joule

Word family

• kilojoule
  nounC1
• megajoule
  nounC2

Synonym discussion

Sense: noun/1

All four words name units of energy, but each belongs to a different practical world. The joule is the scientific standard used in physics classrooms and on European food labels. The calorie is older and is the unit most people meet on diet charts; one calorie equals roughly 4.18 joules, so calorie numbers look smaller. The kilowatt-hour is what an electricity bill counts — one kilowatt-hour is 3.6 million joules, a size convenient for household power use over an hour. The newton-meter is mathematically identical to the joule but is reserved for situations where the focus is the work done by a force, such as a wrench turning a bolt. So a physicist writes joules, a dietitian writes calories, your power company writes kilowatt-hours, and an engineer measuring torque writes newton-meters.

Common collocations

Etymology

Named in 1882 in honour of James Prescott Joule, a British scientist from Manchester who showed in the 1840s that heat and mechanical work are the same kind of thing — energy — and that one can be turned into the other in fixed amounts. His careful experiments with falling weights stirring water gave physics a single unit for any form of energy, which scientists later agreed to call by his name.