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Units of Energy

UNITS OF ENERGY

The joule, symbol J, is the unit of energy in the science community. It is not widely used outside the science community in the United States, but it is elsewhere. For example, in the United States, the energy content of a food product is likely expressed in food calories. This energy could be expressed in joules, and that is being done in many parts of the world. A Diet Coca Cola in Australia is labeled Low Joule rather than Low Cal as in the United States. To obtain a feeling for the size of a joule, consider the following statistics:

  • the sound energy that is in a whisper is about 0.01 joules,
  • the kinetic energy of a 1,000-kilogram car traveling 25 meters per second (55 miles per hour) is about 300,000 joules,
  • the energy from burning a barrel of oil is 6,000,000 joules,
  • the annual energy use in the United States is about 100 billion billion joules, and
  • the daily energy input to the Earth from the sun is about 10,000 billion billion joules.

Even if the use of units of joules were universal, the numbers involved in energy discussions would be large and cumbersome. Therefore, it is customary to use powers of ten notation and prefixes for powers of

Numerical Unit Power of Ten Prefix Symbol
million trillion 1018 exa E
trillion 1012 tera T
billion 109 giga G
million 106 mega M
thousand 103 kilo k
hundredth 10-2 centi c
thousandth 10-3 milli m
millionth 10-6 micro μ

ten. For example, the energy from burning a barrel of oil is 6 × 106 joules, which can be expressed as 6 megajoules or, symbolically, 6 MJ. Table 1 summarizes the powers of ten, prefixes, and symbols usually encountered in energy considerations.

The gasoline tank on an automobile holds about 15 gallons, and the automobile can travel about 300 miles before the tank is empty. Even though the gasoline was purchased for its energy content, the driver probably did not know that the energy content of 15 gallons of gasoline is 2 × 109 J = 2 GJ. For reasons like this, there exists a variety of energy units and energy equivalents. The unit and equivalent depends on the type of energy commodity in question. A homeowner pays an electric utility for electric energy, and the

Special Unit Study area of main use Symbol Equivalent in joules Other units
Kilowatt hour electricity kWh 3,600,000 3413 Btu
Calorie heat cal 4.186  
Kilocalorie (food calorie) heat kcal 4,186 1000 cal
British Thermal unit heat Btu 1,055 252 cal
Electron volt atoms, molecules eV 1.60x10 -19 
Kilo-electron Volt X-rays keV 1.60x10 -16 1000 eV
Mega-electronVolt nuclei, nuclear radiations MeV 1.60x10 -13 1000 keV
Quadrillion energy reserves quad 1.055x10 21 10 15 Btu
Quintillion energy reserves Q 1.055x10 21 10 18 Btu
Energy equivalents
1 gallon of gasoline = 126,000 Btu
1 cubic foot of natural gas = 1030 Btu
1 pound of bituminous coal = 13,100 Btu
1 42-gallon barrel of oil = 5,800,000 Btu
1 therm = 100,000 Btu
Variations of these energy equivalents will appear in the literature. The values listed here are typical.

unit is likely a kilowatt-hour (kWh). A politician interested in imported oil will likely talk in terms of barrels of oil. Table 2 summarizes some special energy units and their equivalents.

Joseph Priest

BIBLIOGRAPHY

Hobson, A. (1995). Physics: Concepts and Connections. Englewood Cliffs, NJ: Prentice-Hall.

Serway, R. A. (1998). Principles of Physics. Fort Worth, TX: Saunders College Publishing.

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