The energy contained in a system is called internal energy. It is denoted by U. It is the sum of following forms of energy-
(i) Kinetic energy due to translational, rotational and vibrational motion of the molecules, all of which depend on the temperature,
(ii) Potential energy due to intermolecular forces, which depends on the separation between the molecules and
(iii) Energy of the electrons and nuclei.
In practice, it is not possible to measure the total internal energy of a system in any given state. Only change of its value can be measured.
Let the state of the system is changed from an initial state 1 to final state 2; due to supply of heat Q to the system.
Let W be the work done by the system during the change, then increase in internal energy is
U2-U1 = Q-W
For ideal gas the internal energy is only the kinetic energy of its molecules. It depends on temperature and hence internal energy is the function of temperature only.
For real gas, the internal energy is the sum of kinetic energy and potential energy of the molecules due to their mutual attraction. The force of attraction between the molecules depends on the intermolecular distance and thus is a function of volume and temperature.
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