Heat Transfer

There are three methods of heat transfer: conduction, convection and radiation.

Conduction

Conduction is the transfer of thermal energy through free electron diffusion or phonon vibration, without a flow of the material medium. In other words, heat is transferred by conduction when adjacent atoms vibrate against one another, or as electrons move from atom to atom. Conduction occurs mainly in solids, where atoms are in constant contact. In liquids and gases, the particles are further apart, giving a lower chance of particles colliding and passing on thermal energy.

Heat transfer is always directed from a higher to a lower temperature. Denser substances are usually better conductors; metals are excellent conductors.

Convention

Convection is a combination of conduction and the transfer of thermal energy by circulation of the heated material medium. That is to say that convection combines conduction with physical movement of the hot particles to cooler areas. Because of this, convection typically occurs from or to a fluid. In solids, molecules are tightly packed and do not exhibit the kind of bulk movement necessary for convection.

Convection occurs in two forms: natural and forced convection.

In natural convection, fluid surrounding a heat source receives heat, becomes less dense and rises. The surrounding, cooler fluid then moves to replace it. This cooler fluid is then heated and the process continues, forming a convection current. The primary driving forces for natural convection are buoyancy and gravity.

Forced convection, by contrast, occurs when pumps, fans or other means are used to propel the fluid and create an artificial convection current.

Radiation

Radiation is transfer of heat through electromagnetic radiation in the heat spectrum. Hot or cold, all objects radiate heat unless they are at absolute zero, which is believed to be unattainable. No medium is necessary for radiation to occur; radiation works even in and through a perfect vacuum. A prime example of this is heat from the Sun, necessary for life on earth, which travels through the vacuum of space before warming the earth.

"Shiny" materials typically reflect radiant heat, just as they reflect visible light; dark materials typically absorb heat, just as they absorb visible light. In actuality, light is another a form of electromagnetic radiation with a shorter wavelength (and therefore a higher frequency) than heat radiation. The difference between visible light and radiant heat is small: they are simply different "colors" of electromagnetic radiation.