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Heat transfer or heat flow is the process whereby heat flows from regions of higher to regions of lower temperature. The flow of heat from one region to another is called a heat current.
The rate of heat flow depends on the mode of heat transfer and on the temperature difference across which the heat flows, as well as on the matter of the system under consideration and its geometry.
Types
There are three types of heat flow:
Conduction
Conduction carries heat through solids, gels, or still liquids by the collision of molecules.
In insulators, the heat current is carried almost entirely by phonon vibrations.
The "electron fluid" of a conductive metallic solid conducts nearly all of the heat current through the solid (phonon currents are still there, but carry less than 1% of the energy). Electrons also conduct electric current through conductive solids, and the thermal and electrical conductivities of most metals have about the same ratio. A good electrical conductor, such as copper, also conducts heat well.
The Peltier-Seebeck effect exhibits the propensity of electrons to conduct heat through an electrically conductive solid. Thermoelectricity is caused by the relationship between electrons, heat currents and electrical currents.
Convection
Convection displaces heat by the motion of hot or cold fluids (gases or liquids).
This is also a form of conduction, but it involves a material which flows because of the ensuing temperature and pressure differences.
Buoyant convection is due to the effects of gravity, and absent in microgravity environments.
Radiation
Radiation carries heat through the emission and absorption of photons (electromagnetic radiation).
For room temperature objects (in the neighborhood of 300 K), the majority of photons emitted (and involved in radiative heat transfer) are in the infrared spectrum, but this is by no means the only frequency range involved in radiation. The frequencies emitted are partially related to black-body radiation. Hotter objects (a campfire is around 700 K, for instance), transfer heat in the visible spectrum or beyond. Whenever EM radiation is emitted and then absorbed, heat is transferred. This principle is used in microwave ovens, laser cutting, and RF hair removal.
Preventing heat transfer
It is often desired to prevent heat transfer.
Weatherization slows convective heat flow in buildings. Insulation slows conductive heat flow. Reflective barriers or radiant barriers slow radiative heat flow, and often couple directly with insulation.
For more on heat transfer, see heat.
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