How Heat Travels
Heat travels by conduction, convection, or radiation, or a combination of these methods.
As molecules move about they frequently bump into each other. According to the second law of thermodynamics, the faster-moving (hotter) molecules will give up some of their heat energy to the slower-moving (colder) molecules whenever collisions occur. The newly heated molecules then will be able to pass on a share of heat to molecules possessing less heat. The process continues in the direction away from the hottest molecules. By this means, heat is conducted (led) from the warm to the cool parts of a substance, or into a cool body that is in contact with a warm body. The heating of an iron rod, as shown in the drawing How Heat Travels, illustrates both: the heat spreads through the rod and also warms the handle that holds the rod.
Substances vary in their ability to conduct heat. Air and water are rather poor conductors. Most metals conduct heat rapidly. Asbestos conducts heat so poorly that it is used as a heat insulator.
When a fluid (a liquid or gas) is heated, the portion of the fluid nearest the heat source will expand as it gains energy. In expanding, this portion becomes less dense (lighter) and is pushed upward by cooler, heavier portions of the surrounding fluid. The displacement brings the cooler portions nearer the heat source, and they in turn gain energy, become lighter, and are pushed upward. The resulting movements, or currents (called convection currents), distribute heat from the source throughout the fluid.
Heat will continue to travel by convection as long as temperature differences exist within the fluid. Examples of convection are the movement of warm air in a room and the circulation of water in a kettle placed over a fire.
All bodies continually give off energy in the form of rays. The rays may be composed of particles or waves. Heat rays, called infrared radiation, are electromagnetic waves that resemble light waves but have somewhat longer wavelengths.
A body emits heat rays as a result of the vibration of its molecules. As the rays are emitted, the molecules lose some of their energy. When another body absorbs the rays, its molecules become more agitated and the body thus gains heat energy. Heat rays can travel through a vacuum. Infrared radiation from the sun, for example, passes through empty space to reach the earth.
The amount of radiation a body will absorb varies with the material out of which it is made and the nature of its surface. In general, materials with a dark or rough surface will absorb more infrared radiation than materials with a white or shiny surface.