Question

While performing an experiment to calculate the emissivity of a material, we assume that the black body has an emissivity of 1.

What are two of the possible errors in the experiment?

Would one be assuming a perfect body exist be an error?

Answer 1

First of all we should understand the real definition of Emmisivity..

We know that Emissivity is defined as the ratio of the energy radiated from a material's surface to that radiated from a perfect emitter, known as a blackbody, at the same temperature and wavelength and under the same viewing conditions. It is a dimensionless number between 0 (for a perfect reflector) and 1 (for a perfect emitter).

​​​​​​​However no body is completely black as of now.

Also We can notice the equation which is widely known as  Stefan-Boltzmann equation-

The radiation energy per unit time from a black body is proportional to the fourth power of the absolute temperature and can be expressed with Stefan-Boltzmann Law as

q = σ T⁴ A       (1)

where

q = heat transfer per unit time (W)

σ = 5.6703 10^-8 (W/m²K⁴) - The Stefan-Boltzmann Constant

T = absolute temperature in kelvins (K)

A = area of the emitting body (m²)

So basically we shoud keep in mind that emmisivity depends on teperature of the surface and wavelength and also angle.So During calculation of emmisivity if we will assume the vale of emmisivity to 1 than a simple approximation will reflect huge fluctuation. However for alculation point of view usually we assume it to be one.

ii) Yes it will reflect error because actually no body is purely black it is assumed to be nearly black. So assuming perfectly black body may cause significant error in real practical sense.

For more deep knowledge you can go through complete mathematical derivation of Stefan boltzman and  weins displacement approximation . or you could ask a question for complete derivation of tese related equations.

Feel free to ask any doubts.