Units of energy

Units used to measure energy

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Energy is defined via work, so the SI unit of energy is the same as the unit of work &#; the joule (J), named in honour of James Prescott Joule[1] and his experiments on the mechanical equivalent of heat. In slightly more fundamental terms, 1 joule is equal to 1 newton metre and, in terms of SI base units

1   J = 1   k g ( m s ) 2 = 1   k g &#; m 2 s 2 {\displaystyle 1\ \mathrm {J} =1\ \mathrm {kg} \left({\frac {\mathrm {m} }{\mathrm {s} }}\right)^{2}=1\ {\frac {\mathrm {kg} \cdot \mathrm {m} ^{2}}{\mathrm {s} ^{2}}}}

An energy unit that is used in atomic physics, particle physics and high energy physics is the electronvolt (eV). One eV is equivalent to 1.×10&#;19 J.[2]

In spectroscopy the unit cm&#;1 &#; 0. eV is used to represent energy since energy is inversely proportional to wavelength from the equation E = h ν = h c / λ {\displaystyle E=h\nu =hc/\lambda } .

In discussions of energy production and consumption, the units barrel of oil equivalent and ton of oil equivalent are often used.

British imperial / US customary units

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The British imperial units and U.S. customary units for both energy and work include the foot-pound force (1. J), the British thermal unit (BTU) which has various values in the region of  J, the horsepower-hour (2. MJ), and the gasoline gallon equivalent (about 120 MJ).

Electricity

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A unit of electrical energy, particularly for utility bills, is the kilowatt-hour (kWh);[3] one kilowatt-hour is equivalent to 3.6 megajoule. Electricity usage is often given in units of kilowatt-hours per year or other time period.[4] This is actually a measurement of average power consumption, meaning the average rate at which energy is transferred. One kilowatt-hour per year is around 0.11 watts.

Natural gas

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Natural gas is often sold in units of energy content or by volume. Common units for selling by energy content are joules or therms. One therm is equal to about 1,055 megajoules. Common units for selling by volume are cubic metre or cubic feet. Natural gas in the US is sold in therms or 100 cubic feet (100 ft3 = 1 Ccf). In Australia, natural gas is sold in cubic metres. One cubic metre contains about 38 megajoules. In the most of the world, natural gas is sold in gigajoules.

Food industry

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The calorie is defined as the amount of thermal energy necessary to raise the temperature of one gram of water by 1 Celsius degree, from a temperature of 14.5 °C, at a pressure of 1 atm. For thermochemistry a calorie of 4.184 J is used, but other calories have also been defined, such as the International Steam Table calorie of 4. J. In many regions, food energy is measured in large calories or kilocalories equalling  calories, sometimes written capitalized as Calories. In the European Union, food energy labeling in joules is mandatory, often with calories as supplementary information.

Atom physics and chemistry

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In physics and chemistry, it is common to measure energy on the atomic scale in the non-SI, but convenient, units electronvolts (eV). 1 eV is equivalent to the kinetic energy acquired by an electron in passing through a potential difference of 1 volt in a vacuum. It is common to use the SI magnitude prefixes (e.g. milli-, mega- etc) with electronvolts. Because of the relativistic equivalence between mass and energy, the eV is also sometimes used as a unit of mass. The Hartree (the atomic unit of energy) is commonly used in the field of computational chemistry since such units arise directly from the calculation algorithms without any need for conversion. Historically Rydberg units have been used.

Spectroscopy

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In spectroscopy and related fields it is common to measure energy levels in units of reciprocal centimetres. These units (cm&#;1) are strictly speaking not energy units but units proportional to energies, with   h c &#; 2 &#; 10 &#; 23   J   c m {\displaystyle \ hc\sim 2\cdot 10^{-23}\ \mathrm {J} \ \mathrm {cm} } being the proportionality constant.[5]

Explosions

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A gram of TNT releases 4,100 to 4,600 joules (980 to 1,100 calories) upon explosion. To define the tonne of TNT, this was standardized to 1 kilocalorie (4,184 joules) giving a value of 4.184 gigajoules (1 billion calories) for the tonne of TNT.[6]

See also

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References

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For starters, the E stands for energy and the m stands for mass, a measurement of the quantity of matter. Energy and matter are interchangeable. Furthermore, it's essential to remember that there's a set amount of energy/matter in the universe.

If you've ever read Dr. Seuss's children's book "The Sneetches," you probably remember how the yellow, birdlike characters in the story go through a machine to change back and forth between "star-bellied sneetches" and "plain-bellied sneetches."

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The number of sneetches remains constant throughout the story, but the ratio between plain- and star-bellied ones changes. It's the same way with energy and matter. The grand total remains constant, but energy regularly changes form into matter and matter into energy.

Now we're getting to the c² part of the equation, which serves the same purpose as the star-on and star-off machines in "The Sneetches." The c stands for the speed of light, a universal constant, so the whole equation breaks down to this: Energy is equal to matter multiplied by the speed of light squared.

Why would you need to multiply matter by the speed of light to produce energy? The reason is that energy, be it light waves or radiation, travels at the speed of light. That breaks down to 186,000 miles per second (300,000 kilometers per second). When we split an atom inside a nuclear power plant or an atomic bomb, the resulting energy released is moving at the speed of light.

But why is the speed of light squared? The reason is that kinetic energy, or the energy of motion, is proportional to mass. When you accelerate an object, the kinetic energy increases to the tune of the speed squared.

You'll find an excellent example of this in any driver's education manual: If you double your speed, the braking distance is four times longer, so the braking distance is equal to the speed squared [source: UNSW Physics: Einsteinlight].

The speed of light squared is a colossal number, illustrating just how much energy there is in even tiny amounts of matter.

A common example of this is that 1 gram of water &#; if its whole mass were converted into pure energy via E=mc² &#; contains energy equivalent to 20,000 tons (18,143 metric tons) of TNT exploding. That's why such a small amount of uranium or plutonium can produce such a massive atomic explosion.

Einstein's equation opened the door for numerous technological advances, from nuclear power and nuclear medicine to the inner workings of the sun. It shows us that matter and energy are one.

This article was updated in conjunction with AI technology, then fact-checked and edited by a HowStuffWorks editor.

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