Measurement of the total internal energy of a body
It is not possible at all to measure the total internal energy of a body. But the change of internal energy due to the application of heat can be measured accurately. Change of internal energy and also work becomes zero at a constant heat.
Dependence of internal energy of a gas: The state of a gas is determined by its pressure, volume, and temperature. So, it is natural to assume that the internal energy of a gas depends on these three terms. Actually, it is not so. After extensive research Joule came to the following conclusion-
The internal energy of a fixed amount of gas depends only on its temperature and not on pressure or volume. It is called Mayer’s hypothesis.
So, the internal energy of a fixed amount of gas can be measured from the change in temperature. Clearly, if the temperature remains constant the internal energy of the gas remains constant even though pressure or volume of the gas changes. Change of internal energy of a system depends on the initial and final condition of the system, but not on the path along which this condition has been reached. The internal energy of a system is identified with the random, disordered motion of molecules; the total (internal) energy in a system includes potential and kinetic energy.