Heat sources. Thermal expansion

The largest natural source of heat is the Sun: its rays warm the Earth and support life. Other sources are the burning of fuel (wood, coal, natural gas, petrol), electric current (electric heaters, irons), friction (rubbing hands warms them; a match), impact and deformation of bodies, the Earth’s internal heat (hot springs, geysers) and nuclear reactions. Heat is part of a body’s internal energy: when the molecules of a body move faster, its internal energy rises. The transfer of energy from a hot body to a colder body is called heat transfer, and the amount of energy transferred is the quantity of heat; it is measured in joules (J). Heat passes spontaneously from a hotter body to a colder one, not the other way. In friction and impact mechanical energy changes into internal energy: a piece of iron gets hot after several blows with a hammer.