Heat can be transferred from one place to another by conduction, convection, radiation or a combination of these methods. Conduction and convection involve the movement of particles, but radiation involves electromagnetic waves.
Conduction occurs as molecules move around and bump into one another. According to the second law of thermodynamics, faster-moving (hotter) molecules give up some of their heat energy to slower-moving (colder) molecules whenever a collision occurs. The newly heated molecules then pass some of that heat to other molecules.
Substances vary in their ability to conduct heat. Air and water are poor conductors. Metals tend to conduct heat rapidly, and asbestos conducts heat so poorly that it was used as a heat insulator.
Convection happens because when a fluid (whether liquid or gas) is heated, the portion nearest the heat source expands as it gains energy. The expanding portion becomes less dense (lighter) and is pushed upward by cooler, heavier portions of the surrounding fluid. This displacement brings the cooler portions nearer the heat source until they gain energy, become lighter and move upward in turn. The resulting movement distributes heat from the source throughout the fluid.
Heat travels by convection as long as temperature differences exist within the fluid. The movement of warm air in a room and the circulation of water in a kettle placed over a fire are examples of convection moving heat.
All objects that are warmer than absolute zero continually give off energy in the form of electromagnetic waves. A body emits heat rays as its molecules vibrate.
As the rays are emitted, the molecules of the initial body lose energy. When another body absorbs the rays, its molecules become more agitated, and it gains heat energy.