscribe to wocomoDOCS: Follow us on Facebook:
The Future of Energy – English Documentary
7 UNBELIEVABLE Solar Powered Creations
Smart Energy Systems: 100% Renewable Energy at a National Level (Full Version)
Smart Energy Systems: 100% Renewable Energy at a National Level (Full Version) Denmark has decided to become independent from fossil fuels. For the sake of the climate, the economy, and in order to ensure security of energy supply. This film shows how this will happen based on research conducted at Aalborg University ( At present wind and solar energy already delivers a good share of Denmark’s energy, but renewable energy is a major challenge for an energy system that is built upon fossil fuels. Energy production from wind and solar fluctuates – it fluctuates as the wind blows. So what renewables are reliable when there is no sun or wind energy available? Another challenge is the transport sector. How do we create an energy system of renewable energy, where also cars, ships and planes can operate on fossil-free energy? A great example of an energy system that will ensure Denmark a 100% renewable energy system is called: Smart Energy Systems – a coherent, fossil-free energy system that will create lots of new jobs and green energy for the Danes, both in terms of electricity, heat and transport. Production courtesy of:…


Renewable energy

Read more : From Wikipedia, the free encyclopedia

Renewable energy is energy that is collected from renewable resources, which are naturally replenished on a human timescale, such as sunlightwindraintideswaves, and geothermal heat.[3] Renewable energy often provides energy in four important areas: electricity generationair and water heating/coolingtransportation, and rural (off-grid) energy services.

World elec­tric­ity gen­er­a­tion by source in 2017. Total gen­er­a­tion was 26 PWh.[1]  Coal (38%)  Natural gas (23%)  Hydro (16%)  Nuclear (10%)  Wind (4%)  Oil (3%)  Solar (2%)  Biofuels (2%)  Other (2%)

Can 100% renewable energy power the world? – Federico Rosei and Renzo Rosei


5 Inventions Showing Us the Future of Solar Energy SciShow When you imagine the energy of the future, solar power is probably in the picture – but in recent years, less than 2% of the world’s electricity has come from solar power. Here are 5 new inventions that are likely to change that.

Breakthrough in renewable energy – VPRO documentary vpro documentary223K subscribersSUBSCRIBEIt’s not in the papers but a silent revolution is moving across the world. Renewable energy is becoming cheaper than from fossil fuels. It means that progressively the choice for wind and solar energy is no longer an ethical one but an economic one. And this will speed up the transfer to renewable energy. In countries like Brazil, Australia, Chile and parts of the United States people consider renewable energy because of financial reasons. The price of solar and wind energy will continue to drop and in more countries renewable energy will occur. A surprising newcomer on the market is Morocco, where the government expects that in 2020 more than 40 percent of the energy could come from solar energy.

#CNBC The Rise Of Solar Power
Solar power is on the rise. You can see the evidence on rooftops and in the desert, where utility-scale solar plants are popping up. The picture is not all rosy, but if the recent past is any indication, solar power is going to help lead the transition to a carbon-free future, and it might do it faster than we all expected. Elon Musk and Tesla promised solar roof tiles in 2016, but the industry might not need an upgrade as its grown significantly with the solar panels currently available. You can see the evidence both on individual rooftops and in the utility-scale solar plants increasingly popping up in deserts across the country. In the United States, of all about 30% of the new power capacity added to the grid in 2018 was from solar.

Is Nuclear Fusion The Answer To Clean Energy?
Nuclear power has a controversial history, but many energy experts say it has a major role to play in our energy future. Some in the industry are working to make standard fission power safer and cheaper. Others are pursuing the holy grail of energy – nuclear fusion, the process that powers the sun and the stars. If we figure out how to harness that power here on earth, it would be a huge game-changer.
Top 5 Best Solar Panels Review in 2020
Trendy Ideas
Best Solar Panels Featured in this Video: 0:25 NO.1. Renogy 100 Watts 12 Volts Monocrystalline Solar Panel – 1:11 NO.2. ECO-WORTHY Monocrystalline Solar Panel 200w Off-Grid Power kit W/ 20A LCD Charger – 2:07 NO.3. SUAOKI 150W 18V 12V Solar Panel Charger Monocrystalline Flexible Cell with MC4 Connector Charging – 3:06 NO.4. 2 Pieces of HQST 100 Watt 12 Volt Polycrystalline Solar Panel – 3:58 NO.5. WindyNation 400 Watt Solar Kit: 4pcs 100 Watt Solar Panels –
Top 4 100w Solar Panels Tested! Renogy vs. HQST vs. Rich Solar vs. NewPowa

DIY Solar Power with Will Prowse

The solar panels in this video may be old or out of stock. Solar panels I currently recommend can be found here:…
Solar Power System For Home: Ultimate Beginners Guide
Solar power for homes is increasing in popularity and decreasing in price. Many homeowners are discovering the advantages of Solar Power and you may have even seen quite a few solar panel systems being installed in your own neighborhood. You’re probably wondering: How much do solar panels cost? How much money will I save on my electricity bill? How solar panels work? Are solar panels worth it? This video is a general overview of solar energy for beginners so you can make the best possible decisions regarding solar power for your home. A solar panel (or photovoltaic panel) is a panel made of solar cells. Solar cells are the essential component by which light is converted into electrical energy and they are usually made of crystalline silicon. Each solar panel usually contains from 32 up to 96 solar cells. Depending on the way solar cells are made, solar panels are categorized as polycrystalline, mono-crystalline, or thin film. The first two categories, which are the most common types of solar panels, are made of crystalline solar cells. The third category (thin film) is made of amorphous silicon. Frameless solar panels have been on the market for more than a decade. Also, solar shingles and solar tiles popularized by the tesla solar roof are two types of products that are becoming more popular among residential applications. How do solar panels work? Solar cells produce electricity by converting the tremendous solar energy that the earth receives every day in the form of sunlight and more specifically in the form of photons. Most typical commercial solar solutions convert sunlight to electrical energy at an average efficiency of 3 – 17% In order to have a complete system that will produce electricity for your home, you need the following components: a) solar panels b) solar panel mounting structures c) an inverter d) an electrical panel with the necessary switches and circuit breakers e) DC and AC electrical cables f) Power meter. Generally, residential solar systems are separated into 2 main categories: off-grid and on-grid. In the case of an off-grid system, the electricity generated by the solar system will be used to cover 100% of the electrical usage of a home since the house is not connected to the electrical grid at all.
5 Best Solar Panels in 2020
?5 – HQST 100 Watt 12 Volt – ?4 – Renogy 100 Watts 12 Volts Monocrystalline Solar Panel – ?3 – SunPower 100 Watt Flexible – ?2 – Canadian Solar 300W Watt Monocrystalline Solar Panel – ?1 – Renogy 300 Watt 24 Volt –

Whether you need to run small appliances off-grid in your RV or looking to significantly reduce your electrical bill for your home or business, solar energy is a great energy alternative. In this video, we’ll be comparing the 5 best solar panels that are designed for different kinds of users. We will take into account performance, features, and price; so you can decide which is best for you. All the products on our list were selected based on their own inherent strengths and features. We’ll be comparing the HQST 100 Watt 12 Volt, Renogy 100 Watts 12 Volts, SunPower 100 Watt Flexible, Canadian Solar 300W Watt, and the Renogy 300 Watt 24 Volt; which are all great options if you’re in the market for solar panels. We’ll break down which solar panels are best for you, and what you can expect to get in return for your money. We’ll help you decide if one of the models on our list seems like a great purchase.


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Petroleum – summary of the modern history of oil
Let’s retrace on an animated map a summary of the modern history of petroleum until the present day.



From Wikipedia, the free encyclopedia

This article is about the scalar physical quantity. For an overview of and topical guide to energy, see Outline of energy. For other uses, see Energy (disambiguation).”Energetic” redirects here. For other uses, see Energetic (disambiguation).

The Sun is the source of energy for most of life on Earth. It derives its energy mainly from nuclear fusion in its core, converting mass to energy as protons are combined to form helium. This energy is transported to the sun’s surface then released into space mainly in the form of radiant (light) energy.
Common symbolsE
SI unitjoule
Other unitskW⋅hBTUcalorieeVergfoot-pound
In SI base unitsJ = kg m2 s−2
DimensionM L2 T−2

In physicsen­ergy is the quan­ti­ta­tive prop­erty that must be trans­ferred to an ob­ject in order to per­form work on, or to heat, the object. En­ergy is a con­served quan­tity; the law of con­ser­va­tion of en­ergy states that en­ergy can be con­verted in form, but not cre­ated or de­stroyed. The SI unit of en­ergy is the joule, which is the en­ergy trans­ferred to an ob­ject by the work of mov­ing it a dis­tance of 1 metre against a force of 1 new­ton.

Com­mon forms of en­ergy in­clude the ki­netic en­ergy of a mov­ing ob­ject, the po­ten­tial en­ergy stored by an ob­ject’s po­si­tion in a force field (grav­i­ta­tionalelec­tric or mag­netic), the elas­tic en­ergy stored by stretch­ing solid ob­jects, the chem­i­cal en­ergy re­leased when a fuel burns, the ra­di­ant en­ergy car­ried by light, and the ther­mal en­ergy due to an ob­ject’s tem­per­a­ture.

Mass and en­ergy are closely re­lated. Due to mass–en­ergy equiv­a­lence, any ob­ject that has mass when sta­tion­ary (called rest mass) also has an equiv­a­lent amount of en­ergy whose form is called rest en­ergy, and any ad­di­tional en­ergy (of any form) ac­quired by the ob­ject above that rest en­ergy will in­crease the ob­ject’s total mass just as it in­creases its total en­ergy. For ex­am­ple, after heat­ing an ob­ject, its in­crease in en­ergy could be mea­sured as a small in­crease in mass, with a sen­si­tive enough scale.

Liv­ing or­gan­isms re­quire en­ergy to stay alive, such as the en­ergy hu­mans get from food. Human civ­i­liza­tion re­quires en­ergy to func­tion, which it gets from en­ergy re­sources such as fos­sil fuelsnu­clear fuel, or re­new­able en­ergy. The processes of Earth’s cli­mate and ecosys­tem are dri­ven by the ra­di­ant en­ergy Earth re­ceives from the sun and the ge­ot­her­mal en­ergy con­tained within the earth.


YouTube Encyclopedic 

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In a typical lightning strike, 500 megajoules of electric potential energy is converted into the same amount of energy in other forms, mostly light energy, sound energy and thermal energy.

In a typical lightning strike, 500 megajoules of electric potential energy is converted into the same amount of energy in other forms, mostly light energysound energy and thermal energy.

Thermal energy is energy of microscopic constituents of matter, which may include both kinetic and potential energy.

Thermal energy is energy of microscopic constituents of matter, which may include both kinetic and potential energy.

The total en­ergy of a sys­tem can be sub­di­vided and clas­si­fied into po­ten­tial en­ergy, ki­netic en­ergy, or com­bi­na­tions of the two in var­i­ous ways. Ki­netic en­ergy is de­ter­mined by the move­ment of an ob­ject – or the com­pos­ite mo­tion of the com­po­nents of an ob­ject – and po­ten­tial en­ergy re­flects the po­ten­tial of an ob­ject to have mo­tion, and gen­er­ally is a func­tion of the po­si­tion of an ob­ject within a field or may be stored in the field it­self.

While these two cat­e­gories are suf­fi­cient to de­scribe all forms of en­ergy, it is often con­ve­nient to refer to par­tic­u­lar com­bi­na­tions of po­ten­tial and ki­netic en­ergy as its own form. For ex­am­ple, macro­scopic me­chan­i­cal en­ergy is the sum of trans­la­tional and ro­ta­tional ki­netic and po­ten­tial en­ergy in a sys­tem ne­glects the ki­netic en­ergy due to tem­per­a­ture, and nu­clear en­ergy which com­bines uti­lize po­ten­tials from the nu­clear force and the weak force), among others.

Type of energyDescription
Mechanicalthe sum of macroscopic translational and rotational kinetic and potential energies
Electricpotential energy due to or stored in electric fields
Magneticpotential energy due to or stored in magnetic fields
Gravitationalpotential energy due to or stored in gravitational fields
Chemicalpotential energy due to chemical bonds
Ionizationpotential energy that binds an electron to its atom or molecule
Nuclearpotential energy that binds nucleons to form the atomic nucleus (and nuclear reactions)
Chromodynamicpotential energy that binds quarks to form hadrons
Elasticpotential energy due to the deformation of a material (or its container) exhibiting a restorative force
Mechanical wavekinetic and potential energy in an elastic material due to a propagated deformational wave
Sound wavekinetic and potential energy in a fluid due to a sound propagated wave (a particular form of mechanical wave)
Radiantpotential energy stored in the fields of propagated by electromagnetic radiation, including light
Restpotential energy due to an object’s rest mass
Thermalkinetic energy of the microscopic motion of particles, a form of disordered equivalent of mechanical energy


Main articles: History of energy and timeline of thermodynamics, statistical mechanics, and random processes

Thomas Young, the first person to use the term "energy" in the modern sense.

Thomas Young, the first person to use the term “energy” in the modern sense.

The word en­ergy de­rives from the An­cient Greek: ἐν­έρ­γεια, ro­man­izedenergeialit. ‘ac­tiv­ity, operation’, which pos­si­bly ap­pears for the first time in the work of Aris­to­tle in the 4th cen­tury BC. In con­trast to the mod­ern de­f­i­n­i­tion, en­ergeia was a qual­i­ta­tive philo­soph­i­cal con­cept, broad enough to in­clude ideas such as hap­pi­ness and plea­sure.

In the late 17th cen­tury, Got­tfried Leib­niz pro­posed the idea of the Latinvis viva, or liv­ing force, which de­fined as the prod­uct of the mass of an ob­ject and its ve­loc­ity squared; he be­lieved that total vis viva was con­served. To ac­count for slow­ing due to fric­tion, Leib­niz the­o­rized that ther­mal en­ergy con­sisted of the ran­dom mo­tion of the con­stituent parts of mat­ter, al­though it would be more than a cen­tury until this was gen­er­ally ac­cepted. The mod­ern ana­log of this prop­erty, ki­netic en­ergy, dif­fers from vis viva only by a fac­tor of two.

In 1807, Thomas Young was pos­si­bly the first to use the term “en­ergy” in­stead of vis viva, in its mod­ern sense. Gus­tave-Gas­pard Cori­o­lis de­scribed “ki­netic en­ergy” in 1829 in its mod­ern sense, and in 1853, William Rank­ine coined the term “po­ten­tial en­ergy“. The law of con­ser­va­tion of en­ergy was also first pos­tu­lated in the early 19th cen­tury, and ap­plies to any iso­lated sys­tem. It was ar­gued for some years whether heat was a phys­i­cal sub­stance, dubbed the caloric, or merely a phys­i­cal quan­tity, such as mo­men­tum. In 1845 James Prescott Joule dis­cov­ered the link be­tween me­chan­i­cal work and the gen­er­a­tion of heat.

These de­vel­op­ments led to the the­ory of con­ser­va­tion of en­ergy, for­mal­ized largely by William Thom­son (Lord Kelvin) as the field of ther­mo­dy­nam­ics. Ther­mo­dy­nam­ics aided the rapid de­vel­op­ment of ex­pla­na­tions of chem­i­cal processes by Rudolf Clau­siusJosiah Willard Gibbs, and Walther Nernst. It also led to a math­e­mat­i­cal for­mu­la­tion of the con­cept of en­tropy by Clau­sius and to the in­tro­duc­tion of laws of ra­di­ant en­ergy by Jožef Ste­fan. Ac­cord­ing to Noe­ther’s the­o­rem, the con­ser­va­tion of en­ergy is a con­se­quence of the fact that the laws of physics do not change over time. Thus, since 1918, the­o­rists have un­der­stood that the law of con­ser­va­tion of en­ergy is the di­rect math­e­mat­i­cal con­se­quence of the trans­la­tional sym­me­try of the quan­tity con­ju­gate to en­ergy, namely time.

Units of measure

Joule's apparatus for measuring the mechanical equivalent of heat. A descending weight attached to a string causes a paddle immersed in water to rotate.

Joule’s apparatus for measuring the mechanical equivalent of heat. A descending weight attached to a string causes a paddle immersed in water to rotate.Main article: Units of energy

In 1843, Joule in­de­pen­dently dis­cov­ered the me­chan­i­cal equiv­a­lent in a se­ries of ex­per­i­ments. The most fa­mous of them used the “Joule ap­pa­ra­tus”: a de­scend­ing weight, at­tached to a string, caused ro­ta­tion of a pad­dle im­mersed in water, prac­ti­cally in­su­lated from heat trans­fer. It showed that the grav­i­ta­tional po­ten­tial en­ergy lost by the weight in de­scend­ing was equal to the in­ter­nal en­ergy gained by the water through fric­tion with the pad­dle.

In the In­ter­na­tional Sys­tem of Units (SI), the unit of en­ergy is the joule, named after James Prescott Joule. It is a de­rived unit. It is equal to the en­ergy ex­pended (or work done) in ap­ply­ing a force of one new­ton through a dis­tance of one metre. How­ever en­ergy is also ex­pressed in many other units not part of the SI, such as ergscalo­riesBritish Ther­mal Unitskilo­watt-hours and kilo­calo­ries, which re­quire a con­ver­sion fac­tor when ex­pressed in SI units.

The SI unit of en­ergy rate (en­ergy per unit time) is the watt, which is a joule per sec­ond. Thus, one joule is one watt-sec­ond, and 3600 joules equal one watt-hour. The CGS en­ergy unit is the erg and the im­pe­r­ial and US cus­tom­ary unit is the foot pound. Other en­ergy units such as the elec­tron­voltfood calo­rie or ther­mo­dy­namic kcal (based on the tem­per­a­ture change of water in a heat­ing process), and BTU are used in spe­cific areas of sci­ence and com­merce.

Scientific use
Classical mechanics
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{\displaystyle {\textbf {F}}={\frac {d}{dt}}(m{\textbf {v}})}Second law of motion
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Categories► Classical mechanics

Main articles: MechanicsMechanical work, and Thermodynamics

In clas­si­cal me­chan­ics, en­ergy is a con­cep­tu­ally and math­e­mat­i­cally use­ful prop­erty, as it is a con­served quan­tity. Sev­eral for­mu­la­tions of me­chan­ics have been de­vel­oped using en­ergy as a core con­cept.

Work, a func­tion of en­ergy, is force times dis­tance.W=\int _{C}\mathbf {F} \cdot \mathrm {d} \mathbf {s}

This says that the work (W) is equal to the line in­te­gral of the force F along a path C; for de­tails see the me­chan­i­cal work ar­ti­cle. Work and thus en­ergy is frame de­pen­dent. For ex­am­ple, con­sider a ball being hit by a bat. In the cen­ter-of-mass ref­er­ence frame, the bat does no work on the ball. But, in the ref­er­ence frame of the per­son swing­ing the bat, con­sid­er­able work is done on the ball.

The total en­ergy of a sys­tem is some­times called the Hamil­ton­ian, after William Rowan Hamil­ton. The clas­si­cal equa­tions of mo­tion can be writ­ten in terms of the Hamil­ton­ian, even for highly com­plex or ab­stract sys­tems. These clas­si­cal equa­tions have re­mark­ably di­rect analogs in non­rel­a­tivis­tic quan­tum mechanics.

An­other en­ergy-re­lated con­cept is called the La­grangian, after Joseph-Louis La­grange. This for­mal­ism is as fun­da­men­tal as the Hamil­ton­ian, and both can be used to de­rive the equa­tions of mo­tion or be de­rived from them. It was in­vented in the con­text of clas­si­cal me­chan­ics, but is gen­er­ally use­ful in mod­ern physics. The La­grangian is de­fined as the ki­netic en­ergy minus the po­ten­tial en­ergy. Usu­ally, the La­grange for­mal­ism is math­e­mat­i­cally more con­ve­nient than the Hamil­ton­ian for non-con­ser­v­a­tive sys­tems (such as sys­tems with fric­tion).

Noe­ther’s the­o­rem (1918) states that any dif­fer­en­tiable sym­me­try of the ac­tion of a phys­i­cal sys­tem has a cor­re­spond­ing con­ser­va­tion law. Noe­ther’s the­o­rem has be­come a fun­da­men­tal tool of mod­ern the­o­ret­i­cal physics and the cal­cu­lus of vari­a­tions. A gen­er­al­i­sa­tion of the sem­i­nal for­mu­la­tions on con­stants of mo­tion in La­grangian and Hamil­ton­ian me­chan­ics (1788 and 1833, re­spec­tively), it does not apply to sys­tems that can­not be mod­eled with a La­grangian; for ex­am­ple, dis­si­pa­tive sys­tems with con­tin­u­ous sym­me­tries need not have a cor­re­spond­ing con­ser­va­tion law.


In the con­text of chem­istry, en­ergy is an at­tribute of a sub­stance as a con­se­quence of its atomic, mol­e­c­u­lar or ag­gre­gate struc­ture. Since a chem­i­cal trans­for­ma­tion is ac­com­pa­nied by a change in one or more of these kinds of struc­ture, it is in­vari­ably ac­com­pa­nied by an in­crease or de­crease of en­ergy of the sub­stances in­volved. Some en­ergy is trans­ferred be­tween the sur­round­ings and the re­ac­tants of the re­ac­tion in the form of heat or light; thus the prod­ucts of a re­ac­tion may have more or less en­ergy than the re­ac­tants. A re­ac­tion is said to be exother­mic or ex­er­gonic if the final state is lower on the en­ergy scale than the ini­tial state; in the case of en­dother­mic re­ac­tions the sit­u­a­tion is the re­verse. Chem­i­cal re­ac­tions are al­most in­vari­ably not pos­si­ble un­less the re­ac­tants sur­mount an en­ergy bar­rier known as the ac­ti­va­tion en­ergy. The speed of a chem­i­cal re­ac­tion (at given tem­per­a­ture T) is re­lated to the ac­ti­va­tion en­ergy E by the Boltz­mann’s pop­u­la­tion fac­tor eE/kT – that is the prob­a­bil­ity of mol­e­cule to have en­ergy greater than or equal to E at the given tem­per­a­ture T. This ex­po­nen­tial de­pen­dence of a re­ac­tion rate on tem­per­a­ture is known as the Ar­rhe­nius equa­tion. The ac­ti­va­tion en­ergy nec­es­sary for a chem­i­cal re­ac­tion can be pro­vided in the form of ther­mal en­ergy.


Main articles: Bioenergetics and Food energyBasic overview of energy and human life.

In bi­ol­ogy, en­ergy is an at­tribute of all bi­o­log­i­cal sys­tems from the bios­phere to the small­est liv­ing or­gan­ism. Within an or­gan­ism it is re­spon­si­ble for growth and de­vel­op­ment of a bi­o­log­i­cal cell or an or­ganelle of a bi­o­log­i­cal or­gan­ism. En­ergy used in res­pi­ra­tion is mostly stored in mol­e­c­u­lar oxy­gen and can be un­locked by re­ac­tions with mol­e­cules of sub­stances such as car­bo­hy­drates (in­clud­ing sug­ars), lipids, and pro­teins stored by cells. In human terms, the human equiv­a­lent (H-e) (Human en­ergy con­ver­sion) in­di­cates, for a given amount of en­ergy ex­pen­di­ture, the rel­a­tive quan­tity of en­ergy needed for human me­tab­o­lism, as­sum­ing an av­er­age human en­ergy ex­pen­di­ture of 12,500 kJ per day and a basal meta­bolic rate of 80 watts. For ex­am­ple, if our bod­ies run (on av­er­age) at 80 watts, then a light bulb run­ning at 100 watts is run­ning at 1.25 human equiv­a­lents (100 ÷ 80) i.e. 1.25 H-e. For a dif­fi­cult task of only a few sec­onds’ du­ra­tion, a per­son can put out thou­sands of watts, many times the 746 watts in one of­fi­cial horse­power. For tasks last­ing a few min­utes, a fit human can gen­er­ate per­haps 1,000 watts. For an ac­tiv­ity that must be sus­tained for an hour, out­put drops to around 300; for an ac­tiv­ity kept up all day, 150 watts is about the maximum. The human equiv­a­lent as­sists un­der­stand­ing of en­ergy flows in phys­i­cal and bi­o­log­i­cal sys­tems by ex­press­ing en­ergy units in human terms: it pro­vides a “feel” for the use of a given amount of energy.

Sun­light’s ra­di­ant en­ergy is also cap­tured by plants as chem­i­cal po­ten­tial energy in pho­to­syn­the­sis, when car­bon diox­ide and water (two low-en­ergy com­pounds) are con­verted into car­bo­hy­drates, lipids, and pro­teins and high-en­ergy com­pounds like oxy­gen and ATP. Car­bo­hy­drates, lipids, and pro­teins can re­lease the en­ergy of oxy­gen, which is uti­lized by liv­ing or­gan­isms as an elec­tron ac­cep­tor. Re­lease of the en­ergy stored dur­ing pho­to­syn­the­sis as heat or light may be trig­gered sud­denly by a spark, in a for­est fire, or it may be made avail­able more slowly for an­i­mal or human me­tab­o­lism, when or­ganic mol­e­cules are in­gested, and ca­tab­o­lism is trig­gered by en­zyme ac­tion.

Any liv­ing or­gan­ism re­lies on an ex­ter­nal source of en­ergy – ra­di­ant en­ergy from the Sun in the case of green plants, chem­i­cal en­ergy in some form in the case of an­i­mals – to be able to grow and re­pro­duce. The daily 1500–2000 Calo­ries (6–8 MJ) rec­om­mended for a human adult are taken as a com­bi­na­tion of oxy­gen and food mol­e­cules, the lat­ter mostly car­bo­hy­drates and fats, of which glu­cose (C6H12O6) and stearin (C57H110O6) are con­ve­nient ex­am­ples. The food mol­e­cules are ox­i­dised to car­bon diox­ide and water in the mi­to­chon­dria{\displaystyle {\ce {C6H12O6 + 6O2 -> 6CO2 + 6H2O}}}” src=””><img alt= 57CO2 + 55H2O}}}” src=””>

and some of the en­ergy is used to con­vert ADP into ATP.ADP + HPO42− → ATP + H2O

The rest of the chem­i­cal en­ergy in O2 and the car­bo­hy­drate or fat is con­verted into heat: the ATP is used as a sort of “en­ergy cur­rency”, and some of the chem­i­cal en­ergy it con­tains is used for other me­tab­o­lism when ATP re­acts with OH groups and even­tu­ally splits into ADP and phos­phate (at each stage of a meta­bolic path­way, some chem­i­cal en­ergy is con­verted into heat). Only a tiny frac­tion of the orig­i­nal chem­i­cal en­ergy is used for work:gain in kinetic energy of a sprinter during a 100 m race: 4 kJgain in gravitational potential energy of a 150 kg weight lifted through 2 metres: 3 kJDaily food intake of a normal adult: 6–8 MJ

It would ap­pear that liv­ing or­gan­isms are re­mark­ably in­ef­fi­cient (in the phys­i­cal sense) in their use of the en­ergy they re­ceive (chem­i­cal or ra­di­ant en­ergy), and it is true that most real ma­chines man­age higher ef­fi­cien­cies. In grow­ing or­gan­isms the en­ergy that is con­verted to heat serves a vital pur­pose, as it al­lows the or­gan­ism tis­sue to be highly or­dered with re­gard to the mol­e­cules it is built from. The sec­ond law of ther­mo­dy­nam­ics states that en­ergy (and mat­ter) tends to be­come more evenly spread out across the uni­verse: to con­cen­trate en­ergy (or mat­ter) in one spe­cific place, it is nec­es­sary to spread out a greater amount of en­ergy (as heat) across the re­main­der of the uni­verse (“the surroundings”). Sim­pler or­gan­isms can achieve higher en­ergy ef­fi­cien­cies than more com­plex ones, but the com­plex or­gan­isms can oc­cupy eco­log­i­cal niches that are not avail­able to their sim­pler brethren. The con­ver­sion of a por­tion of the chem­i­cal en­ergy to heat at each step in a meta­bolic path­way is the phys­i­cal rea­son be­hind the pyra­mid of bio­mass ob­served in ecol­ogy: to take just the first step in the food chain, of the es­ti­mated 124.7 Pg/a of car­bon that is fixed by pho­to­syn­the­sis, 64.3 Pg/a (52%) are used for the me­tab­o­lism of green plants, i.e. re­con­verted into car­bon diox­ide and heat.

Earth sciences

In ge­ol­ogycon­ti­nen­tal driftmoun­tain rangesvol­ca­noes, and earth­quakes are phe­nom­ena that can be ex­plained in terms of en­ergy trans­for­ma­tions in the Earth’s interior, while me­te­o­ro­log­i­cal phe­nom­ena like wind, rain, hail, snow, light­ning, tor­na­does and hur­ri­canes are all a re­sult of en­ergy trans­for­ma­tions brought about by solar en­ergy on the at­mos­phere of the planet Earth.

Sun­light may be stored as grav­i­ta­tional po­ten­tial en­ergy after it strikes the Earth, as (for ex­am­ple) water evap­o­rates from oceans and is de­posited upon moun­tains (where, after being re­leased at a hy­dro­elec­tric dam, it can be used to drive tur­bines or gen­er­a­tors to pro­duce elec­tric­ity). Sun­light also dri­ves many weather phe­nom­ena, save those gen­er­ated by vol­canic events. An ex­am­ple of a so­lar-me­di­ated weather event is a hur­ri­cane, which oc­curs when large un­sta­ble areas of warm ocean, heated over months, give up some of their ther­mal en­ergy sud­denly to power a few days of vi­o­lent air move­ment.

In a slower process, ra­dioac­tive decay of atoms in the core of the Earth re­leases heat. This ther­mal en­ergy dri­ves plate tec­ton­ics and may lift moun­tains, via oro­ge­n­e­sis. This slow lift­ing rep­re­sents a kind of grav­i­ta­tional po­ten­tial en­ergy stor­age of the ther­mal en­ergy, which may be later re­leased to ac­tive ki­netic en­ergy in land­slides, after a trig­ger­ing event. Earth­quakes also re­lease stored elas­tic po­ten­tial en­ergy in rocks, a store that has been pro­duced ul­ti­mately from the same ra­dioac­tive heat sources. Thus, ac­cord­ing to pre­sent un­der­stand­ing, fa­mil­iar events such as land­slides and earth­quakes re­lease en­ergy that has been stored as po­ten­tial en­ergy in the Earth’s grav­i­ta­tional field or elas­tic strain (me­chan­i­cal po­ten­tial en­ergy) in rocks. Prior to this, they rep­re­sent re­lease of en­ergy that has been stored in heavy atoms since the col­lapse of long-de­stroyed su­per­nova stars cre­ated these atoms.


In cos­mol­ogy and as­tron­omy the phe­nom­ena of starsnovasu­per­novaquasars and gamma-ray bursts are the uni­verse’s high­est-out­put en­ergy trans­for­ma­tions of mat­ter. All stel­lar phe­nom­ena (in­clud­ing solar ac­tiv­ity) are dri­ven by var­i­ous kinds of en­ergy trans­for­ma­tions. En­ergy in such trans­for­ma­tions is ei­ther from grav­i­ta­tional col­lapse of mat­ter (usu­ally mol­e­c­u­lar hy­dro­gen) into var­i­ous classes of as­tro­nom­i­cal ob­jects (stars, black holes, etc.), or from nu­clear fu­sion (of lighter el­e­ments, pri­mar­ily hy­dro­gen). The nu­clear fu­sion of hy­dro­gen in the Sun also re­leases an­other store of po­ten­tial en­ergy which was cre­ated at the time of the Big Bang. At that time, ac­cord­ing to the­ory, space ex­panded and the uni­verse cooled too rapidly for hy­dro­gen to com­pletely fuse into heav­ier el­e­ments. This meant that hy­dro­gen rep­re­sents a store of po­ten­tial en­ergy that can be re­leased by fu­sion. Such a fu­sion process is trig­gered by heat and pres­sure gen­er­ated from grav­i­ta­tional col­lapse of hy­dro­gen clouds when they pro­duce stars, and some of the fu­sion en­ergy is then trans­formed into sun­light.

Quantum mechanics

Main article: Energy operator

In quan­tum me­chan­ics, en­ergy is de­fined in terms of the en­ergy op­er­a­tor as a time de­riv­a­tive of the wave func­tion. The Schrödinger equa­tion equates the en­ergy op­er­a­tor to the full en­ergy of a par­ti­cle or a sys­tem. Its re­sults can be con­sid­ered as a de­f­i­n­i­tion of mea­sure­ment of en­ergy in quan­tum me­chan­ics. The Schrödinger equa­tion de­scribes the space- and time-de­pen­dence of a slowly chang­ing (non-rel­a­tivis­tic) wave func­tion of quan­tum sys­tems. The so­lu­tion of this equa­tion for a bound sys­tem is dis­crete (a set of per­mit­ted states, each char­ac­ter­ized by an en­ergy level) which re­sults in the con­cept of quanta. In the so­lu­tion of the Schrödinger equa­tion for any os­cil­la­tor (vi­bra­tor) and for elec­tro­mag­netic waves in a vac­uum, the re­sult­ing en­ergy states are re­lated to the fre­quency by Planck’s re­la­tionE=h\nu  (where h is Planck’s con­stant and \nu  the fre­quency). In the case of an elec­tro­mag­netic wave these en­ergy states are called quanta of light or pho­tons.


When cal­cu­lat­ing ki­netic en­ergy (work to ac­cel­er­ate a mas­sive body from zero speed to some fi­nite speed) rel­a­tivis­ti­cally – using Lorentz trans­for­ma­tions in­stead of New­ton­ian me­chan­ics – Ein­stein dis­cov­ered an un­ex­pected by-prod­uct of these cal­cu­la­tions to be an en­ergy term which does not van­ish at zero speed. He called it rest en­ergy: en­ergy which every mas­sive body must pos­sess even when being at rest. The amount of en­ergy is di­rectly pro­por­tional to the mass of the body:{\displaystyle E_{0}=mc^{2}},

wherem is the mass of the body,c is the speed of light in vacuum, is the rest energy.

For ex­am­ple, con­sider elec­tronpositron an­ni­hi­la­tion, in which the rest en­ergy of these two in­di­vid­ual par­ti­cles (equiv­a­lent to their rest mass) is con­verted to the ra­di­ant en­ergy of the pho­tons pro­duced in the process. In this sys­tem the mat­ter and an­ti­mat­ter (elec­trons and positrons) are de­stroyed and changed to non-mat­ter (the pho­tons). How­ever, the total mass and total en­ergy do not change dur­ing this in­ter­ac­tion. The pho­tons each have no rest mass but nonethe­less have ra­di­ant en­ergy which ex­hibits the same in­er­tia as did the two orig­i­nal par­ti­cles. This is a re­versible process – the in­verse process is called pair cre­ation – in which the rest mass of par­ti­cles is cre­ated from the ra­di­ant en­ergy of two (or more) an­ni­hi­lat­ing pho­tons.

In gen­eral rel­a­tiv­ity, the stress–en­ergy ten­sor serves as the source term for the grav­i­ta­tional field, in rough anal­ogy to the way mass serves as the source term in the non-rel­a­tivis­tic New­ton­ian approximation.

En­ergy and mass are man­i­fes­ta­tions of one and the same un­der­ly­ing phys­i­cal prop­erty of a sys­tem. This prop­erty is re­spon­si­ble for the in­er­tia and strength of grav­i­ta­tional in­ter­ac­tion of the sys­tem (“mass man­i­fes­ta­tions”), and is also re­spon­si­ble for the po­ten­tial abil­ity of the sys­tem to per­form work or heat­ing (“en­ergy man­i­fes­ta­tions”), sub­ject to the lim­i­ta­tions of other phys­i­cal laws.

In clas­si­cal physics, en­ergy is a scalar quan­tity, the canon­i­cal con­ju­gate to time. In spe­cial rel­a­tiv­ity en­ergy is also a scalar (al­though not a Lorentz scalar but a time com­po­nent of the en­ergy–mo­men­tum 4-vec­tor). In other words, en­ergy is in­vari­ant with re­spect to ro­ta­tions of space, but not in­vari­ant with re­spect to ro­ta­tions of space-time (= boosts).


Main article: Energy transformation

Type of transfer processDescription
Heatthat amount of thermal energy in transit spontaneously towards a lower-temperature object
Workthat amount of energy in transit due to a displacement in the direction of an applied force
Transfer of materialthat amount of energy carried by matter that is moving from one system to another

A turbo generator transforms the energy of pressurised steam into electrical energy

turbo generator transforms the energy of pressurised steam into electrical energy

En­ergy may be trans­formed be­tween dif­fer­ent forms at var­i­ous ef­fi­cien­cies. Items that trans­form be­tween these forms are called trans­duc­ers. Ex­am­ples of trans­duc­ers in­clude a bat­tery, from chem­i­cal en­ergy to elec­tric en­ergy; a dam: grav­i­ta­tional po­ten­tial en­ergy to ki­netic en­ergy of mov­ing water (and the blades of a tur­bine) and ul­ti­mately to elec­tric en­ergy through an elec­tric gen­er­a­tor; or a heat en­gine, from heat to work.

Ex­am­ples of en­ergy trans­for­ma­tion in­clude gen­er­at­ing elec­tric en­ergy from heat en­ergy via a steam tur­bine, or lift­ing an ob­ject against grav­ity using elec­tri­cal en­ergy dri­ving a crane motor. Lift­ing against grav­ity per­forms me­chan­i­cal work on the ob­ject and stores grav­i­ta­tional po­ten­tial en­ergy in the ob­ject. If the ob­ject falls to the ground, grav­ity does me­chan­i­cal work on the ob­ject which trans­forms the po­ten­tial en­ergy in the grav­i­ta­tional field to the ki­netic en­ergy re­leased as heat on im­pact with the ground. Our Sun trans­forms nu­clear po­ten­tial en­ergy to other forms of en­ergy; its total mass does not de­crease due to that in it­self (since it still con­tains the same total en­ergy even if in dif­fer­ent forms), but its mass does de­crease when the en­ergy es­capes out to its sur­round­ings, largely as ra­di­ant en­ergy.

There are strict lim­its to how ef­fi­ciently heat can be con­verted into work in a cyclic process, e.g. in a heat en­gine, as de­scribed by Carnot’s the­o­rem and the sec­ond law of ther­mo­dy­nam­ics. How­ever, some en­ergy trans­for­ma­tions can be quite ef­fi­cient. The di­rec­tion of trans­for­ma­tions in en­ergy (what kind of en­ergy is trans­formed to what other kind) is often de­ter­mined by en­tropy (equal en­ergy spread among all avail­able de­grees of free­dom) con­sid­er­a­tions. In prac­tice all en­ergy trans­for­ma­tions are per­mit­ted on a small scale, but cer­tain larger trans­for­ma­tions are not per­mit­ted be­cause it is sta­tis­ti­cally un­likely that en­ergy or mat­ter will ran­domly move into more con­cen­trated forms or smaller spaces.

En­ergy trans­for­ma­tions in the uni­verse over time are char­ac­ter­ized by var­i­ous kinds of po­ten­tial en­ergy that has been avail­able since the Big Bang later being “re­leased” (trans­formed to more ac­tive types of en­ergy such as ki­netic or ra­di­ant en­ergy) when a trig­ger­ing mech­a­nism is avail­able. Fa­mil­iar ex­am­ples of such processes in­clude nu­clear decay, in which en­ergy is re­leased that was orig­i­nally “stored” in heavy iso­topes (such as ura­nium and tho­rium), by nu­cle­osyn­the­sis, a process ul­ti­mately using the grav­i­ta­tional po­ten­tial en­ergy re­leased from the grav­i­ta­tional col­lapse of su­per­novae, to store en­ergy in the cre­ation of these heavy el­e­ments be­fore they were in­cor­po­rated into the solar sys­tem and the Earth. This en­ergy is trig­gered and re­leased in nu­clear fis­sion bombs or in civil nu­clear power gen­er­a­tion. Sim­i­larly, in the case of a chem­i­cal ex­plo­sionchem­i­cal po­ten­tial en­ergy is trans­formed to ki­netic en­ergy and ther­mal en­ergy in a very short time. Yet an­other ex­am­ple is that of a pen­du­lum. At its high­est points the ki­netic en­ergy is zero and the grav­i­ta­tional po­ten­tial en­ergy is at max­i­mum. At its low­est point the ki­netic en­ergy is at max­i­mum and is equal to the de­crease of po­ten­tial en­ergy. If one (un­re­al­is­ti­cally) as­sumes that there is no fric­tion or other losses, the con­ver­sion of en­ergy be­tween these processes would be per­fect, and the pen­du­lum would con­tinue swing­ing for­ever.

En­ergy is also trans­ferred from po­ten­tial en­ergy (E_{p}) to ki­netic en­ergy (E_{k}) and then back to po­ten­tial en­ergy con­stantly. This is re­ferred to as con­ser­va­tion of en­ergy. In this closed sys­tem, en­ergy can­not be cre­ated or de­stroyed; there­fore, the ini­tial en­ergy and the final en­ergy will be equal to each other. This can be demon­strated by the fol­low­ing:

E_{pi}+E_{ki}=E_{pF}+E_{kF}    (4)

The equa­tion can then be sim­pli­fied fur­ther since E_{p}=mgh (mass times ac­cel­er­a­tion due to grav­ity times the height) and E_{k}={\frac {1}{2}}mv^{2} (half mass times ve­loc­ity squared). Then the total amount of en­ergy can be found by adding E_{p}+E_{k}=E_{total}.

Conservation of energy and mass in transformation

En­ergy gives rise to weight when it is trapped in a sys­tem with zero mo­men­tum, where it can be weighed. It is also equiv­a­lent to mass, and this mass is al­ways as­so­ci­ated with it. Mass is also equiv­a­lent to a cer­tain amount of en­ergy, and like­wise al­ways ap­pears as­so­ci­ated with it, as de­scribed in mass-en­ergy equiv­a­lence. The for­mula E = mc², de­rived by Al­bert Ein­stein (1905) quan­ti­fies the re­la­tion­ship be­tween rest-mass and rest-en­ergy within the con­cept of spe­cial rel­a­tiv­ity. In dif­fer­ent the­o­ret­i­cal frame­works, sim­i­lar for­mu­las were de­rived by J.J. Thom­son (1881), Henri Poin­caré (1900), Friedrich Hasenöhrl (1904) and oth­ers (see Mass-en­ergy equiv­a­lence#His­tory for fur­ther in­for­ma­tion).

Part of the rest en­ergy (equiv­a­lent to rest mass) of mat­ter may be con­verted to other forms of en­ergy (still ex­hibit­ing mass), but nei­ther en­ergy nor mass can be de­stroyed; rather, both re­main con­stant dur­ing any process. How­ever, since c^{2} is ex­tremely large rel­a­tive to or­di­nary human scales, the con­ver­sion of an every­day amount of rest mass (for ex­am­ple, 1 kg) from rest en­ergy to other forms of en­ergy (such as ki­netic en­ergy, ther­mal en­ergy, or the ra­di­ant en­ergy car­ried by light and other ra­di­a­tion) can lib­er­ate tremen­dous amounts of en­ergy (~9\times 10^{16} joules = 21 mega­tons of TNT), as can be seen in nu­clear re­ac­tors and nu­clear weapons. Con­versely, the mass equiv­a­lent of an every­day amount en­ergy is mi­nus­cule, which is why a loss of en­ergy (loss of mass) from most sys­tems is dif­fi­cult to mea­sure on a weigh­ing scale, un­less the en­ergy loss is very large. Ex­am­ples of large trans­for­ma­tions be­tween rest en­ergy (of mat­ter) and other forms of en­ergy (e.g., ki­netic en­ergy into par­ti­cles with rest mass) are found in nu­clear physics and par­ti­cle physics.

Reversible and non-reversible transformations

Ther­mo­dy­nam­ics di­vides en­ergy trans­for­ma­tion into two kinds: re­versible processes and ir­re­versible processes. An ir­re­versible process is one in which en­ergy is dis­si­pated (spread) into empty en­ergy states avail­able in a vol­ume, from which it can­not be re­cov­ered into more con­cen­trated forms (fewer quan­tum states), with­out degra­da­tion of even more en­ergy. A re­versible process is one in which this sort of dis­si­pa­tion does not hap­pen. For ex­am­ple, con­ver­sion of en­ergy from one type of po­ten­tial field to an­other, is re­versible, as in the pen­du­lum sys­tem de­scribed above. In processes where heat is gen­er­ated, quan­tum states of lower en­ergy, pre­sent as pos­si­ble ex­ci­ta­tions in fields be­tween atoms, act as a reser­voir for part of the en­ergy, from which it can­not be re­cov­ered, in order to be con­verted with 100% ef­fi­ciency into other forms of en­ergy. In this case, the en­ergy must partly stay as heat, and can­not be com­pletely re­cov­ered as us­able en­ergy, ex­cept at the price of an in­crease in some other kind of heat-like in­crease in dis­or­der in quan­tum states, in the uni­verse (such as an ex­pan­sion of mat­ter, or a ran­domi­sa­tion in a crys­tal).

As the uni­verse evolves in time, more and more of its en­ergy be­comes trapped in ir­re­versible states (i.e., as heat or other kinds of in­creases in dis­or­der). This has been re­ferred to as the in­evitable ther­mo­dy­namic heat death of the uni­verse. In this heat death the en­ergy of the uni­verse does not change, but the frac­tion of en­ergy which is avail­able to do work through a heat en­gine, or be trans­formed to other us­able forms of en­ergy (through the use of gen­er­a­tors at­tached to heat en­gines), grows less and less.

Conservation of energy

Main article: Conservation of energy

The fact that en­ergy can be nei­ther cre­ated nor be de­stroyed is called the law of con­ser­va­tion of en­ergy. In the form of the first law of ther­mo­dy­nam­ics, this states that a closed sys­tem‘s en­ergy is con­stant un­less en­ergy is trans­ferred in or out by work or heat, and that no en­ergy is lost in trans­fer. The total in­flow of en­ergy into a sys­tem must equal the total out­flow of en­ergy from the sys­tem, plus the change in the en­ergy con­tained within the sys­tem. When­ever one mea­sures (or cal­cu­lates) the total en­ergy of a sys­tem of par­ti­cles whose in­ter­ac­tions do not de­pend ex­plic­itly on time, it is found that the total en­ergy of the sys­tem al­ways re­mains constant.

While heat can al­ways be fully con­verted into work in a re­versible isother­mal ex­pan­sion of an ideal gas, for cyclic processes of prac­ti­cal in­ter­est in heat en­gines the sec­ond law of ther­mo­dy­nam­ics states that the sys­tem doing work al­ways loses some en­ergy as waste heat. This cre­ates a limit to the amount of heat en­ergy that can do work in a cyclic process, a limit called the avail­able en­ergy. Me­chan­i­cal and other forms of en­ergy can be trans­formed in the other di­rec­tion into ther­mal en­ergy with­out such limitations. The total en­ergy of a sys­tem can be cal­cu­lated by adding up all forms of en­ergy in the sys­tem.

Richard Feyn­man said dur­ing a 1961 lecture:

There is a fact, or if you wish, a law, gov­ern­ing all nat­ural phe­nom­ena that are known to date. There is no known ex­cep­tion to this law – it is exact so far as we know. The law is called the con­ser­va­tion of en­ergy. It states that there is a cer­tain quan­tity, which we call en­ergy, that does not change in man­i­fold changes which na­ture un­der­goes. That is a most ab­stract idea, be­cause it is a math­e­mat­i­cal prin­ci­ple; it says that there is a nu­mer­i­cal quan­tity which does not change when some­thing hap­pens. It is not a de­scrip­tion of a mech­a­nism, or any­thing con­crete; it is just a strange fact that we can cal­cu­late some num­ber and when we fin­ish watch­ing na­ture go through her tricks and cal­cu­late the num­ber again, it is the same.— The Feynman Lectures on Physics

Most kinds of en­ergy (with grav­i­ta­tional en­ergy being a no­table exception) are sub­ject to strict local con­ser­va­tion laws as well. In this case, en­ergy can only be ex­changed be­tween ad­ja­cent re­gions of space, and all ob­servers agree as to the vol­u­met­ric den­sity of en­ergy in any given space. There is also a global law of con­ser­va­tion of en­ergy, stat­ing that the total en­ergy of the uni­verse can­not change; this is a corol­lary of the local law, but not vice versa.

This law is a fun­da­men­tal prin­ci­ple of physics. As shown rig­or­ously by Noe­ther’s the­o­rem, the con­ser­va­tion of en­ergy is a math­e­mat­i­cal con­se­quence of trans­la­tional sym­me­try of time, a prop­erty of most phe­nom­ena below the cos­mic scale that makes them in­de­pen­dent of their lo­ca­tions on the time co­or­di­nate. Put dif­fer­ently, yes­ter­day, today, and to­mor­row are phys­i­cally in­dis­tin­guish­able. This is be­cause en­ergy is the quan­tity which is canon­i­cal con­ju­gate to time. This math­e­mat­i­cal en­tan­gle­ment of en­ergy and time also re­sults in the un­cer­tainty prin­ci­ple – it is im­pos­si­ble to de­fine the exact amount of en­ergy dur­ing any def­i­nite time in­ter­val. The un­cer­tainty prin­ci­ple should not be con­fused with en­ergy con­ser­va­tion – rather it pro­vides math­e­mat­i­cal lim­its to which en­ergy can in prin­ci­ple be de­fined and mea­sured.

Each of the basic forces of na­ture is as­so­ci­ated with a dif­fer­ent type of po­ten­tial en­ergy, and all types of po­ten­tial en­ergy (like all other types of en­ergy) ap­pears as sys­tem mass, when­ever pre­sent. For ex­am­ple, a com­pressed spring will be slightly more mas­sive than be­fore it was com­pressed. Like­wise, when­ever en­ergy is trans­ferred be­tween sys­tems by any mech­a­nism, an as­so­ci­ated mass is trans­ferred with it.

In quan­tum me­chan­ics en­ergy is ex­pressed using the Hamil­ton­ian op­er­a­tor. On any time scales, the un­cer­tainty in the en­ergy is by\Delta E\Delta t\geq {\frac {\hbar }{2}}

which is sim­i­lar in form to the Heisen­berg Un­cer­tainty Prin­ci­ple (but not re­ally math­e­mat­i­cally equiv­a­lent thereto, since H and t are not dy­nam­i­cally con­ju­gate vari­ables, nei­ther in clas­si­cal nor in quan­tum me­chan­ics).

In par­ti­cle physics, this in­equal­ity per­mits a qual­i­ta­tive un­der­stand­ing of vir­tual par­ti­cles which carry mo­men­tum, ex­change by which and with real par­ti­cles, is re­spon­si­ble for the cre­ation of all known fun­da­men­tal forces (more ac­cu­rately known as fun­da­men­tal in­ter­ac­tions). Vir­tual pho­tons (which are sim­ply low­est quan­tum me­chan­i­cal en­ergy state of pho­tons) are also re­spon­si­ble for elec­tro­sta­tic in­ter­ac­tion be­tween elec­tric charges (which re­sults in Coulomb law), for spon­ta­neous ra­dia­tive decay of ex­ited atomic and nu­clear states, for the Casimir force, for van der Waals bond forces and some other ob­serv­able phe­nom­ena.

Energy transfer
Closed systems

En­ergy trans­fer can be con­sid­ered for the spe­cial case of sys­tems which are closed to trans­fers of mat­ter. The por­tion of the en­ergy which is trans­ferred by con­ser­v­a­tive forces over a dis­tance is mea­sured as the work the source sys­tem does on the re­ceiv­ing sys­tem. The por­tion of the en­ergy which does not do work dur­ing the trans­fer is called heat. En­ergy can be trans­ferred be­tween sys­tems in a va­ri­ety of ways. Ex­am­ples in­clude the trans­mis­sion of elec­tro­mag­netic en­ergy via pho­tons, phys­i­cal col­li­sions which trans­fer ki­netic en­ergy, and the con­duc­tive trans­fer of ther­mal en­ergy.

En­ergy is strictly con­served and is also lo­cally con­served wher­ever it can be de­fined. In ther­mo­dy­nam­ics, for closed sys­tems, the process of en­ergy trans­fer is de­scribed by the first law:

\Delta {}E=W+Q    (1)

where E is the amount of en­ergy trans­ferred, W  rep­re­sents the work done on the sys­tem, and Q rep­re­sents the heat flow into the sys­tem. As a sim­pli­fi­ca­tion, the heat term, Q, is some­times ig­nored, es­pe­cially when the ther­mal ef­fi­ciency of the trans­fer is high.

\Delta {}E=W    (2)

This sim­pli­fied equa­tion is the one used to de­fine the joule, for ex­am­ple.

Open systems

Be­yond the con­straints of closed sys­tems, open sys­tems can gain or lose en­ergy in as­so­ci­a­tion with mat­ter trans­fer (both of these process are il­lus­trated by fu­el­ing an auto, a sys­tem which gains in en­ergy thereby, with­out ad­di­tion of ei­ther work or heat). De­not­ing this en­ergy by E, one may write

\Delta {}E=W+Q+E.    (3)
Internal energy

In­ter­nal en­ergy is the sum of all mi­cro­scopic forms of en­ergy of a sys­tem. It is the en­ergy needed to cre­ate the sys­tem. It is re­lated to the po­ten­tial en­ergy, e.g., mol­e­c­u­lar struc­ture, crys­tal struc­ture, and other geo­met­ric as­pects, as well as the mo­tion of the par­ti­cles, in form of ki­netic en­ergy. Ther­mo­dy­nam­ics is chiefly con­cerned with changes in in­ter­nal en­ergy and not its ab­solute value, which is im­pos­si­ble to de­ter­mine with ther­mo­dy­nam­ics alone.

First law of thermodynamics

The first law of ther­mo­dy­nam­ics as­serts that en­ergy (but not nec­es­sar­ily ther­mo­dy­namic free en­ergy) is al­ways conserved and that heat flow is a form of en­ergy trans­fer. For ho­mo­ge­neous sys­tems, with a well-de­fined tem­per­a­ture and pres­sure, a com­monly used corol­lary of the first law is that, for a sys­tem sub­ject only to pres­sure forces and heat trans­fer (e.g., a cylin­der-full of gas) with­out chem­i­cal changes, the dif­fer­en­tial change in the in­ter­nal en­ergy of the sys­tem (with a gain in en­ergy sig­ni­fied by a pos­i­tive quan­tity) is given as\mathrm {d} E=T\mathrm {d} S-P\mathrm {d} V\,,

where the first term on the right is the heat trans­ferred into the sys­tem, ex­pressed in terms of tem­per­a­ture T and en­tropy S (in which en­tropy in­creases and the change dS is pos­i­tive when the sys­tem is heated), and the last term on the right hand side is iden­ti­fied as work done on the sys­tem, where pres­sure is P and vol­ume V (the neg­a­tive sign re­sults since com­pres­sion of the sys­tem re­quires work to be done on it and so the vol­ume change, dV, is neg­a­tive when work is done on the sys­tem).

This equa­tion is highly spe­cific, ig­nor­ing all chem­i­cal, elec­tri­cal, nu­clear, and grav­i­ta­tional forces, ef­fects such as ad­vec­tion of any form of en­ergy other than heat and pV-work. The gen­eral for­mu­la­tion of the first law (i.e., con­ser­va­tion of en­ergy) is valid even in sit­u­a­tions in which the sys­tem is not ho­mo­ge­neous. For these cases the change in in­ter­nal en­ergy of a closed sys­tem is ex­pressed in a gen­eral form by\mathrm {d} E=\delta Q+\delta W

where \delta Q is the heat sup­plied to the sys­tem and \delta W is the work ap­plied to the sys­tem.

Equipartition of energy

The en­ergy of a me­chan­i­cal har­monic os­cil­la­tor (a mass on a spring) is al­ter­na­tively ki­netic and po­ten­tial en­ergy. At two points in the os­cil­la­tion cycle it is en­tirely ki­netic, and at two points it is en­tirely po­ten­tial. Over the whole cycle, or over many cy­cles, net en­ergy is thus equally split be­tween ki­netic and po­ten­tial. This is called equipar­ti­tion prin­ci­ple; total en­ergy of a sys­tem with many de­grees of free­dom is equally split among all avail­able de­grees of free­dom.

This prin­ci­ple is vi­tally im­por­tant to un­der­stand­ing the be­hav­iour of a quan­tity closely re­lated to en­ergy, called en­tropy. En­tropy is a mea­sure of even­ness of a dis­tri­b­u­tion of en­ergy be­tween parts of a sys­tem. When an iso­lated sys­tem is given more de­grees of free­dom (i.e., given new avail­able en­ergy states that are the same as ex­ist­ing states), then total en­ergy spreads over all avail­able de­grees equally with­out dis­tinc­tion be­tween “new” and “old” de­grees. This math­e­mat­i­cal re­sult is called the sec­ond law of ther­mo­dy­nam­ics. The sec­ond law of ther­mo­dy­nam­ics is valid only for sys­tems which are near or in equi­lib­rium state. For non-equi­lib­rium sys­tems, the laws gov­ern­ing sys­tem’s be­hav­ior are still de­bat­able. One of the guid­ing prin­ci­ples for these sys­tems is the prin­ci­ple of max­i­mum en­tropy pro­duc­tion. It states that non­equi­lib­rium sys­tems be­have in such a way to max­i­mize its en­tropy production


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“Summer Sunshine” Peaceful Relaxing Instrumental Music, Meditation Nature Music “Summer Sunshine” by Tim Janis


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From Wikipedia, the free encyclopedia

Sunlight is a portion of the electromagnetic radiation given off by the Sun, in particular infraredvisible, and ultraviolet light. On Earth, sunlight is scattered and filtered through Earth’s atmosphere, and is obvious as daylight when the Sun is above the horizon. When direct solar radiation is not blocked by clouds, it is experienced as sunshine, a combination of bright light and radiant heat. When blocked by clouds or reflected off other objects, sunlight is diffused. The World Meteorological Organization uses the term “sunshine duration” to mean the cumulative time during which an area receives direct irradiance from the Sun of at least 120 watts per square meter.[1] Other sources indicate an “Average over the entire earth” of “164 Watts per square meter over a 24 hour day”.[2]

The ultraviolet radiation in sunlight has both positive and negative health effects, as it is both a requisite for vitamin D3 synthesis and a mutagen.

Sunlight takes about 8.3 minutes to reach Earth from the surface of the Sun. A photon starting at the center of the Sun and changing direction every time it encounters a charged particle would take between 10,000 and 170,000 years to get to the surface.[3]

Sunlight is a key factor in photosynthesis, the process used by plants and other autotrophic organisms to convert light energy, normally from the Sun, into chemical energy that can be used to synthesize carbohydrates and to fuel the organisms’ activities.


  • ✪ Radiation from the Sun and Earth
  • ✪ Sunlight through a prism (aka electromagnetic radiation in the visible spectrum)
  • ✪ Unit 4 Climate Ch-9 Solar Radiation heat Budget and Temperature part 2
  • ✪ The Earth’s Energy Balance
  • ✪ 106 – Grasshopper & Ladybug – Verifying Parametric Shading Blocks Sunlight with Radiation Analysis


Researchers can measure the intensity of sunlight using a sunshine recorderpyranometer, or pyrheliometer. To calculate the amount of sunlight reaching the ground, both the eccentricity of Earth’s elliptic orbit and the attenuation by Earth’s atmosphere have to be taken into account. The extraterrestrial solar illuminance (Eext), corrected for the elliptic orbit by using the day number of the year (dn), is given to a good approximation by[4]E_{\rm {ext}}=E_{\rm {sc}}\cdot \left(1+0.033412\cdot \cos \left(2\pi {\frac {{\rm {dn}}-3}{365}}\right)\right),

where dn=1 on January 1; dn=32 on February 1; dn=59 on March 1 (except on leap years, where dn=60), etc. In this formula dn–3 is used, because in modern times Earth’s perihelion, the closest approach to the Sun and, therefore, the maximum Eext occurs around January 3 each year. The value of 0.033412 is determined knowing that the ratio between the perihelion (0.98328989 AU) squared and the aphelion (1.01671033 AU) squared should be approximately 0.935338.

The solar illuminance constant (Esc), is equal to 128×103 lux. The direct normal illuminance (Edn), corrected for the attenuating effects of the atmosphere is given by:E_{\rm {dn}}=E_{\rm {ext}}\,e^{-cm},

where c is the atmospheric extinction and m is the relative optical airmass. The atmospheric extinction brings the number of lux down to around 100 000 lux.

The total amount of energy received at ground level from the Sun at the zenith depends on the distance to the Sun and thus on the time of year. It is about 3.3% higher than average in January and 3.3% lower in July (see below). If the extraterrestrial solar radiation is 1367 watts per square meter (the value when the Earth–Sun distance is 1 astronomical unit), then the direct sunlight at Earth’s surface when the Sun is at the zenith is about 1050 W/m2, but the total amount (direct and indirect from the atmosphere) hitting the ground is around 1120 W/m2.[5] In terms of energy, sunlight at Earth’s surface is around 52 to 55 percent infrared (above 700 nm), 42 to 43 percent visible (400 to 700 nm), and 3 to 5 percent ultraviolet (below 400 nm).[6] At the top of the atmosphere, sunlight is about 30% more intense, having about 8% ultraviolet (UV),[7] with most of the extra UV consisting of biologically damaging short-wave ultraviolet.[8]

Direct sunlight has a luminous efficacy of about 93 lumens per watt of radiant flux. Multiplying the figure of 1050 watts per square meter by 93 lumens per watt indicates that bright sunlight provides an illuminance of approximately 98 000 lux (lumens per square meter) on a perpendicular surface at sea level. The illumination of a horizontal surface will be considerably less than this if the Sun is not very high in the sky. Averaged over a day, the highest amount of sunlight on a horizontal surface occurs in January at the South Pole (see insolation).

Dividing the irradiance of 1050 W/m2 by the size of the Sun’s disk in steradians gives an average radiance of 15.4 MW per square metre per steradian. (However, the radiance at the center of the sun’s disk is somewhat higher than the average over the whole disk due to limb darkening.) Multiplying this by π gives an upper limit to the irradiance which can be focused on a surface using mirrors: 48.5 MW/m2.

Composition and power

Solar irradiance spectrum above atmosphere and at surface. Extreme UV and X-rays are produced (at left of wavelength range shown) but comprise very small amounts of the Sun’s total output power.See also: UltravioletInfrared, and Light

The spectrum of the Sun’s solar radiation is close to that of a black body[9][10] with a temperature of about 5,800 K.[11] The Sun emits EM radiation across most of the electromagnetic spectrum. Although the Sun produces gamma rays as a result of the nuclear-fusion process, internal absorption and thermalization convert these super-high-energy photons to lower-energy photons before they reach the Sun’s surface and are emitted out into space. As a result, the Sun does not emit gamma rays from this process, but it does emit gamma rays from solar flares.[12] The Sun also emits X-raysultravioletvisible lightinfrared, and even radio waves;[13] the only direct signature of the nuclear process is the emission of neutrinos.

Although the solar corona is a source of extreme ultraviolet and X-ray radiation, these rays make up only a very small amount of the power output of the Sun (see spectrum at right). The spectrum of nearly all solar electromagnetic radiation striking the Earth’s atmosphere spans a range of 100 nm to about 1 mm (1,000,000 nm). This band of significant radiation power can be divided into five regions in increasing order of wavelengths:[14]

  • Ultraviolet C or (UVC) range, which spans a range of 100 to 280 nm. The term ultraviolet refers to the fact that the radiation is at higher frequency than violet light (and, hence, also invisible to the human eye). Due to absorption by the atmosphere very little reaches Earth’s surface. This spectrum of radiation has germicidal properties, as used in germicidal lamps.
  • Ultraviolet B or (UVB) range spans 280 to 315 nm. It is also greatly absorbed by the Earth’s atmosphere, and along with UVC causes the photochemical reaction leading to the production of the ozone layer. It directly damages DNA and causes sunburn, but is also required for vitamin D synthesis in the skin of mammals.[15]
  • Ultraviolet A or (UVA) spans 315 to 400 nm. This band was once held to be less damaging to DNA, and hence is used in cosmetic artificial sun tanning (tanning booths and tanning beds) and PUVA therapy for psoriasis. However, UVA is now known to cause significant damage to DNA via indirect routes (formation of free radicals and reactive oxygen species), and can cause cancer.[16]
  • Visible range or light spans 380 to 700 nm.[17] As the name suggests, this range is visible to the naked eye. It is also the strongest output range of the Sun’s total irradiance spectrum.
  • Infrared range that spans 700 nm to 1,000,000 nm (1 mm). It comprises an important part of the electromagnetic radiation that reaches Earth. Scientists divide the infrared range into three types on the basis of wavelength:
    • Infrared-A: 700 nm to 1,400 nm
    • Infrared-B: 1,400 nm to 3,000 nm
    • Infrared-C: 3,000 nm to 1 mm.

Published tables

Tables of direct solar radiation on various slopes from 0 to 60 degrees north latitude, in calories per square centimetre, issued in 1972 and published by Pacific Northwest Forest and Range Experiment Station, Forest Service, U.S. Department of Agriculture, Portland, Oregon, USA, appear on the web.[18]

Solar constant

Main article: Solar constant

Solar irradiance spectrum at top of atmosphere, on a linear scale and plotted against wavenumber

Solar irradiance spectrum at top of atmosphere, on a linear scale and plotted against wavenumber

The solar constant is a measure of flux density, is the amount of incoming solar electromagnetic radiation per unit area that would be incident on a plane perpendicular to the rays, at a distance of one astronomical unit (AU) (roughly the mean distance from the Sun to Earth). The “solar constant” includes all types of solar radiation, not just the visible light. Its average value was thought to be approximately 1366 W/m²,[19] varying slightly with solar activity, but recent recalibrations of the relevant satellite observations indicate a value closer to 1361 W/m² is more realistic.[20]

Total solar irradiance (TSI) and spectral solar irradiance (SSI) upon Earth

Since 1978 a series of overlapping NASA and ESA satellite experiments have measured total solar irradiance (TSI) – the amount of solar radiation received at the top of Earth’s atmosphere – as 1.365 kilo⁠watts per square meter (kW/m²).[19][21][22][23] TSI observations continue with the ACRIMSAT/ACRIM3, SOHO/VIRGO and SORCE/TIM satellite experiments.[24] Observations have revealed variation of TSI on many timescales, including the solar magnetic cycle[25] and many shorter periodic cycles.[26] TSI provides the energy that drives Earth’s climate, so continuation of the TSI time-series database is critical to understanding the role of solar variability in climate change.

Since 2003 the SORCE Spectral Irradiance Monitor (SIM) has monitored Spectral solar irradiance (SSI) – the spectral distribution of the TSI. Data indicate that SSI at UV (ultraviolet) wavelength corresponds in a less clear, and probably more complicated fashion, with Earth’s climate responses than earlier assumed, fueling broad avenues of new research in “the connection of the Sun and stratosphere, troposphere, biosphere, ocean, and Earth’s climate”.[27]

Intensity in the Solar System

Sunlight on Mars is dimmer than on Earth. This photo of a Martian sunset was imaged by Mars Pathfinder.

Sunlight on Mars is dimmer than on Earth. This photo of a Martian sunset was imaged by Mars Pathfinder.

Different bodies of the Solar System receive light of an intensity inversely proportional to the square of their distance from Sun.

A table comparing the amount of solar radiation received by each planet in the Solar System at the top of its atmosphere:[28]

Planet or dwarf planetdistance (AU)Solar radiation (W/m²)

The actual brightness of sunlight that would be observed at the surface also depends on the presence and composition of an atmosphere. For example, Venus’s thick atmosphere reflects more than 60% of the solar light it receives. The actual illumination of the surface is about 14,000 lux, comparable to that on Earth “in the daytime with overcast clouds”.[29]

Sunlight on Mars would be more or less like daylight on Earth during a slightly overcast day, and, as can be seen in the pictures taken by the rovers, there is enough diffuse sky radiation that shadows would not seem particularly dark. Thus, it would give perceptions and “feel” very much like Earth daylight. The spectrum on the surface is slightly redder than that on Earth, due to scattering by reddish dust in the Martian atmosphere.

For comparison, sunlight on Saturn is slightly brighter than Earth sunlight at the average sunset or sunrise (see daylight for comparison table). Even on Pluto, the sunlight would still be bright enough to almost match the average living room. To see sunlight as dim as full moonlight on Earth, a distance of about 500 AU (~69 light-hours) is needed; only a handful of objects in the Solar System have been discovered that are known to orbit farther than such a distance, among them 90377 Sedna and (87269) 2000 OO67.

Surface illumination

Sunlight shining through clouds, giving rise to crepuscular rays

Sunlight shining through clouds, giving rise to crepuscular rays

The spectrum of surface illumination depends upon solar elevation due to atmospheric effects, with the blue spectral component dominating during twilight before and after sunrise and sunset, respectively, and red dominating during sunrise and sunset. These effects are apparent in natural light photography where the principal source of illumination is sunlight as mediated by the atmosphere.

While the color of the sky is usually determined by Rayleigh scattering, an exception occurs at sunset and twilight. “Preferential absorption of sunlight by ozone over long horizon paths gives the zenith sky its blueness when the sun is near the horizon”.[30]

See diffuse sky radiation for more details.

Spectral composition of sunlight at Earth’s surface

The Sun may be said to illuminate, which is a measure of the light within a specific sensitivity range. Many animals (including humans) have a sensitivity range of approximately 400–700 nm,[31] and given optimal conditions the absorption and scattering by Earth’s atmosphere produces illumination that approximates an equal-energy illuminant for most of this range.[32] The useful range for color vision in humans, for example, is approximately 450–650 nm. Aside from effects that arise at sunset and sunrise, the spectral composition changes primarily in respect to how directly sunlight is able to illuminate. When illumination is indirect, Rayleigh scattering in the upper atmosphere will lead blue wavelengths to dominate. Water vapour in the lower atmosphere produces further scattering and ozone, dust and water particles will also absorb particular wavelengths.[33][34]Spectrum of the visible wavelengths at approximately sea level; illumination by direct sunlight compared with direct sunlight scattered by cloud cover and with indirect sunlight by varying degrees of cloud cover. The yellow line shows the spectrum of direct illumination under optimal conditions. The other illumination conditions are scaled to show their relation to direct illumination. The units of spectral power are simply raw sensor values (with a linear response at specific wavelengths).

Variations in solar irradiance
Seasonal and orbital variation

Further information: Insolation and Sunshine duration

On Earth, the solar radiation varies with the angle of the Sun above the horizon, with longer sunlight duration at high latitudes during summer, varying to no sunlight at all in winter near the pertinent pole. When the direct radiation is not blocked by clouds, it is experienced as sunshine. The warming of the ground (and other objects) depends on the absorption of the electromagnetic radiation in the form of heat.

The amount of radiation intercepted by a planetary body varies inversely with the square of the distance between the star and the planet. Earth’s orbit and obliquity change with time (over thousands of years), sometimes forming a nearly perfect circle, and at other times stretching out to an orbital eccentricity of 5% (currently 1.67%). As the orbital eccentricity changes, the average distance from the Sun (the semimajor axis does not significantly vary, and so the total insolation over a year remains almost constant due to Kepler’s second law,{\tfrac {2A}{r^{2}}}dt=d\theta ,

where A is the “areal velocity” invariant. That is, the integration over the orbital period (also invariant) is a constant.\int _{0}^{T}{\tfrac {2A}{r^{2}}}dt=\int _{0}^{2\pi }d\theta =\mathrm {constant} .

If we assume the solar radiation power P as a constant over time and the solar irradiation given by the inverse-square law, we obtain also the average insolation as a constant.

But the seasonal and latitudinal distribution and intensity of solar radiation received at Earth’s surface does vary.[35] The effect of Sun angle on climate results in the change in solar energy in summer and winter. For example, at latitudes of 65 degrees, this can vary by more than 25% as a result of Earth’s orbital variation. Because changes in winter and summer tend to offset, the change in the annual average insolation at any given location is near zero, but the redistribution of energy between summer and winter does strongly affect the intensity of seasonal cycles. Such changes associated with the redistribution of solar energy are considered a likely cause for the coming and going of recent ice ages (see: Milankovitch cycles).

Solar intensity variation

Further information: Solar variation

Space-based observations of solar irradiance started in 1978. These measurements show that the solar constant is not constant. It varies on many time scales, including the 11-year sunspot solar cycle.[25] When going further back in time, one has to rely on irradiance reconstructions, using sunspots for the past 400 years or cosmogenic radionuclides for going back 10,000 years. Such reconstructions have been done.[36][37][38][39] These studies show that in addition to the solar irradiance variation with the solar cycle (the (Schwabe) cycle), the solar activity varies with longer cycles, such as the proposed 88 year (Gleisberg cycle), 208 year (DeVries cycle) and 1,000 year (Eddy cycle).

Life on Earth

The existence of nearly all life on Earth is fueled by light from the Sun. Most autotrophs, such as plants, use the energy of sunlight, combined with carbon dioxide and water, to produce simple sugars—a process known as photosynthesis. These sugars are then used as building-blocks and in other synthetic pathways that allow the organism to grow.

Heterotrophs, such as animals, use light from the Sun indirectly by consuming the products of autotrophs, either by consuming autotrophs, by consuming their products, or by consuming other heterotrophs. The sugars and other molecular components produced by the autotrophs are then broken down, releasing stored solar energy, and giving the heterotroph the energy required for survival. This process is known as cellular respiration.

In prehistory, humans began to further extend this process by putting plant and animal materials to other uses. They used animal skins for warmth, for example, or wooden weapons to hunt. These skills allowed humans to harvest more of the sunlight than was possible through glycolysis alone, and human population began to grow.

During the Neolithic Revolution, the domestication of plants and animals further increased human access to solar energy. Fields devoted to crops were enriched by inedible plant matter, providing sugars and nutrients for future harvests. Animals that had previously provided humans with only meat and tools once they were killed were now used for labour throughout their lives, fueled by grasses inedible to humans.

The more recent discoveries of coalpetroleum and natural gas are modern extensions of this trend. These fossil fuels are the remnants of ancient plant and animal matter, formed using energy from sunlight and then trapped within Earth for millions of years. Because the stored energy in these fossil fuels has accumulated over many millions of years, they have allowed modern humans to massively increase the production and consumption of primary energy. As the amount of fossil fuel is large but finite, this cannot continue indefinitely, and various theories exist as to what will follow this stage of human civilization (e.g., alternative fuelsMalthusian catastrophenew urbanismpeak oil).

Cultural aspects

Eduard Manet: Le déjeuner sur l'herbe (1862-63)

Eduard ManetLe déjeuner sur l’herbe (1862-63)

The effect of sunlight is relevant to painting, evidenced for instance in works of Eduard Manet and Claude Monet on outdoor scenes and landscapes.

Téli verőfény ("Winter Sunshine") by László Mednyánszky, early 20th century

Téli verőfény (“Winter Sunshine”) by László Mednyánszky, early 20th century

Many people find direct sunlight to be too bright for comfort, especially when reading from white paper upon which the sunlight is directly shining. Indeed, looking directly at the Sun can cause long-term vision damage. To compensate for the brightness of sunlight, many people wear sunglassesCars, many helmets and caps are equipped with visors to block the Sun from direct vision when the Sun is at a low angle. Sunshine is often blocked from entering buildings through the use of wallswindow blindsawningsshutterscurtains, or nearby shade trees. Sunshine exposure is needed biologically for the creation of Vitamin D in the skin, a vital compound needed to make strong bone and muscle in the body.

In colder countries, many people prefer sunnier days and often avoid the shade. In hotter countries, the converse is true; during the midday hours, many people prefer to stay inside to remain cool. If they do go outside, they seek shade that may be provided by trees, parasols, and so on.

In many world religions, such as Hinduism, the Sun is considered to be a god, as it is the source of life and energy on Earth. It also formed the basis for religion in Ancient Egypt.


Main article: Sun tanning

Sunbathing is a popular leisure activity in which a person sits or lies in direct sunshine. People often sunbathe in comfortable places where there is ample sunlight. Some common places for sunbathing include beaches, open air swimming poolsparksgardens, and sidewalk cafes. Sunbathers typically wear limited amounts of clothing or some simply go nude. For some, an alternative to sunbathing is the use of a sunbed that generates ultraviolet light and can be used indoors regardless of weather conditions. Tanning beds have been banned in a number of states in the world.

For many people with light skin, one purpose for sunbathing is to darken one’s skin color (get a sun tan), as this is considered in some cultures to be attractive, associated with outdoor activity, vacations/holidays, and health. Some people prefer naked sunbathing so that an “all-over” or “even” tan can be obtained, sometimes as part of a specific lifestyle.

Controlled heliotherapy, or sunbathing, has been used as a treatment for psoriasis and other maladies.

Skin tanning is achieved by an increase in the dark pigment inside skin cells called melanocytes, and is an automatic response mechanism of the body to sufficient exposure to ultraviolet radiation from the Sun or from artificial sunlamps. Thus, the tan gradually disappears with time, when one is no longer exposed to these sources.

Effects on human health

Main article: Health effects of sunlight exposure

The ultraviolet radiation in sunlight has both positive and negative health effects, as it is both a principal source of vitamin D3 and a mutagen.[40] A dietary supplement can supply vitamin D without this mutagenic effect,[41] but bypasses natural mechanisms that would prevent overdoses of vitamin D generated internally from sunlight. Vitamin D has a wide range of positive health effects, which include strengthening bones[42] and possibly inhibiting the growth of some cancers.[43][44] Sun exposure has also been associated with the timing of melatonin synthesis, maintenance of normal circadian rhythms, and reduced risk of seasonal affective disorder.[45]

Long-term sunlight exposure is known to be associated with the development of skin cancerskin agingimmune suppression, and eye diseases such as cataracts and macular degeneration.[46] Short-term overexposure is the cause of sunburnsnow blindness, and solar retinopathy.

UV rays, and therefore sunlight and sunlamps, are the only listed carcinogens that are known to have health benefits,[47] and a number of public health organizations state that there needs to be a balance between the risks of having too much sunlight or too little.[48] There is a general consensus that sunburn should always be avoided.

Epidemiological data shows that people who have more exposure to sunlight have less high blood pressure and cardiovascular-related mortality. While sunlight (and its UV rays) are a risk factor for skin cancer, “sun avoidance may carry more of a cost than benefit for over-all good health.”[49] A study found that there is no evidence that UV reduces lifespan in contrast to other risk factors like smoking, alcohol and high blood pressure.[49]

Effect on plant genomes

Elevated solar UV-B doses increase the frequency of DNA recombination in Arabidopsis thaliana and tobacco (Nicotiana tabacum) plants.[50] These increases are accompanied by strong induction of an enzyme with a key role in recombinational repair of DNA damage. Thus the level of terrestrial solar UV-B radiation likely affects genome stability in plants.


Classes of fire
  • Class A – fires involving solid materials such as wood, paper or textiles.
  • Class B – fires involving flammable liquids such as petrol, diesel or oils.
  • Class C – fires involving gases.
  • Class D – fires involving metals.
  • Class E – fires involving live electrical apparatus. (Technically ‘Class E’ doesn’t exists however this is used for convenience here)
  • Class F – fires involving cooking oils such as in deep-fat fryers.



From Wikipedia, the free encyclopedia

For other uses, see Fire (disambiguation) and Firing (disambiguation).

An outdoor wood fire

An outdoor wood fireFile:Feu-de-paille-couverture.ogvPlay mediaThe ignition and extinguishing of a pile of wood shavingsFile:MOD14A1 M FIRE.ogvPlay mediaThe fire maps show the locations of actively burning fires around the world on a monthly basis, based on observations from the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite. The colors are based on a count of the number (not size) of fires observed within a 1,000-square-kilometer area. White pixels show the high end of the count—as many as 100 fires in a 1,000-square-kilometer area per day. Yellow pixels show as many as 10 fires, orange shows as many as five fires, and red areas as few as one fire per day.

Fire is the rapid ox­i­da­tion of a ma­te­r­ial in the exother­mic chem­i­cal process of com­bus­tion, re­leas­ing heatlight, and var­i­ous re­ac­tion prod­ucts. Fire is hot be­cause the con­ver­sion of the weak dou­ble bond in mol­e­c­u­lar oxy­gen, O2, to the stronger bonds in the com­bus­tion prod­ucts car­bon diox­ide and water re­leases en­ergy (418 kJ per 32 g of O2); the bond en­er­gies of the fuel play only a minor role here. At a cer­tain point in the com­bus­tion re­ac­tion, called the ig­ni­tion point, flames are pro­duced. The flame is the vis­i­ble por­tion of the fire. Flames con­sist pri­mar­ily of car­bon diox­ide, water vapor, oxy­gen and ni­tro­gen. If hot enough, the gases may be­come ion­ized to pro­duce plasma. De­pend­ing on the sub­stances alight, and any im­pu­ri­ties out­side, the color of the flame and the fire’s in­ten­sity will be dif­fer­ent.

Fire in its most com­mon form can re­sult in con­fla­gra­tion, which has the po­ten­tial to cause phys­i­cal dam­age through burn­ing. Fire is an im­por­tant process that af­fects eco­log­i­cal sys­tems around the globe. The pos­i­tive ef­fects of fire in­clude stim­u­lat­ing growth and main­tain­ing var­i­ous eco­log­i­cal sys­tems. Its neg­a­tive ef­fects in­clude haz­ard to life and prop­erty, at­mos­pheric pol­lu­tion, and water contamination. If fire re­moves pro­tec­tive veg­e­ta­tion, heavy rain­fall may lead to an in­crease in soil ero­sion by water. Also, when veg­e­ta­tion is burned, the ni­tro­gen it con­tains is re­leased into the at­mos­phere, un­like el­e­ments such as potas­sium and phos­pho­rus which re­main in the ash and are quickly re­cy­cled into the soil. This loss of ni­tro­gen caused by a fire pro­duces a long-term re­duc­tion in the fer­til­ity of the soil, but this fe­cun­dity can po­ten­tially be re­cov­ered as mol­e­c­u­lar ni­tro­gen in the at­mos­phere is “fixed” and con­verted to am­mo­nia by nat­ural phe­nom­ena such as light­ning and by legu­mi­nous plants that are “ni­tro­gen-fix­ing” such as cloverpeas, and green beans.

Fire has been used by hu­mans in rit­u­als, in agri­cul­ture for clear­ing land, for cook­ing, gen­er­at­ing heat and light, for sig­nal­ing, propul­sion pur­poses, smelt­ingforg­ingin­cin­er­a­tion of waste, cre­ma­tion, and as a weapon or mode of de­struc­tion.


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Physical properties


Main article: CombustionThe fire tetrahedron

Fires start when a flam­ma­ble or a com­bustible ma­te­r­ial, in com­bi­na­tion with a suf­fi­cient quan­tity of an ox­i­dizer such as oxy­gen gas or an­other oxy­gen-rich com­pound (though non-oxy­gen ox­i­diz­ers exist), is ex­posed to a source of heat or am­bi­ent tem­per­a­ture above the flash point for the fuel/ox­i­dizer mix, and is able to sus­tain a rate of rapid ox­i­da­tion that pro­duces a chain re­ac­tion. This is com­monly called the fire tetra­he­dron. Fire can­not exist with­out all of these el­e­ments in place and in the right pro­por­tions. For ex­am­ple, a flam­ma­ble liq­uid will start burn­ing only if the fuel and oxy­gen are in the right pro­por­tions. Some fuel-oxy­gen mixes may re­quire a cat­a­lyst, a sub­stance that is not con­sumed, when added, in any chem­i­cal re­ac­tion dur­ing com­bus­tion, but which en­ables the re­ac­tants to com­bust more read­ily.

Once ig­nited, a chain re­ac­tion must take place whereby fires can sus­tain their own heat by the fur­ther re­lease of heat en­ergy in the process of com­bus­tion and may prop­a­gate, pro­vided there is a con­tin­u­ous sup­ply of an ox­i­dizer and fuel.

If the ox­i­dizer is oxy­gen from the sur­round­ing air, the pres­ence of a force of grav­ity, or of some sim­i­lar force caused by ac­cel­er­a­tion, is nec­es­sary to pro­duce con­vec­tion, which re­moves com­bus­tion prod­ucts and brings a sup­ply of oxy­gen to the fire. With­out grav­ity, a fire rapidly sur­rounds it­self with its own com­bus­tion prod­ucts and non-ox­i­diz­ing gases from the air, which ex­clude oxy­gen and ex­tin­guish the fire. Be­cause of this, the risk of fire in a space­craft is small when it is coast­ing in in­er­tial flight. This does not apply if oxy­gen is sup­plied to the fire by some process other than ther­mal con­vec­tion.

Fire can be ex­tin­guished by re­mov­ing any one of the el­e­ments of the fire tetra­he­dron. Con­sider a nat­ural gas flame, such as from a stove-top burner. The fire can be ex­tin­guished by any of the fol­low­ing:

  • turning off the gas supply, which removes the fuel source;
  • covering the flame completely, which smothers the flame as the combustion both uses the available oxidizer (the oxygen in the air) and displaces it from the area around the flame with CO2;
  • application of water, which removes heat from the fire faster than the fire can produce it (similarly, blowing hard on a flame will displace the heat of the currently burning gas from its fuel source, to the same end), or
  • application of a retardant chemical such as Halon to the flame, which retards the chemical reaction itself until the rate of combustion is too slow to maintain the chain reaction.

In con­trast, fire is in­ten­si­fied by in­creas­ing the over­all rate of com­bus­tion. Meth­ods to do this in­clude bal­anc­ing the input of fuel and ox­i­dizer to sto­i­chio­met­ric pro­por­tions, in­creas­ing fuel and ox­i­dizer input in this bal­anced mix, in­creas­ing the am­bi­ent tem­per­a­ture so the fire’s own heat is bet­ter able to sus­tain com­bus­tion, or pro­vid­ing a cat­a­lyst, a non-re­ac­tant medium in which the fuel and ox­i­dizer can more read­ily react.


Main article: FlameSee also: Flame test

A candle's flame

candle‘s flame

Northwest Crown Fire Experiment, Canada

Northwest Crown Fire Experiment, Canada

Photo of a fire taken with a 1/4000th of a second exposure

Photo of a fire taken with a 1/4000th of a second exposure

Fire is affected by gravity. Left: Flame on Earth; Right: Flame on the ISS

Fire is affected by gravity. Left: Flame on Earth; Right: Flame on the ISS

A flame is a mix­ture of re­act­ing gases and solids emit­ting vis­i­ble, in­frared, and some­times ul­tra­vi­o­let light, the fre­quency spec­trum of which de­pends on the chem­i­cal com­po­si­tion of the burn­ing ma­te­r­ial and in­ter­me­di­ate re­ac­tion prod­ucts. In many cases, such as the burn­ing of or­ganic mat­ter, for ex­am­ple wood, or the in­com­plete com­bus­tion of gas, in­can­des­cent solid par­ti­cles called soot pro­duce the fa­mil­iar red-or­ange glow of “fire”. This light has a con­tin­u­ous spec­trum. Com­plete com­bus­tion of gas has a dim blue color due to the emis­sion of sin­gle-wave­length ra­di­a­tion from var­i­ous elec­tron tran­si­tions in the ex­cited mol­e­cules formed in the flame. Usu­ally oxy­gen is in­volved, but hy­dro­gen burn­ing in chlo­rine also pro­duces a flame, pro­duc­ing hy­dro­gen chlo­ride (HCl). Other pos­si­ble com­bi­na­tions pro­duc­ing flames, amongst many, are flu­o­rine and hy­dro­gen, and hy­drazine and ni­tro­gen tetrox­ide. Hy­dro­gen and hydrazine/UDMH flames are sim­i­larly pale blue, while burn­ing boron and its com­pounds, eval­u­ated in mid-20th cen­tury as a high en­ergy fuel for jet and rocket en­gines, emits in­tense green flame, lead­ing to its in­for­mal nick­name of “Green Dragon”.

The glow of a flame is com­plex. Black-body ra­di­a­tion is emit­ted from soot, gas, and fuel par­ti­cles, though the soot par­ti­cles are too small to be­have like per­fect black­bod­ies. There is also pho­ton emis­sion by de-ex­cited atoms and mol­e­cules in the gases. Much of the ra­di­a­tion is emit­ted in the vis­i­ble and in­frared bands. The color de­pends on tem­per­a­ture for the black-body ra­di­a­tion, and on chem­i­cal makeup for the emis­sion spec­tra. The dom­i­nant color in a flame changes with tem­per­a­ture. The photo of the for­est fire in Canada is an ex­cel­lent ex­am­ple of this vari­a­tion. Near the ground, where most burn­ing is oc­cur­ring, the fire is white, the hottest color pos­si­ble for or­ganic ma­te­r­ial in gen­eral, or yel­low. Above the yel­low re­gion, the color changes to or­ange, which is cooler, then red, which is cooler still. Above the red re­gion, com­bus­tion no longer oc­curs, and the un­com­busted car­bon par­ti­cles are vis­i­ble as black smoke.

The com­mon dis­tri­b­u­tion of a flame under nor­mal grav­ity con­di­tions de­pends on con­vec­tion, as soot tends to rise to the top of a gen­eral flame, as in a can­dle in nor­mal grav­ity con­di­tions, mak­ing it yel­low. In micro grav­ity or zero grav­ity, such as an en­vi­ron­ment in outer space, con­vec­tion no longer oc­curs, and the flame be­comes spher­i­cal, with a ten­dency to be­come more blue and more ef­fi­cient (al­though it may go out if not moved steadily, as the CO2 from com­bus­tion does not dis­perse as read­ily in micro grav­ity, and tends to smother the flame). There are sev­eral pos­si­ble ex­pla­na­tions for this dif­fer­ence, of which the most likely is that the tem­per­a­ture is suf­fi­ciently evenly dis­trib­uted that soot is not formed and com­plete com­bus­tion occurs. Ex­per­i­ments by NASA re­veal that dif­fu­sion flames in micro grav­ity allow more soot to be com­pletely ox­i­dized after they are pro­duced than dif­fu­sion flames on Earth, be­cause of a se­ries of mech­a­nisms that be­have dif­fer­ently in micro grav­ity when com­pared to nor­mal grav­ity conditions. These dis­cov­er­ies have po­ten­tial ap­pli­ca­tions in ap­plied sci­ence and in­dus­try, es­pe­cially con­cern­ing fuel ef­fi­ciency.

In com­bus­tion en­gines, var­i­ous steps are taken to elim­i­nate a flame. The method de­pends mainly on whether the fuel is oil, wood, or a high-en­ergy fuel such as jet fuel.

Flame temperatures

Temperatures of flames by appearance

It is true that ob­jects at spe­cific tem­per­a­tures do ra­di­ate vis­i­ble light. Ob­jects whose sur­face is at a tem­per­a­ture above ap­prox­i­mately 470 °C (878 °F) will glow, emit­ting light at a color that in­di­cates the tem­per­a­ture of that sur­face. See the sec­tion on red heat for more about this ef­fect. It is a mis­con­cep­tion that one can judge the tem­per­a­ture of a fire by the color of its flames or the sparks in the flames. For many rea­sons, chem­i­cally and op­ti­cally, these col­ors may not match the red/or­ange/yel­low/white heat tem­per­a­tures on the chart. Bar­ium ni­trate burns a bright green, for in­stance, and this is not pre­sent on the heat chart.

Typical temperatures of flames

Main article: adiabatic flame temperature

The “adi­a­batic flame tem­per­a­ture” of a given fuel and ox­i­dizer pair in­di­cates the tem­per­a­ture at which the gases achieve sta­ble com­bus­tion.

Fire ecology

Main article: Fire ecology

Every nat­ural ecosys­tem has its own fire regime, and the or­gan­isms in those ecosys­tems are adapted to or de­pen­dent upon that fire regime. Fire cre­ates a mo­saic of dif­fer­ent habi­tat patches, each at a dif­fer­ent stage of suc­ces­sion. Dif­fer­ent species of plants, an­i­mals, and mi­crobes spe­cial­ize in ex­ploit­ing a par­tic­u­lar stage, and by cre­at­ing these dif­fer­ent types of patches, fire al­lows a greater num­ber of species to exist within a land­scape.

Fossil record

Main article: Fossil record of fire

The fos­sil record of fire first ap­pears with the es­tab­lish­ment of a land-based flora in the Mid­dle Or­dovi­cian pe­riod, 470 mil­lion years ago, per­mit­ting the ac­cu­mu­la­tion of oxy­gen in the at­mos­phere as never be­fore, as the new hordes of land plants pumped it out as a waste prod­uct. When this con­cen­tra­tion rose above 13%, it per­mit­ted the pos­si­bil­ity of wild­fire. Wild­fire is first recorded in the Late Sil­urian fos­sil record, 420 mil­lion years ago, by fos­sils of char­coal­i­fied plants. Apart from a con­tro­ver­sial gap in the Late De­von­ian, char­coal is pre­sent ever since. The level of at­mos­pheric oxy­gen is closely re­lated to the preva­lence of char­coal: clearly oxy­gen is the key fac­tor in the abun­dance of wildfire. Fire also be­came more abun­dant when grasses ra­di­ated and be­came the dom­i­nant com­po­nent of many ecosys­tems, around 6 to 7 mil­lion years ago; this kin­dling pro­vided tin­der which al­lowed for the more rapid spread of fire. These wide­spread fires may have ini­ti­ated a pos­i­tive feed­back process, whereby they pro­duced a warmer, drier cli­mate more con­ducive to fire.

Human control

Main article: Control of fire by early humans

Bushman starting a fire in Namibia

Bushman starting a fire in Namibia

Process of ignition of a match

Process of ignition of a match

The abil­ity to con­trol fire was a dra­matic change in the habits of early hu­mans. Mak­ing fire to gen­er­ate heat and light made it pos­si­ble for peo­ple to cook food, si­mul­ta­ne­ously in­creas­ing the va­ri­ety and avail­abil­ity of nu­tri­ents and re­duc­ing dis­ease by killing or­gan­isms in the food. The heat pro­duced would also help peo­ple stay warm in cold weather, en­abling them to live in cooler cli­mates. Fire also kept noc­tur­nal preda­tors at bay. Ev­i­dence of cooked food is found from 1.9 mil­lion years ago, al­though fire was prob­a­bly not used in a con­trolled fash­ion until 400,000 years ago. There is some ev­i­dence that fire may have been used in a con­trolled fash­ion about 1 mil­lion years ago. Ev­i­dence be­comes wide­spread around 50 to 100 thou­sand years ago, sug­gest­ing reg­u­lar use from this time; in­ter­est­ingly, re­sis­tance to air pol­lu­tion started to evolve in human pop­u­la­tions at a sim­i­lar point in time. The use of fire be­came pro­gres­sively more so­phis­ti­cated, with it being used to cre­ate char­coal and to con­trol wildlife from ‘tens of thou­sands’ of years ago.

Fire has also been used for cen­turies as a method of tor­ture and ex­e­cu­tion, as ev­i­denced by death by burn­ing as well as tor­ture de­vices such as the iron boot, which could be filled with water, oil, or even lead and then heated over an open fire to the agony of the wearer.

By the Ne­olithic Rev­o­lu­tion, dur­ing the in­tro­duc­tion of grain-based agri­cul­ture, peo­ple all over the world used fire as a tool in land­scape man­age­ment. These fires were typ­i­cally con­trolled burns or “cool fires”, as op­posed to un­con­trolled “hot fires”, which dam­age the soil. Hot fires de­stroy plants and an­i­mals, and en­dan­ger com­mu­ni­ties. This is es­pe­cially a prob­lem in the forests of today where tra­di­tional burn­ing is pre­vented in order to en­cour­age the growth of tim­ber crops. Cool fires are gen­er­ally con­ducted in the spring and au­tumn. They clear un­der­growth, burn­ing up bio­mass that could trig­ger a hot fire should it get too dense. They pro­vide a greater va­ri­ety of en­vi­ron­ments, which en­cour­ages game and plant di­ver­sity. For hu­mans, they make dense, im­pass­able forests tra­vers­a­ble. An­other human use for fire in re­gards to land­scape man­age­ment is its use to clear land for agri­cul­ture. Slash-and-burn agri­cul­ture is still com­mon across much of trop­i­cal Africa, Asia and South Amer­ica. “For small farm­ers, it is a con­ve­nient way to clear over­grown areas and re­lease nu­tri­ents from stand­ing veg­e­ta­tion back into the soil”, said Miguel Pinedo-Vasquez, an ecol­o­gist at the Earth In­sti­tute’s Cen­ter for En­vi­ron­men­tal Re­search and Con­ser­va­tion. How­ever this use­ful strat­egy is also prob­lem­atic. Grow­ing pop­u­la­tion, frag­men­ta­tion of forests and warm­ing cli­mate are mak­ing the earth’s sur­face more prone to ever-larger es­caped fires. These harm ecosys­tems and human in­fra­struc­ture, cause health prob­lems, and send up spi­rals of car­bon and soot that may en­cour­age even more warm­ing of the at­mos­phere – and thus feed back into more fires. Glob­ally today, as much as 5 mil­lion square kilo­me­tres – an area more than half the size of the United States – burns in a given year.

There are nu­mer­ous mod­ern ap­pli­ca­tions of fire. In its broad­est sense, fire is used by nearly every human being on earth in a con­trolled set­ting every day. Users of in­ter­nal com­bus­tion ve­hi­cles em­ploy fire every time they drive. Ther­mal power sta­tions pro­vide elec­tric­ity for a large per­cent­age of hu­man­ity.

Hamburg after four fire-bombing raids in July 1943, which killed an estimated 50,000 people[24]

Hamburg after four fire-bombing raids in July 1943, which killed an estimated 50,000 people

The use of fire in war­fare has a long his­tory. Fire was the basis of all early ther­mal weaponsHomer de­tailed the use of fire by Greek sol­diers who hid in a wooden horse to burn Troy dur­ing the Tro­jan war. Later the Byzan­tine fleet used Greek fire to at­tack ships and men. In the First World War, the first mod­ern flamethrow­ers were used by in­fantry, and were suc­cess­fully mounted on ar­moured ve­hi­cles in the Sec­ond World War. In the lat­ter war, in­cen­di­ary bombs were used by Axis and Al­lies alike, no­tably on Tokyo, Rot­ter­dam, Lon­don, Ham­burg and, no­to­ri­ously, at Dres­den; in the lat­ter two cases firestorms were de­lib­er­ately caused in which a ring of fire sur­round­ing each city was drawn in­ward by an up­draft caused by a cen­tral clus­ter of fires. The United States Army Air Force also ex­ten­sively used in­cen­di­aries against Japan­ese tar­gets in the lat­ter months of the war, dev­as­tat­ing en­tire cities con­structed pri­mar­ily of wood and paper houses. The use of na­palm was em­ployed in July 1944, to­wards the end of the Sec­ond World War; al­though its use did not gain pub­lic at­ten­tion until the Viet­nam WarMolo­tov cock­tails were also used.

Use as fuel

A coal-fired power station in the People's Republic of China

coal-fired power station in the People’s Republic of ChinaDisability-adjusted life year for fires per 100,000 inhabitants in 2004  no data  less than 50  50–100  100–150  150–200  200–250  250–300  300–350  350–400  400–450  450–500  500–600  more than 600

Set­ting fuel aflame re­leases us­able en­ergy. Wood was a pre­his­toric fuel, and is still vi­able today. The use of fos­sil fuels, such as pe­tro­leumnat­ural gas, and coal, in power plants sup­plies the vast ma­jor­ity of the world’s elec­tric­ity today; the In­ter­na­tional En­ergy Agency states that nearly 80% of the world’s power came from these sources in 2002. The fire in a power sta­tion is used to heat water, cre­at­ing steam that dri­ves tur­bines. The tur­bines then spin an elec­tric gen­er­a­tor to pro­duce elec­tric­ity. Fire is also used to pro­vide me­chan­i­cal work di­rectly, in both ex­ter­nal and in­ter­nal com­bus­tion en­gines.

The un­burn­able solid re­mains of a com­bustible ma­te­r­ial left after a fire is called clinker if its melt­ing point is below the flame tem­per­a­ture, so that it fuses and then so­lid­i­fies as it cools, and ash if its melt­ing point is above the flame tem­per­a­ture.

Protection and prevention

Main articles: Wildfire and Fire protection

Wild­fire pre­ven­tion pro­grams around the world may em­ploy tech­niques such as wild­land fire use and pre­scribed or con­trolled burnsWild­land fire use refers to any fire of nat­ural causes that is mon­i­tored but al­lowed to burn. Con­trolled burns are fires ig­nited by gov­ern­ment agen­cies under less dan­ger­ous weather conditions.

Fire fight­ing ser­vices are pro­vided in most de­vel­oped areas to ex­tin­guish or con­tain un­con­trolled fires. Trained fire­fight­ers use fire ap­pa­ra­tus, water sup­ply re­sources such as water mains and fire hy­drants or they might use A and B class foam de­pend­ing on what is feed­ing the fire.

Fire pre­ven­tion is in­tended to re­duce sources of ig­ni­tion. Fire pre­ven­tion also in­cludes ed­u­ca­tion to teach peo­ple how to avoid caus­ing fires. Build­ings, es­pe­cially schools and tall build­ings, often con­duct fire drills to in­form and pre­pare cit­i­zens on how to react to a build­ing fire. Pur­posely start­ing de­struc­tive fires con­sti­tutes arson and is a crime in most jurisdictions.

Model build­ing codes re­quire pas­sive fire pro­tec­tion and ac­tive fire pro­tec­tion sys­tems to min­i­mize dam­age re­sult­ing from a fire. The most com­mon form of ac­tive fire pro­tec­tion is fire sprin­klers. To max­i­mize pas­sive fire pro­tec­tion of build­ings, build­ing ma­te­ri­als and fur­nish­ings in most de­vel­oped coun­tries are tested for fire-re­sis­tance, com­bustibil­ity and flam­ma­bil­ityUp­hol­sterycar­pet­ing and plas­tics used in ve­hi­cles and ves­sels are also tested.

Where fire pre­ven­tion and fire pro­tec­tion have failed to pre­vent dam­age, fire in­sur­ance can mit­i­gate the fi­nan­cial impact.File:Fires observed in the United States From July 2002 through July 2011.ogvPlay mediaThis visualization shows fires detected in the United States from July 2002 through July 2011. Look for fires that reliably burn each year in western states and across the Southeast.


Dif­fer­ent restora­tion meth­ods and mea­sures are used de­pend­ing on the type of fire dam­age that oc­curred. Restora­tion after fire dam­age can be per­formed by prop­erty man­age­ment teams, build­ing main­te­nance per­son­nel, or by the home­own­ers them­selves; how­ever, con­tact­ing a cer­ti­fied pro­fes­sional fire dam­age restora­tion spe­cial­ist is often re­garded as the safest way to re­store fire dam­aged prop­erty due to their train­ing and ex­ten­sive experience. Most are usu­ally listed under “Fire and Water Restora­tion” and they can help speed re­pairs, whether for in­di­vid­ual home­own­ers or for the largest of institutions.

Fire and Water Restora­tion com­pa­nies are reg­u­lated by the ap­pro­pri­ate state’s De­part­ment of Con­sumer Af­fairs – usu­ally the state con­trac­tors li­cense board. In Cal­i­for­nia, all Fire and Water Restora­tion com­pa­nies must reg­is­ter with the Cal­i­for­nia Con­trac­tors State Li­cense Board. Presently, the Cal­i­for­nia Con­trac­tors State Li­cense Board has no spe­cific clas­si­fi­ca­tion for “water and fire dam­age restora­tion.” Hence, the Con­trac­tor’s State Li­cense Board re­quires both an as­bestos cer­ti­fi­ca­tion (ASB) as well as a de­mo­li­tion clas­si­fi­ca­tion (C-21) in order to per­form Fire and Water Restora­tion work.



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