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Joule, James (Prescott)

heat energy thomson amount

[jowl] (1818–89) British physicist; established the mechanical theory of heat.

Joule grew up in a wealthy Manchester brewing family, a shy and delicate child. He received home tuition from in elementary science and mathematics. He was attracted to physics and especially to the problems of heat and began experimental work in a laboratory near the brewery. Joule’s skill enabled him to measure heat and temperature changes accurately and he was later encouraged to pursue his work.

When he was 18, Joule began his study of the heat developed by an electric current and by 1840 he had deduced the law connecting the current and resistance of a wire to the heat generated (Joule’s Law). Between 1837 and 1847 his work established the principle of conservation of energy and the equivalence of heat and other forms of energy. J R Mayer (1814–78) arrived at the idea of conservation in the 1840s but in an unclear form, and W Thomson and also were major contributors, but Joule made it a precise and explicit concept. The amount of mechanical work required to produce a given amount of heat was determined by Joule in 1843. He measured the small amount of heat produced in water by the rotation of paddles driven by falling weights.

Thomson and Joule collaborated for 7 years from 1852 in a series of experiments, mainly on the Joule-Thomson effect, whereby an expanding gas is cooled as work is done to separate the molecules. Joule also produced a paper on the kinetic theory of gases that included the first estimation of the speed of gas molecules (1848). He was over-modest and made himself into an assistant to Thomson rather than following his own lines of thought; and he became unwell when he was 55 and did little more afterwards.

Joule remains one of the foremost experimentalists of his century; his main work was done before he was 30, on one problem of great importance, the mechanical equivalent of heat. He attacked this with ingenuity, made precise measurements and tenaciously located sources of error. The SI unit of energy, the joule (J) (pronounced ‘jool’) is the energy expended when the point of application of a force of 1 newton moves through 1 metre in the direction of the force, so 1 J = 1 Nm = 1 kg m 2 s –2 . Heat and work are measured in the same units.

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