Kirchhoff's law - overview

1. A PowerPoint presentation of Kirchhoff's law of thermal radiations Prepared by Jayam chemistry adda

2. Kirchhoff's law It states that the proportion of emissive and absorbing powers of any material object is constant at every wavelength in thermal equilibrium. And the emissive power of a perfect blackbody at that wavelength and temperature gives its value. At a temperature T, the amount of heat Kirchhoff's formula radiation taken or emitted by the body is wavelength-specific. The quantum of thermal radiation exhaled by a substance at a particular wavelength is proportional to e and a are emissive and absorptive powers of the body at the wavelength λ. E is the blackbody emissive power at the same wavelength. the sum of heat radiation absorbed by the substance at the same wavelength.

3. Absorptivity The ratio between the quantity of light absorbed to the total incident light is called absorptivity. The body's surface that absorbs 100% of the Heat absorption incident light is a good absorber. And its absorptivity is one. Similarly, if a body's surface reflects off a portion of the incident light falling on it, its absorptivity is less than one. And are termed gray bodies. Eventually, the absorptivity of gray bodies ranges between 0 and 1. So, lamp black and platinum black are approximate blackbodies with absorptivities of nearly 0.98. As we know, an ideal blackbody is inexistent.

4. Emissive power At constant temperature T for a particular wavelength λ, the emissive power is the energy radiated in a vacuum per unit time per unit area per unit range of wavelength lying between (λ-½) to (λ+½). If a substance is a good absorber, it is an excellent emitter. The blackbody absorbs all incident light. Thus, it has good emissive power. Hence, the emissive power of a perfect blackbody is a universal constant at each wavelength of the electromagnetic spectrum in thermal equilibrium. It gives the value of the ratio of emissive and absorptive powers of gray bodies at that wavelength and temperature conditions.

5. Thermal equilibrium Thermal equilibrium is the primary condition of Kirchhoff's law. In thermal equilibrium conditions, net heat flow is zero. Gustav Kirchhoff experimented in a constant temperature Temperature-T enclosure with white and black metallic balls that are identical in every respect. The black ball is a good absorber that takes a high quantum of radiation than the white ball. So, the black ball should be hotter than the white one. But, it did not happen. We know the perfect absorbers are good emitters. Consequently, the black sphere emits all absorbed energy to the box in temperature fluctuations. It maintains thermal equilibrium in the closed vessel.

6. Relationship between absorptivity & emissivity "Kirchhoff's law states that the body's emissivity numerically equals its absorptivity at thermal equilibrium conditions." 'a' is the absorptive power of the body at the wavelength λ. 'e' is its emissivity at the same wavelength. By Kirchhoff's law of thermal radiation, both are the same when expressed in numbers. =e A perfect blackbody can absorb all the incident light. Hence, its absorptivity is one. Consequently, its emissivity is also one. Gray body's emissivities vary between 0 and 1. A body with the highest emissivity can be a good absorber of thermal radiation.

7. Emissivity It is the ratio between the gray body and the perfect blackbody radiant emittances at temperature T. It defines the gray body's efficacy in emitting thermal radiation by comparing it with the black body at a particular wavelength and temperature. Gray bodies are poor absorbers and emitters of thermal radiation compared with black bodies. So, they possess low emissivities. As a result, polished silver coated metal surface emissivity is less than the lampblack coated metal surface at the same wavelength and temperature conditions.

8. Applications of Kirchhoff's law In spectroscopy In astronomy Considering the sun as a blackbody, The atomic spectra consist of Kirchhoff explained Fraunhofer's lines absorption and emission lines at the of the solar absorption spectrum. same positions. By Kirchhoff's law, the These dark absorption lines in the body's absorptivity is numerically sun's outer atmosphere are due to equal to its emissivity. thermal energy exchanges.

9. Examples of Kirchhoff's law Shiny outer walls of the thermos flask Bottom black coated cooking utensils White-colored cricket apparel Colored decorations in dark rooms

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