For example, take the metal bar that was described above. When someone says "50 J of heat were added to the metal bar," what he or she is really saying is that the bar was heated so that it had 50 J more heat. It was probably placed in a hot water bath or heated in a bunsen burner flame, for example. The atoms in the bar were bombarded by the water molecules or ions in the burner flame. On average, the atoms in the bar started to move faster than they were, while the water molecules or flame ions started to move more slowly. Thus the kinetic energy of the metal atoms increased while the water molecules or ions in the flame lost kinetic energy. Thus heat energy was transferred from the water or flame to the metal.
Imagine that you have two metal bars with the same composition and the same mass. The only difference between them is that one bar has more heat (a higher average kinetic energy) than the other. What do you think you would feel if you were to touch the two bars?
Click on the metal bars below to see the macroscopic effect of an object's higher or lower heat. Click on the mouse icon at left to reset the image.