The Science Book

138

See also: Isaac Newton 62–69 ■ Joseph Black 76–77 ■ Joseph Fourier 122–23

T

he principle of the

conservation of energy

states that energy is never

lost but only changed in form. But

in the 1840s, scientists had only a

vague idea of what energy was.

It was a British brewer’s son,

James Joule, who showed that

heat, mechanical movement, and

electricity are interchangeable

forms of energy, and that when

one is changed to another the

total energy remains the same.

Converting energy

Joule began his experiments in

a laboratory in the family home.

In 1841, he figured out how much

heat an electric current generates.

He experimented with converting

mechanical movement into heat,

and developed an experiment in

which a falling weight turns a

paddle wheel in water, heating

the water. By measuring the rise

in temperature of the water, Joule

was able to figure out the exact

amount of heat a certain amount of

mechanical work would create. He

went on to assert that no energy

was ever lost in this conversion.

His ideas were largely ignored

until 1847, when German physicist

Hermann Helmholtz published a

paper summarizing the theory of

the conservation of energy, and

Joule then presented his work at

the British Association in Oxford.

The standard unit of energy, a joule,

is named after him. ■

LIVING FORCE MAY BE

CONVERTED INTO HEAT

JAMES JOULE (1818–1889)

IN CONTEXT

BRANCH

Physics

BEFORE

1749 French mathematician

Émilie du Châtelet derives her

law of the conservation of

energy from Newton’s laws.

1824 French engineer Sadi

Carnot states that there are no

reversible processes in nature,

paving the way for the second

law of thermodynamics.

1834 French physicist Émile

Clapeyron develops Carnot’s

work, stating a version of the

second law of thermodynamics.

AFTER

1850 German physicist Rudolf

Clausius gives the first clear

statement of the first and second

laws of thermodynamics.

1854 Scottish engineer

William Rankine adds the

concept that is later named

entropy (a measure of disorder)

in the transformation of energy.

In Joule’s experiment, a falling

weight drove a paddle that turned

inside a bucket of water. The energy of

the movement was changed into heat.