JEE Main & Advanced Physics Transmission of Heat Convection

Mode of transfer of heat by means of migration of material particles of medium is called convection. It is of two types.

(1) Natural convection : This arise due to difference of densities at two places and is a consequence of gravity because on account of gravity the hot light particles rise up and cold heavy particles try setting down. It mostly occurs on heating a liquid/fluid.

(2) Forced convection : If a fluid is forced to move to take up heat from a hot body then the convection process is called forced convection. In this case Newton's law of cooling holds good. According to which rate of loss of heat from a hot body due to moving fluid is directly proportional to the surface area of body and excess temperature of body over its surroundings i.e.

 \[\frac{Q}{t}\propto A(T-{{T}_{0}})\]\[\Rightarrow \] \[\frac{Q}{t}=h\,A(T-{{T}_{0}})\]

where h = Constant of proportionality called convection coefficient, T = Temperature of body and \[{{T}_{0}}=\] Temperature of surrounding

Convection coefficient (h) depends on properties of fluid such as density, viscosity, specific heat and thermal conductivity.

(3) Natural convection takes place from bottom to top while forced convection in any direction.

(4) In case of natural convection, convection currents move warm air upwards and cool air downwards. That is why heating is done from base, while cooling from the top.

(5) Natural convection plays an important role in ventilation, in changing climate and weather and in forming land and sea breezes and trade winds.

(6) Natural convection is not possible in a gravity free region such as a free falling lift or an orbiting satellite.

(7) The force of blood in our body by heart helps in keeping the temperature of body constant.

(8) If liquids and gases are heated from the top (so that convection is not possible) they transfer heat (from top to bottom) by conduction.

(9) Mercury though a liquid is heated by conduction and not by convection.