Conservation of Energy
Energy and Thermal Physics

James Prescott Joule and energy conservation

Teaching Guidance for 14-16 PRACTICAL PHYISCS

As the son of a Manchester brewer, privately tutored for several years by the chemist John Dalton, James Joule took an early interest in all the technical equipment associated with brewing. In 1840, at the age of 22, he experimentally discovered that the heat generated in a coil of wire is proportional to the square of the current through it. This effect is now called ‘Joule heating’.

In the following decade, Joule sought to unify electrical, chemical and thermal phenomena by conducting dozens of different experiments to demonstrate their inter-convertibility and quantitative equivalence. He gave particular attention to the conversion of heat into mechanical work, regarding this as fundamental to the theory of steam engines.

Churning fluids

Joules’ best-known experiment involved a paddle wheel which rotated so that it churned water in a large cylinder. The wheel itself was driven by falling weights so that he could determine the work done. The same experiment was repeated using whale oil and then mercury. Joule worked meticulously, constantly improving the accurately of his results. He took great care to minimise heat losses to the surroundings and introduced appropriate correction factors.

With the help of scientific instrument-maker John Dancer, Joule was also able to use very sensitive thermometers. Some contemporaries doubted the claim that he could measure temperatures to within 1/200 of a degree Fahrenheit.


Famously, Joule packed thermometers for his honeymoon in the Swiss Alps, so that he could compare the temperatures of water at the top and bottom of a waterfall near Chamonix. In fact this particular experiment failed, because the water produced too much spray at the bottom of its fall.

SI unit of energy

In recognition of Joules’ importance in demonstrating energy conservation, the SI unit of energy is named after him.


Some diagrams and a chronology of increasingly accurate experiments to determine the energy needed to increase the temperature of 1 kilogram of water by 1 °C, carried out by Joule and others during the period 1842 – 1939. This is a six-page extract from a Nuffield Physics pupil textbook.

Conservation of Energy
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