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1 When the burner of a stovetop is very hot, it is a source of heat energy. 2  Anything placed onto the stovetop and warmed, whether a pot of tea or a skillet for frying eggs, also become sources of heat energy. 3  Automobile fuels such as gasoline are sources of heat energy, as is the hot engine of a racecar or a school bus.
The core is earth's source of internal heat because it contains radioactive materials which release heat as they break down into more stable substances. The core is divided into two different zones. The outer core is a liquid because the temperatures there are adequate to melt the iron-nickel alloy.