MECHANISM OF HEAT TRANSFER

 HEAT TRANSFER  Occurs only between regions that are at different temperature and its direction is always from higher to lower temperature

 CONDUCTION  Occurs within a body or between two bodies in contact HEAT CURRENT (H) – defined as the quantity of heat transfer through the rod per unit time - proportional to the cross- section area A and the temperature difference (T H - T C ) and inversely proportional to the length of the rod L.

 CONVECTION  Transfer of heat by mass motion of a fluid from one region of space to the other Two Types 1.Free Convection – involves heat flow that is caused by the difference in the density due to the thermal expansion 2.Forced Convection – involves heat flow caused by outside influences, like a blower or a pump

 RADIATION  mechanism of heat transfer via electromagnetic waves  can still occur even if there were nothing between the two bodies but vacuum  Every body, even at ordinary temperature, emits energy in a form of electromagnetic radiation  The rate of this energy radiation from the surface is proportional to the surface A  Also depends with the 4 th power of the absolute temperature and the nature of the surface

 A body that is a good absorber must also be a good emitter  An ideal radiator, with an emissivity of unity, is also an ideal absorber, absorbing all of the radiation that strikes it. Such an ideal surface is called an ideal black body or simply a blackbody.  An ideal reflector, which absorbs no radiation at all, is also a very ineffective radiator.

 This is the reason for the silver coatings on vacuum (“thermos”) bottles, invented by Sir James Dewar ( )  A vacuum bottle has double glass walls – the air is pumped out of the spaces between the walls; this eliminates nearly all heat transfer by conduction and convection  The silver coatings on the walls reflects most of the radiation from the contents back into the container, and the wall itself is a very poor emitter.