Stefan's law, emissivity, emissive power - explanation by advguruji/Physics Advocate

Stefan's law

All objects radiate energy continuously in the form of electromagnetic waves. The rate at which an object radiates energy is proportional to the fourth power of its absolute temperature. This is known as the Stefan's law and is expressed in equation form as -

[   I = σeT⁴   ]

Here I is the intensity of radiation in W/m² radiated by the object, T is the absolute temperature in kelvin (K), e is a fraction between 0 and 1 called the emissivity of the object and σ is a universal constant called Stefan's constant, which has the value -

σ = 5.67 × 10-⁸ W/m²/K⁴

This law is also known as Stefan-Boltzmann law.

Confusion in emissivity (e) ?

I can explain-

Emissivity

The ratio of emissive power (E) of a body to the emissive power (ε) of a black body (a body that absorbs all the radiations falling on it) at same temperature is called emissivity. It is denoted by e.

Thus, emissivity -

          [ e = E/ε ]

It is a dimensionless quantity.

(i) For highly polished body, e = 0

(ii) For practical bodies, 0 < e < 1

Note :- Emissive power is basically the intensity of radiation emitted by a body.

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