# Black Body vs Non-Black Body: What Sets Them Apart in Kirchoff's Law?

• Kqwert
In summary, Kirchoff's law states that a black body and a non-black body have equal emissivity and absorptivity at the same temperature. This means that both types of bodies can absorb and emit radiation at all wavelengths. However, the difference lies in the fact that a black body can emit freely at all wavelengths, while a non-black body may only be able to emit at certain wavelengths depending on its temperature.
Kqwert

## Homework Statement

For a black body all absorbed radiation is emitted. Kirchoff´s law states that at the same temperature T1 the emissivity and absorptivity of a surface are equal, which holds for nonblack bodies as well. So, what really differs a black body and a non-black body in this case?

Kqwert said:
what really differs a black body and a non-black body in this case?
That it can absorb and emit freely at all wavelengths.

haruspex said:
That it can absorb and emit freely at all wavelengths.
Alright, but the attribute of emitting all absorbed radiation is not unique for black bodies, rather it holds for all bodies?

Kqwert said:
Alright, but the attribute of emitting all absorbed radiation is not unique for black bodies, rather it holds for all bodies?
That it can emit at all the wavelengths at which it absorbs, yes, but that is not quite what you wrote.
One can imagine a cool body only able to emit and absorb at a short wavelength. Incoming short wavelength radiation will warm the body, but it might not be able to emit much until its temperature rises.

## What is Kirchoff's Law?

Kirchoff's Law, also known as Kirchoff's Radiation Law, is a fundamental principle in physics that describes the relationship between the emission and absorption of thermal radiation by a material or object. It states that the ratio of emissive power to absorptive power of a material at a given temperature is equal to the ratio of its emissivity to its absorptivity.

## How does Kirchoff's Law relate to blackbody radiation?

Kirchoff's Law is a key component of the theory of blackbody radiation, which states that a perfect blackbody, which absorbs all incident radiation, also emits radiation at a maximum rate for a given temperature. This is described by the blackbody curve, which is based on Kirchoff's Law.

## What is the significance of Kirchoff's Law in thermodynamics?

In thermodynamics, Kirchoff's Law is significant because it helps to explain the behavior of thermal radiation in systems such as engines and heat transfer processes. It is also used in the calculation of thermodynamic properties, such as the emissivity and absorptivity of materials.

## Can Kirchoff's Law be applied to non-thermal radiation?

While Kirchoff's Law was originally formulated for thermal radiation, it can also be applied to non-thermal radiation, such as electromagnetic radiation. In this case, the ratio of emissive power to absorptive power is equal to the ratio of the material's reflectivity to its transmissivity.

## How is Kirchoff's Law different from other laws of radiation?

Kirchoff's Law differs from other laws of radiation, such as Stefan-Boltzmann Law and Wien's Displacement Law, in that it is a general principle that can be applied to all types of radiation. It does not provide specific equations or formulas, but rather a fundamental understanding of the relationship between emission and absorption of radiation by materials.

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