- #1
em3ry
Gold Member
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Is there a form of Kirchhoff's law of thermal radiation that applies to infinitesimal volumes rather than optical surfaces of materials?
Kirchhoff's law of thermal radiation states that the ratio of emissive power to absorptive power of a material at thermal equilibrium is constant and independent of temperature or wavelength.
Kirchhoff's law can be applied to infinitesimal volumes by considering the emission and absorption of radiation within a small volume. The law states that the ratio of the emission and absorption coefficients for a given material will be the same for all wavelengths and temperatures within this volume.
Kirchhoff's law is significant in thermal radiation studies because it provides a fundamental understanding of the relationship between emission and absorption of radiation in materials. It allows for the prediction of thermal radiation behavior and can be used to design and optimize thermal systems.
Kirchhoff's law is related to the Stefan-Boltzmann law, which states that the total emissive power of a blackbody is proportional to the fourth power of its absolute temperature. This is because Kirchhoff's law states that the emissivity of a material is equal to its absorptivity, and a blackbody is a material with an emissivity and absorptivity of 1.
While Kirchhoff's law is generally applicable to most materials, there are some exceptions. Some materials, such as selective surfaces, have different emissivity and absorptivity values for different wavelengths, violating the law. Additionally, Kirchhoff's law does not hold for non-equilibrium systems or for materials with strong electric or magnetic fields.