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Radiation, convection, and conduction are three different ways in which heat can be transferred. Convection and conduction require matter to transfer heat. Radiation transfers heat through space in the form of energy, as waves. Although these three methods of heat transfer involve different principles, they can all be understood based on the physics of heat, or thermal energy.
Matter is made up of particles, which interact with each other to transfer thermal energy. When a material with a higher temperature comes into contact with a material of lower temperature, heat flows from the hotter to the colder material. This process will continue until the two materials are at the same temperature and have reached a state of thermal equilibrium.
In conduction, a hotter piece of matter comes into contact with a colder piece of matter, and heat flows from the hotter to the colder region. The heat is conducted because fast-moving particles of the hotter matter transfer energy to the colder, slower-moving molecules of the colder matter. The ability of a material to conduct heat depends on its molecular structure and consistency. For example, metals are better conductors of heat than wood, and solids are better conductors of heat than liquids.
Convection transfers heat based on a different principle of particle movement. When particles possess a large amount of thermal energy, this energy causes them to move faster and spread out, making the material less dense. Particles in a colder region have less energy and move slowly, leading to greater density. In fluids and gases, this principle results in colder regions of the material sinking to the bottom, while hotter regions rise to the top.
A current is formed by the circulation of fluid or gas in this pattern. This is called a convection current. In the atmosphere, for example, cold air sinks while warm air rises, producing circulation.
The third method of heat transfer, radiation, requires no matter and does not depend on the interaction of particles. An example is solar radiation. Heat from the sun reaches the earth despite traveling through the vacuum of space. In the case of radiation, thermal energy exists in the form of waves. It is a type of electromagnetic radiation, like visible light.
Atoms absorb the energy of radiation through their electrons, which use the energy to move to a higher level within the atom. This energy can be emitted again when the electron falls to its original level. The temperature of an object in the presence of radiation depends on how much energy it absorbs versus how much it emits, so an object that absorbs more energy than it emits will rise in temperature.