Heat Transfer Notes

Heat transfer from one place to another place in different ways.

Conduction

 The heat energy flow from a body at a higher temperature to a body at a lower temperature without the actual movement of molecules of a medium. In metaL conduction takes place by the movement of free electrons.

 1. Steady-state and non-steady state

 Non-steady state
Two ends of a rod are at different temperature whose body is exposed to air the rate of flow of heat remain different across different cross do section. The temperature gradient varies (dƟ / dl ) from point to point in this case heat loss takes place from the body of the conductor.

Steady-state
 The body of the conductor is well insulated from its surrounding then no heat loss takes place from the body of the conductor is called a steady state. The rate of flow of heat remains the same across any cross-section. The temperature gradient (dƟ / dl ) remains the same.

Radiation

The process of transfer of heat in which medium is not needed. Heat transfer in the form electromagnetic wave of speed 3km/s. 

1. Black body.

 Body that can absorb all the heat radiations incident on it. It also can radiate all the wavelengths if heated at high temperature. 

2. Emissive power
It is energy radiated per unit area per unit time at a temperature.

 Absorptive power
It is the ratio of heat energy absorbed in a given time by given area and amount of heat energy incident on the surface for same time.

Kirchhoff"s law
The ratio of emissive power and absorptive power at a temperature is constant for all bodies is equal to emissive power of perfectly black body.