0Whenever there is a temperature difference, there is a transfer of heat energy occurring between two material bodies.
Conditions for the transfer of heat energy to occur:
Condition 1:
Material bodies in contact with a difference in their temperatures.
Heat energy is always transferred from a higher temperature region to a lower temperature region.
Mechanism of conduction in a gas :
In a higher temperature region, the molecules of a particular gas have higher velocities, move in a continuous random motion, colliding with each other and exchanging a significant amount of energy and momentum.
Therefore if a molecule moves from a higher temperature region to a lower temperature region, kinetic energy is transferred to the lower temperature region, and this energy transfer takes place until the system attains an equilibrium temperature.
Condition 2:
Material bodies may not be in contact with each other.
Mechanism of convection between a hot metal plate and air/fan :
Hot metal plate kept under a fan cools faster when compared to the hot metal plate in still air.
Here, heat is convected.
The temperature gradient is dependent upon the rate at which the fluid carries the heat away.
Heat transferred from the hot metal plate to the fluid at the wall of the plate is a conduction process, but overall the entire transfer of heat is a convective process.
Convection heat transfer depends upon the viscosity of the fluid and also the thermal properties such as conductivity, specific heat, density etc.
Condition 3:
Heat transfer also occurs in a region where perfect vacuum exists.
Mechanism of radiation:
Electromagnetic radiation which occurs due to temperature difference is called thermal radiation.
