Energy

Energy is a fundamental quantity that every physical system possesses. Energy of physical system in a certain given state is defined as the amount of work W needed to change the state of the system from some initial state (called reference state or reference level) to the given state.

Units

SI and related units

The SI unit for both energy and work is the joule (J), named in honour of James Prescott Joule 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 is equal to 1 kg m2 s−2.

Related Topics:
SI - Joule - James Prescott Joule - Mechanical equivalent of heat - 1 joule - Newton - Metre - SI base unit - Kg - S

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An energy unit that is used in particle physics is the electronvolt (eV). One eV  is equivalent to 1.60217653×10−19 J.

Related Topics:
Particle physics - Electronvolt - 1.60217653×10⁻¹⁹ J

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(Note that torque, which is typically expressed in newton-metres, has the same dimension and this is not a simple coincidence: a torque of 1 newton-metre applied on 1 radian requires exactly 1 newton-metre=joule of energy.)

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Other units of energy

In cgs units, one erg is 1 g cm2 s−2, equal to 1.0×10−7 J. Another obsolete metric unit is the litre-atmosphere (101.325 J).

Related Topics:
Cgs - Erg - G - Cm - S - 1.0×10⁻⁷ J

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The imperial/US units for both energy and work include the foot-pound force (1.3558 J), the British thermal unit (Btu) which has various values in the region of 1055 J, and the horsepower-hour (2.6845 MJ).

Related Topics:
Imperial - US units - Foot-pound force - British thermal unit - Horsepower

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The energy unit used for everyday electricity, particularly for utility bills, is the kilowatt-hour (kW h), and one kW h is equivalent to 3.6×106 J  (3600 kJ or 3.6 MJ; the metric units usually are self-consistent, and this particular one may seem arbitrary; it's not, the metric measurement for time is the second, and there are 3,600 seconds in an hour -- in other words, 1 kW second = 1 kJ, but the kW h is a more convenient unit for everyday use).

Related Topics:
Electricity - Kilowatt-hour - 3.6×10⁶ J 

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The calorie is mainly used in nutrition and equals the amount of heat necessary to raise the temperature of one kilogram of water by 1 degree Celsius, at a pressure of 1 atm. This amount of heat depends somewhat on the initial temperature of the water, which results in various different units sharing the name of "calorie" but having slightly different energy values. It is equal to 4.1868 kJ.

Related Topics:
Calorie - Heat - Temperature - Kilogram - Water - Celsius - Pressure - Atm - 4.1868 kJ

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The calories used for food energy in nutrition are the large calories based on the kilogram rather than the gram, often identified as food calories. These are sometimes called kilocalories with that calorie being the small calorie based on the gram, and as a result the prefixes are generally avoided for the large calories (i.e., 1 kcal is 4.184 kJ, never 4.184 MJ, even if "calories" are also used for the other, larger unit in the same document or the same nutrition label). Food calories are sometimes noted as Calories (1000 calories) or simply abbreviated Cal with the capital C, but that convention is more often found in chemistry or physics textbooks—which do not use these large calories—than it is in real-world applications by those who do use these calories. (This convention is also, of course, useless when the word calorie appears in a location where it would ordinarily be capitalized, as at the beginning of a sentence or in the first column of a nutrition label as a substitute for the quantity being measured, which is energy, when all the other quantities such as "Iron" and "Sugars" are also capitalized.)

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