|We have already seen that the average kinetic energy of the molecules in a gas is proportional to the absolute temperature of the gas. In a gas the atoms are moving randomly. In a solid, the atoms can move randomly about their equilibrium positions. In addition, the solid as a whole can move with a given velocity and have ordered kinetic energy. Only the kinetic energy associated with the random motion of the atoms is proportional to absolute temperature of the solid. The average kinetic energy associated with the random motion of a any substance is proportional to the absolute temperature of the substance. Thermal energy is disordered energy. Temperature is a measure of this internal, disordered energy. While in ideal gases the disordered energy is all kinetic energy, in solids it is a combination of kinetic and potential energy. If we model the atoms in a solid as being held together by tiny springs, the random kinetic energy of each atom constantly switches between kinetic energy and elastic potential energy.|
Two objects with different temperatures can exchange energy, if they are in thermal contact. The energy exchanged between object because they are in thermal contact is called heat. If two objects are in thermal contact and do not exchange heat, then they are in thermal equilibrium.
The zeroth law of thermodynamics states that two object, which are separately in thermal equilibrium with a third object, are in thermal equilibrium with each other. Two objects in thermal equilibrium with each other are at the same temperature.