The physical processes that are described by the words radiation, convection, and conduction all involve the transportation of heat energy. The difference between these processes is that each is associated with a particular type of medium and therefore a particular mode of heat transportation.
1
Radiation involves the transportation of heat through empty space, or the vacuum. Electromagnetic energy, such as light and radio waves, is transported by radiation. The light that warms your face from the Sun is a form of electromagnetic radiation that has traveled through 93 million miles of empty space. It is not necessary to have a complete vacuum present, however. When you heat a cup of tea in a microwave oven, the water is heated directly by microwave radiation, that is, by electromagnetic energy.
2
Convection involves the transportation of heat through fluids (either liquids or gases). Suppose that a portion of a fluid is heated. If there is another portion of the fluid that is colder, then the properties of most fluids dictate that a flow develops between the two regions that tends to mix them and bring them into thermal equilibrium. Suppose that you remove the hot tea from the microwave oven, and then you pour some cream into the water. Since water and cream are fluids, the method of heat transfer between them is convection. As the cream heats up and the tea cools down slightly, a flow develops and the cream mixes with the tea.
3
Conduction involves the transportation of heat through solids. In a solid, the constituent atoms are in fixed positions with respect to each other and flow is not possible. When a portion of the solid is heated, the atoms increase their rate of vibration at that point. This vibrating motion propagates to neighboring atoms—ones that are not vibrating as much—and thus propagates away from the source of the heat. Concluding with our hot cup of tea, suppose that you place a metal spoon in the cup and stir. The handle of the spoon that extends out of the tea heats up by conduction.
