HEAT, HEAT ENERGY AND INTERNAL ENERGY

• The molecules of a substance possess momentum and kinetic energy due to their random motion. Due to randomness of motion, probability of motion in all directions is the same. Hence, the total momentum associated with all the molecules of the substance, int p , is zero, but the total kinetic energy, int K , is not zero. The total kinetic energy associated with random motion of molecules of a substance ( i.e., energy associated with zero momentum state ) is called heat ( or thermal ) energy possessed by the substance.’

• Besides this, the molecules also possess potential energy, int U , due to mutual interactions. The sum of these two energies, i.e., int K + int U , is called the internal energy of the substance.

• Moreover, when a body as a whole is moving in the gravitational field, it also possesses gravitational potential energy, ext U due to its position and kinetic energy, ext K , due to its motion. The sum of these two energies, i.e., ext K + ext U , is called the mechanical energy of the substance.

• When two bodies at different temperatures are brought in thermal contact with each other, the exchange of heat energy that takes place between them is called heat.