Spread the love

Emissivity | What Is Emissivity

Emissivity | What Is Emissivity :- When an object is heated, it emits thermal radiation. The efficiency with which a surface emits this radiation compared to an ideal black body at the same temperature is called emissivity.

Emissivity (ε) is defined as :

The ratio of the radiant energy emitted per unit area per unit time by a surface to the radiant energy emitted by an ideal black body at the same temperature.

$\displaystyle \varepsilon =\frac{Radiant~energy~emitted~by~the~surface~per~unit~area~per~unit~time}{Radiant~energy~emitted~by~a~black~body~at~the~same~temperature}$

$\displaystyle \Rightarrow \varepsilon =\frac{{{e}_{GB}}}{{{E}_{IBB}}}=\frac{{{Q}_{GB}}}{{{Q}_{IBB}}}$

Emissivity :

A Unitless and Dimensionless quantity.
For an ideal black body ε = 1
Range 0 < ε ≤ 1

Factors Affecting Emissivity

Material type : Metals generally have low emissivity (0.05 – 0.2), while non-metals and painted surfaces have higher values.
Surface finish : Smooth, polished surfaces have lower emissivity than rough or oxidized ones.
Temperature : For some materials, emissivity changes with temperature.
Wavelength : Emissivity can vary across different parts of the spectrum.

Emissivity | What Is Emissivity

Factors Affecting Emissivity

Like this: