The internal energy of an object is the energy of its atoms or molecules due to their movement and positions.The internal energy of an object is the sum of the random distribution of the kinetic and potential energies of its molecules. The internal energy can change as a result of heat transfer (by conduction, convection or radiation) or work done on the object or by the object. Increasing the internal energy increases the random kinetic and potential energies.Solid particles vibrate about a fixed point and energy supplied increases the kinetic energy. The heat energy supplied to melt the solid increases the potential energy of the solid molecules as they vibrate enough to break free from the electrostatic forces holding them together.