Mechanical Equivalent of Heat

Mechanical Equivalent of Heat:

Heat is defined as energy in transit. Heat is manifested in the form of motion i.e., flow at a molecular level. In other words, it is equivalent to the kinetic energy of the molecules. Hence, mechanical work can be directly converted to heat. At the macroscopic level, it is possible to determine the work done by a thermodynamic system by the amount of heat it has utilized James Prescott Joule experimented on this principle for about 10 years to arrive at the conclusion that a certain amount of work, in whatever form, is to be performed to develop a specific amount of heat energy.

Later Maxwell defined this experimental inference of Joule into a law string- when heat is converted to mechanical work (energy) or work into heat, the energy involved is equivalent to the energy being converted. Helmholtz later showed that this law is applicable to all forms of energy conversion into heat. If 0 is the amount of heat produced due to the complete conversion of W amount of work, then from Joule’s law (later named by Maxwell)-

W ∝ Q
or W = JQ

Where J is constant and is called Joule’s equivalent of heat.

J = W/Q
J = W if Q = 1

Thus, J, the Joules equivalent or the mechanical equivalent of heat can also be defined as the amount of work done or received due to complete conversion of unit quantity of heat. Conversely, it is also defined as the amount of work done in order to produce a unit quantity of heat. Barnes, Callender, Joule, etc. showed on experimentally that when 1 caloric (gm-cal) of heat is converted to work 4.18 Joule of energy is evolved, i.e., the mechanical equivalent of heat J is 4.18 Joules/calorie. Thus, in the S.I. system,

J = 4.18 Joules/Calorie
J = 4180 Joules/Kilocal.

In the C.G.S. system, J = 4.18 x 107 ergs/calorie.

A number of methods are available for determining the mechanical equivalent of heat (J). An accurate method for determining J is the constant flow method suggested by Callender and Barnes. The apparatus used is known as a constant flow calorimeter.

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