Sources of heat include the sun, earth, wood and biomass, coal, petroleum, natural gas, propane, oil, electricity, heat pumps, ground source heat pumps, solar energy, chemical energy, electrical energy, atomic energy, air, and friction
. The sun is the primary source of heat energy on earth
. Heat energy can be produced by changing other forms of energy, such as electrical energy and chemical energy, into heat energy
. Examples of sources of heat energy include electrical appliances, burning wood, eating food, and friction
