Theories of Heat Transfer

Physics describes heat transfer or the movement of heat energy in three ways. These are conduction, convection and radiation.

When there is a temperature difference within a piece of material, the heat from the part that has a high temperature will flow to the part with the lower temperature. This is known as the general conduction theory of heat transfer. Higher temperature denotes higher kinetic energy - the atoms in the part of the material that is hotter are moving faster. These fast moving atoms will naturally collide with slow moving ones in the part of the material that is colder. The result is that the fast moving atoms lose energy and the slow moving ones gain energy. This whole energy transfer process is conduction heat transfer.

The mathematical description of this phenomenon was formulated by the French physicist Joseph Fourier and the equation is called Fourier's Law. This equation states that the rate of heat transfer can be determined given the temperature profile of the material and its thermal conductivity. Temperature profile refers to the difference of temperatures. Thermal conductivity on the other hand is based largely on the kind of material used and its physical properties. Some materials such as copper metal wires easily and quickly allow heat to flow through it and so are good thermal conductors. Thermal insulators refer to materials that have the opposite characteristic and examples are fiberglass or rubber. Physical dimensions directly affect thermal conductivity. Energy transfer will naturally take longer in big or irregularly shaped materials compared to small, streamlined or symmetrically shaped materials.

While conduction deals with the kinetic energy of atoms and serves as the general description of heat transfer, convection is about the movement of heat and matter from a high temperature area to a low one. Convection heat transfer involves the properties of pressure and density. When a substance is heated it generally becomes less dense and in terms of gases and liquids, this affects their buoyancy. A boiling pot of water is a simple example of convection heat transfer. The water at the bottom of the pot is the first to be heated, becomes less dense, and rises to the surface producing bubbles. The water at the top, being cold yet and so more dense, sinks to the bottom of the pot and is heated in turn. What occurs then is a continuous flow of heat and matter between hot and cold areas. This flow is called convection currents.

Radiation is about the transfer of heat without an intervening medium. In both conduction and convection, heated matter moves from one area to another or heat energy flows within a material. With radiation however the energy of heat moves through mostly empty space. What flows or radiates is not necessarily energy in the form of heat although it may initially have been so. When you light a match, the heat creates light, and light is a form of energy that can travel either as a particle or a wave and strike any material across a distance. Thus the energy transfer occurs even if a material is not directly attached to the source of heat.


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