Thermal Energy

Potential mechanical and kinetic energy of random motion on a microscopic scale is known as thermal energy. In the case of solids this random motion is the result of atomic vibrations. However, in the case of liquids the molecules have sufficient energy of motion to overcome the forces that tend to restrict solids to a definite shape. In the case of a gas the velocities of the molecules are such that the distances between them become so great that the forces between molecules are negligible and the gas fills the space to which it is confined by an enclosing vessel.