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 English physicist who was initially
interested in the efficiency of electric motors. He discovered the
heat
 dissipated
by a resistor
is given
by Q = I2Rt (where I is the
current,
R
the resistance,
and
t the time
), a result
now known as Joule's
law.
Motivated by theological beliefs, Joule began attempting to
demonstrate the unity of forces in nature. He determined the
mechanical equivalent of heat
by
measuring change in temperature produced by the friction
of a
paddlewheel attached to a falling weight in the 1840s. He made a
series of measurements and found that, on average, a weight of 772
pounds falling through a distance of one foot would raise the temperature
of one
pound of water by 1° F. This corresponds to (772 ft lbs)(1.356 J/ft
lb) = 59 453.6 Calories,
or 1 cal =
4.15 Joules,
in close
agreement with the current accepted value of 1 cal = 4.184 J. Joule
was not the first person to establish the mechanical equivalence of
heat, but it was his demonstration that eventually came to be
accepted. He did not claim, however, to have formulated a general
Law of Conservation of Energy. Nevertheless, his experiments were
certainly fundamental in bringing that formulation about. In
addition, Joule's experiments showed that heat is produced by
motion, contradicting the caloric
theory.
Author: Eric W. Weisstein
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