Concept of Grey body and Black body

Absorptivity

Reflectivity

Transmissivity

Emissivity

Grey Body

Black Body

Laws of black body radiation

Stefan-Boltzmann law

Planck’s Law

Wiens Displacement law

Conclusion

1. Process Heat Transfer (2140503) TOPIC Concept of Grey body and Black body Presented by Paranami Chintan (160170105043) Rajput Mayank (160170105045) Rana Pranav (160170105046)

2. CONTAINS Basic Terms Absorptivity Reflectivity Transmissivity Emissivity Grey Body Black Body Laws of black body radiation Stefan-Boltzmann law Planck’s Law Wiens Displacement law Conclusion

3. Before the understanding concept of grey body and black body we should understand some terms like Absorptivity Reflectivity Transmissivity Emissivity Monochromic Emissivity

4. The Fraction of incident radiation on a body that is absorbed by the body is known as the absorptivity . It may ne denoted by the letter “a” . The Fraction of incident radiation on a body that is REFLECTED by the body is known as the REFLECTIVITY . It may ne denoted by the letter “r” . The Fraction of incident radiation on a body that is transmitted by the body is known as the TRANSMISSIVITY . It may ne denoted by the letter “ τ ” . It is define as the ratio of emissive power E of the body to that of a black body E b at the same temperature. It may be denoted by the letter “e” . ABSORPTIVITY REFLECTIVITY TRANSMISSIVITY EMISSIVITY

5. Grey Body A body having the same value of the monochromatic emissivity at all the wave lengths is called grey body. A Grey body is the one of which emissivity is independent of wavelength. MONOCHOMATIC EMISSIVITY It is the ratio of the monochromatic emissive power of a surface to that of a black surface at the same wavelength.

6. BLACK BODY A body for which a=1, r= t= 0, i.e which absorbs all the incident radiant energy, is called a black body. It neither reflects nor transmits but absorbs all the radiation incident on it, so it treated as ideal radiation receiver. It is not necessary that surface is black in colour . It absorbs 96% of visible light Both absorptivity and emissivity of a perfectly black body are unity.

7. DEFINATION OF BLACK BODY A black body is an ideal body which allows the whole of the incident radiation to pass into itself without reflecting the energy and absorbs within itself this whole incident radiation without passing on the energy . This property is valid for all wavelengths and to all angels of incidence. Therefore, the blackbody is an ideal absorber of incident radiation.

8. BLACK BODY RADIATION LAW Stefan-Boltzmann law: It states that the total emissive power of a black body is directly proportional to the fourth power of its absolute temperature. E b α T 4 E b = σ T 4 Where T = Temperature in K σ = Stefan-Boltzmann constant = 5.67 * 10 -8 W/m 2 .K 4

9. 2. Planck’s Law This law gives a relationship between the monochromatic emissive power of a black body, absolute temperature and the corresponding wavelength. Eb , λ = ------- Where : Eb , λ = monochromatic emissive power of the body surface, W/(m 2 μ m). h= Planck's constant (6.625 * 10 -3 ) K = Boltzmann constant C = speed of light T = absolute Temperature λ = wavelength 2 π hc 2 λ -5 e hc /k λ T -1

10. 3. Wiens Displacement law: It states that the wavelength at which the maximum monochromatic emissive power is obtained in inversely proportional to the absolute temp., or T λ max = C Where : λ max = micrometers T = temp in K C = constant (2890) This law gives a relationship between the wavelength at which maximum emissive power is attained and the absolute temperature.

11. CONCLUSION As the temperature increase , the peak wavelength emitted by the black body decreases. As temperature increases, the total energy emitted increases , because the total area under the curve increases. The curve gets infinitely close to the x- axis but never touches it.