UY1: Radiation

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.

$$\frac{dQ}{dt} = \sigma A e T^{4}$$
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 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

Previous: Convective Heat Transfer

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