## Saturday, July 12, 2008

### Heat Transfer Laws

Prevost Theory of Exchange

According to this theory all bodies radiate at all temperatures. The amount of thermal radiation radiated per unit time depends on the nature of the emitting surface, its area and its temperature.

Every body absorbs part of the thermal radiation emitted by the surrounding bodies when this radiation falls on it.

If a body radiates more than what it absorbs, its temperature falls.

Kirchoff’s law

The ratio of emissive power to absorptive power is the same for all bodies at a given temperature and is equal to the emissive power of a blackbody at that temperature.

Thus
E(body)/a(body) = E(blackbody)

Stefan-Boltzmann Law

The energy of thermal radiation emitted by per unit time by a black body of surface area A is given by

U = σAT4

Where
σ = Stefan Boltzmann constant = 5.67*10-8 W/m²-K4

Newton’s law of cooling

dT/dt = -bA(T-T0)

b = a constant depends on the nature of the surface involved
a = surface area exposed of the body
T- T0) = temperature difference between the body and surrounding