The atoms of matter are always in motion and as a result possess kinetic energies that they use to do work. When molecular work is done on or by systems, some amount of the total molecular kinetic energy is dissipated to the surroundings as heat. In general, energy is transferred between systems and surroundings as work or as heat. The presence of heat energy in matter establishes hotness in the matter while the absence of heat energy establishes coldness. Temperature is the measure of the degree of hotness or coldness of a substance. In other words, temperature measures the average kinetic energy of the molecules of a substance. The faster the molecules are moving, the higher the temperature. A lower temperature implies that the molecules are moving more slowly.
Heat transfer is conditioned on the presence of a temperature differential between systems and surroundings. Heat is transferred by conduction, convection, and radiation.