6 2 Internal Energy – Class 11TH Physics

6.2 Internal Energy

The sum of all forms of molecular energies (kinetic and potential) of a substance is termed as its internal energy. In the study of thermodynamics, usually ideal gas is considered as a working substance. The molecules of an ideal gas are mere mass points which exert no forces on one another. So, the internal energy of an ideal gas system is generally the translational K.E. of its molecules. Since the temperature of a system is defined as the average K.E. of its molecules, thus for an ideal gas system, the internal energy is directly proportional to its temperature.

According to the kinetic theory of gases, the average kinetic energy of gas molecules is given by:

\langle\mathrm{K.E.}\rangle = \tfrac{3}{2}k_{B}T \quad \text{or} \quad \langle\tfrac{1}{2}mv^{2}\rangle = \tfrac{3}{2}k_{B}T

where $k_{B}$ is Boltzmann constant.

Therefore, the rise in temperature of an object reflects an increase in the internal kinetic energy of its particles. This increase in internal energy can occur due to the absorption of heat energy, which raises the average kinetic energy of the particles and thus increases the temperature of the object.

6.2 Internal Energy

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