This is the mechanism by which heat is transferred by continual emission of electromagnetic energy (also called thermal radiation) from the surface of a body. These electromagnetic waves travel at the speed of light and are transmitted through vacuum as well as through air. When they fall on a body that is not transparent, they are absorbed, resulting in a transfer of heat to the absorbing material.

The thermal energy emitted by a surface depends on the nature of the surface and on its temperature.

Stefan-Boltzmann Law

The rate at which an object radiates electromagnetic energy is proportional to the fourth power of its absolute temperature.

$$\frac{dQ}{dt} = \sigma A e T^{4}$$
,where
σ = 5.67 x 10-8 W.m-2.K-4 (Stefan-Boltzmann constant)
A = surface area of the object
e = emissivity of object (dimensionless)
T = surface temperature of the object

Radiation loss quickly becomes an important heat loss mechanism as the temperature increases.

Emissivity

Emissivity is the fraction of the incident radiation that the surface absorbs: 0 < e  ≤ 1.

For any radiation falling on a body, absorption + transmission + reflection = 1

– Emissivity depends on the properties of the surface of the object, generally larger for dark and rough surfaces than for light and smooth ones.

– Emissivity also depends on wavelength of the radiation.

– A good emitter (large emissivity) is also a good absorber but a poor reflector. Typical organic stuff has e ≈ 0.95.

– A poor emitter is also a poor absorber but a good reflector (e.g. polished metals: e < 0.1).

– An “ideal” absorber (e = 1) is also an “ideal” emitter. It reflects no thermal energy, and hence would appear black. Such an ideal absorber is called blackbody (if the absorber is non-ideal, it is called a greybody).

Next: Planck radiation law and Wien displacement law

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