Question

Define emissivity and absorptivity. What is Kirchhoff's law of radiation?

Short answer

Question: Define emissivity and absorptivity, and briefly explain Kirchhoff's law of radiation. Answer: Emissivity is a measure of the effectiveness of a material in emitting thermal radiation, compared to a perfect blackbody, with values ranging from 0 to 1. Absorptivity is a measure of how well a material absorbs incident thermal radiation compared to a perfect blackbody, with values also ranging from 0 to 1. Kirchhoff's law of radiation states that the emissivity and absorptivity of a given material and wavelength are equal at the same temperature, represented by the formula \(ε(λ, T) = α(λ, T)\). This law highlights the relationship between thermal radiation properties and the temperature and wavelength of a material in thermal equilibrium.

Step-by-step solution

Definition of Emissivity

Emissivity is a measure of the effectiveness of a material in emitting thermal radiation, compared to a perfect blackbody. It is a dimensionless quantity, represented by the symbol \(ε\) and varies between 0 (no emission) and 1 (perfect emitter).

Definition of Absorptivity

Absorptivity is a measure of how well a material absorbs incident thermal radiation compared to a perfect blackbody. It is a dimensionless quantity, represented by the symbol \(α\), and also varies between 0 (no absorption) and 1 (perfect absorber).

Kirchhoff's Law of Radiation

Kirchhoff's law of radiation states that, for a given material and wavelength, the emissivity (the ability to emit thermal radiation) and absorptivity (the ability to absorb incident thermal radiation) are equal at the same temperature. Mathematically, it is represented by the formula: $$ε(λ, T) = α(λ, T)$$ Where \(ε\) is the emissivity, \(α\) is the absorptivity, \(λ\) is the wavelength, and \(T\) is the temperature. The law emphasizes that the properties of a material regarding thermal radiation are determined by its temperature and wavelength, and it holds true for materials in thermal equilibrium.