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Heat

Heat, or thermal energy, is the sum of the kinetic energy of atoms or molecules. In thermodynamics, heat means energy which is moved between two things, when one of them has a higher temperature than the other thing.

Adding heat to something increases its temperature, but heat is not the same as temperature. The temperature of an object is the measure of the average speed of the moving particles in it. The energy of the particles is called the internal energy. When an object is heated, its internal energy can increase to make the object hotter. The first law of thermodynamics says that the increase in internal energy is equal to the heat added minus the work done on the surroundings.