Energy Units

The basic energy unit is the erg, defined as the work done by a force of 1 dyne moving a distance of 1 cm. The dyne is the force which, acting on a mass of 1 gram produce an acceleration of 1 cm per sec. A joule (J) is 10 ergs, and is also defined as the kinetic energy of a mass of 1 kg moving at 1 metre per second. Since this is very small for practical purposes, large multiples of the joule are frequently used, particularly the megajoule (MJ) (10(6)J), the gigajoule (GJ) (10(9)J), the pentajoule (10(15)J) and the exajoule (EJ) (10 (18) J). In the oil industry, the unit is the tonne of oil equivalent (TOE). A tonne is 1000 kg. 1 EJ = 22.7 TOE, or 1 TOE = 44 GJ. Also, 7.3 barrels = 12 tonnes of oil. One barrel of oil per day is 50 TOE per year. Rates of heat production are measured in watts. A watt is the rate of working of one joule per second, so the watt has 'per second' built into it. A kilowatt (kW) is 1000 watts, a megawatt (MW) is 10 (6) watts, a gigawatt (GW) is 10 (9) watts and a terawatt (TW) is 10 (12) watts. One kilowatt hour (kWh) is 3.6 MJ. 1 EJ per year is 32.2 GJ per second or 32.2 gigawatts. An energy unit used in nuclear physics is the electron volt (eV). One eV is 1.6 x 10 (-19) J. A million electron volts (1 MeV) is 1.6 x 10 (13) J. Each fission of a uranium nucleus releases 200 MeV = 3.2 x 10 (-13) J. One gram of uranium undergoing fission releases 82,000 MJ.

In discussion of energy the terms 'energy' and 'power' are often used indiscriminately. It is important to note that energy is power multiplied by time. Thus for example if an electric light bulb has a power of 100 watts and it is switched on for one hour it delivers 100Wh of energy.

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