Stefan-Boltzmann Law

The thermal energy radiated by a blackbody radiator per second per unit area is proportional to the fourth power of the absolute temperature and is given by

For hot objects other than ideal radiators, the law is expressed in the form:

where e is the emissivity of the object (e = 1 for ideal radiator). If the hot object is radiating energy to its cooler surroundings at temperature T_c, the net radiation loss rate takes the form

The Stefan-Boltzmann relationship is also related to the energy density in the radiation in a given volume of space.

Heat Radiation

Thermal radiation is energy transfer by the emission of electromagnetic waves which carry energy away from the emitting object. For ordinary temperatures (less than red hot"), the radiation is in the infrared region of the electromagnetic spectrum. The relationship governing the net radiation from hot objects is called the Stefan-Boltzmann law:

Calculation

While the typical situation envisioned here is the radiation from a hot object to its cooler surroundings, the Stefan-Boltzmann law is not limited to that case. If the surroundings are at a higher temperature (T_C > T) then you will obtain a negative answer, implying net radiative transfer to the object.

Radiation Calculation

The Sun at 5800K and a hot campfire at perhaps 800 K give off radiation at a rate proportional to the 4th power of the temperature.

Discussion

= =	Emissivity = (e = 1 for ideal radiator) Radiating area = = x 10^m2 =m2 = cm2 = ft2

Radiation rate =

x 10^watts

= watts
=calories/s = BTU/hr