Emissive Power : E

The rate of radiation per unit surface area of an object is called its emissive power.

Emissivity or Coefficient of emission : e

The ratio of emissive power of any object (E) to the corresponding emissive power of a perfectly black body (E_b) at same temperature is called emissivity.

Stefan's law of heat radiation

The emissive power of a perfectly black body is proportional to the fourth power of its absolute temperature.

i.e. E = s T⁴

where s = Stefan's constant = 5.67 ´ 10^-8 watt / m² ´ K⁴
For any other object E = e s T⁴

Newton's Law of cooling

The rate of cooling of an object is directly proportional to the excess temperature of the object over that of its surrounding objects; provided that the excess temperature is sufficiently small.

K > q & constant for the object under a given temperature of the surroundings
q : temperature of the object is ⁰C
q ₀ : temperature of the surrounding objects in ⁰C
(The surrounding object is assumed to be perfectly black bodies)

If T << 2T₀ where T and T₀ are temperatures of the object and the surrounding objects in ⁰K respectively, then from Stefan's law we can show that

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Index

14.1 Conduction
14.2 Convection
14.3 Radiation

  Chapter 15