Physicists Biographies

James Prescott Joule

James Prescott Joule was a British physicist who was born in 1818. His main contribution was the discovery of the transformation of electrical energy into heat.

Joule’s main contributions were in relation to the conversion of mechanical energy and electrical energy into heat*

James Prescott Joule was a British physicist, the son of a successful brewer . He was born in 1818 and died, aged 71, in 1889. Joule was educated through private lessons. In that time, he had contact with some great names of science, of which John Dalton stands out . Science was a hobby in Joule’s life, but his contributions were very significant.


From several experiments related to the study of heat , Joule determined the relationship between mechanical work and heat, showing that the energy spent in carrying out an activity can be converted into heat . This relationship was decisive for the formulation of the First Law of Thermodynamics , a law that gave impetus to studies referring to thermal machines , equipment that contributed to the Industrial Revolution .

The experiment outlined above is authored by Joule . The water inside the container was thermally insulated and in contact with a series of paddles, which, in turn, were connected to weights by means of wires and pulleys. Each time the weights were dropped from a certain height, the blades kicked in and moved the water. After dropping the weights several times, Joule noticed that the thermometer attached to the system indicated an increase in temperature. This was proof that mechanical energy can be converted into heat.

Q = i 2 .R.Δt

Q = Amount of heat generated;

i = Electric current;

R = Electrical resistance;

Δt = Time variation.

Joule also defined a law that determined the relationship between the internal energy of a gas and the change in temperature. Joule’s law for perfect gases can be written as follows:

ΔU= 3 .NRT

ΔU = Variation of internal energy;

N = Number of Mol;

R = Universal gas constant;

T = Temperature.

Joule Unit

After his death in 1889, the unit of measurement for energy and work was determined by the International System of Units as the Joule (J) unit. The unit Joule (J) is defined as newton times meter (Nm) or watt times second (Ws).

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