Define emissive power and coefficient of emmision of a body.
Emissive power of a body at a given temperature is the quantity of radiant energy emitted by the body per unit time per unit surface area of the body at that temperature.
If ‘Q’ is the amount of radiant energy emitted, ‘A’ is the surface area of the body and ‘t’ is the time for which body radiates energy, then the emissive power is
Coefficient of emission of a body is the ratio of the emissive power of the body at agiven temperature to the emissive power of a perfectly black body at the same temperature.
Coefficient of emission, `e=E/E_b`
The dimensions of emissive power are
(A) [M1 L-2 T-3 ]
(B) [M1 L2 T-3 ]
(C) [M1 L0 T-3 ]
(D) [M1 L0 T-2 ]
A pinhole is made in a hollow sphere of radius 5 cm whose inner wall is at temperature 727oC. Find the power radiated per unit area. [Stefan’s constant σ = 5.7 x 10-8 J/m2 s K4 , emissivity (e) = 0.2]
Compute the temperature at which the r.m.s. speed of nitrogen molecules is 832 m/s. [Universal gas constant, R = 8320 J/k mole K, molecular weight of nitrogen = 28.]
The light from the Sun is found to have a maximum intensity near the wavelength of 470 nm. Assuming the surface of the Sun as a black body, the temperature of the Sun is .................
[Wien's constant b = 2 .898 x l0- 3mK]
(a) 5800 K
(b) 6050 K
(c) 6166 K
(d) 6500 K