Joule

The joule (symbol = J) is the SI unit of energy. One joule equals equals the work done (or energy expended) by a force of one newton (symbol = N) acting over a distance of one meter (symbol = m). One newton equals a force that produces an acceleration of one meter per second per second on a one kilogram (kg) mass. Therefore, one joule equals one newton•meter.

In terms of SI basic units:

$\mathrm {J} =\mathrm {m} ^{2}\cdot \mathrm {kg} \cdot \mathrm {s} ^{-2}={\frac {\mathrm {m} ^{2}\cdot \mathrm {kg} }{\mathrm {s^{2}} }}$.

'Joule' is variously pronounced 'jool' (rhymes with 'jewel') or 'joul' (rhymes with 'jowl').

One joule of work or energy expended per second gives the unit of power equal to one watt, the unit that rates many electrical devices. A 100 watt light bulb produces 100 joules of energy per second.

The joule is also used to measure thermal energy. In nutritional terms, burning 1000 calories (one kilocalorie) of food equals the energy of 4.184 kilojoules.

One joule also measures the energy released by an electric charge of one coulomb dropping one volt in electrical potential.

The joule is named for James Prescott Joule (1818 - 1889), who studied the relation between mechanical and heat energy discovered earlier by count Rumford.

Practical use

• A baseball pitched by a fast Major League pitcher has a kinetic energy of approximately 230 J.
• One "dietary calorie" (as used in the United States) is 4186 J (4.186 kJ).
• One gram of gasoline releases about 50 kJ on combustion in air.
• A small car (about 1200 kg) travelling at 65 miles per hour (105 km/h) has a kinetic energy of 1 megajoule (1 MJ).
• One "kilowatt-hour" is 3.6 MJ.
• One ton of TNT releases 4.184 GJ on exploding.