Question

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

Short answer

Question: Define emissivity, absorptivity, and explain Kirchhoff's law of radiation. Answer: Emissivity (\(\epsilon\)) is a measure of how effectively a material emits thermal radiation compared to a perfect blackbody radiator. Absorptivity (\(\alpha\)) represents the fraction of incident electromagnetic radiation absorbed by a material compared to the total incident radiation. Kirchhoff's law of radiation states that, for an object in thermal equilibrium, the emissivity and absorptivity are equal at the same temperature and wavelength (i.e., \(\epsilon = \alpha\)). This law implies that good absorbers are also good emitters, while poor absorbers are poor emitters.

Step-by-step solution

Definition of Emissivity

Emissivity, represented by the symbol \(\epsilon\), is a measure of how effectively a material emits thermal radiation as compared to a perfect blackbody radiator. It is a dimensionless quantity with values ranging from 0 to 1. A higher value of emissivity indicates more effective emission of thermal radiation by the material.

Definition of Absorptivity

Absorptivity, represented by the symbol \(\alpha\), is the fraction of incident electromagnetic radiation absorbed by a material compared to the total incident radiation. It is also a dimensionless quantity, with values ranging from 0 to 1. A higher value of absorptivity indicates more effective absorption of incident radiation by the material.

Kirchhoff's Law of Radiation

Kirchhoff's law of radiation states that, for an object in thermal equilibrium, the emissivity and absorptivity are equal at the same temperature and wavelength. Mathematically, it is expressed as: $$\epsilon = \alpha$$ This law implies that good absorbers are also good emitters of radiation and that poor absorbers are poor emitters. This principle is essential in understanding the behavior of materials in various applications such as thermal insulation, solar panels, and radiative cooling systems.