Metric (SI) unit of work and energy. (1) As a unit or work, it is defined as the work done by a force of one newton acting on an object to move it through a distance of one meter in the direction the force is applied. One joule per second equals 1 watt or 0.737 foot pounds, or 9.47 x 10^-4 British thermal units (Btu) (one Btu equals about 1,055 joules). (2) As a unit of energy, it is defined as the amount of energy required to raise the temperature of one kilogram of water by one degree Celsius. Or the energy expended in one second by one ampere current against a resistance of one ohm.
In terms of kinetic energy, one joule is equal to the energy of a mass of two kilograms moving at a velocity of one meter per second. It equals about 0.24 calories, or roughly the energy required to raise a spoonful of food to mouth. Named after UK physicist James Prescott Joule (1818-89) who in 1843 discovered the mechanical equivalent of heat.

Use joule in a sentence

  • The machine had to be powered by several hundred joules to work and required a very powerful energy source to be operated.

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  • Accurately communicating the number of joules involved in a process is a necessary part of the physics calculations done by engineers launching rockets into space.

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  • The physicist carefully recorded the measurements of his experiment, taking note of precisely how many joules of force was needed to get the desired effect.

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