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If a temperature difference exists between two points heat will flow from the point at the higher temperature to the point at the lower temperature. This flow will continue until equilibrium has been reached. The transmission of heat may be due to three different processes.
  • Conduction
This is the process by which heat flows from the hotter region of a substance to the colder region without there being any net flow of the material itself.
  • Convection
The heat is transfered from the solid or liquid by the surrounding gas. If the flow of gas away from the object starts by itself it is called natural convection. However, if the gas is forced past the object it is called forced convection. Note: convection may also occur within a fluid carrying heat away from a solid.
  • Radiation
This is the electromagnetic radiation emitted by a body as a function of its temperature. The idealised body is a black body, this being one which absorbs all radiation that impinges on it. Stefan's law states that the total energy radiated per unit time per unit surface area of a black body is proportional to the fourth power of the temperature (kelvins) of the body.
where s = Stefan's constant = 5.67x10-8Wm-2K-4

Heat Content
The SI unit for heat content is the Joule per cubic metre.
1 J/m3 = 26.8392x10-6 Btu/ft3 = 4.30886x10-11 therm/UK gal
Heat Flow Rate
The SI unit of heat flow rate or power is the watt.
1W = 3.41214Btu/h
Btu = British thermal units