Heat Transfer

11. ccording to Kirchoff's law, the ratio of emissive power to absorptivity for all bodies is equal to the emissive power of a

  1. grey body
  2. brilliant white polished body
  3. red hot body
  4. black body
  5. none of the above.

Correct answer: (D)
black body

12. ccording to Stefan-Boltzmann law, ideal radiators emit radiant energy at a rate proportional to

  1. absolute temperature
  2. square of temperature
  3. fourth power of absolute temperature
  4. fourth power of temperature
  5. cube of absolute temperature.

Correct answer: (C)
fourth power of absolute temperature

13. ccording to Stefan's law, the total radiation from a black body per second per unit area is proportional to

  1. absolute temperature
  2. T2
  3. T5
  4. t
  5. l/T.

Correct answer: (D)
t

14. ccording to Wien's law, the wavelength corresponding to maximum energy is proportion to

  1. absolute temperature (T)
  2. I2
  3. f
  4. t
  5. 1/r.

Correct answer: (A)
absolute temperature (T)

15. Cork is a good insulator because it has

  1. free electrons
  2. atoms colliding frequency
  3. low density
  4. porous body
  5. all of the above.

Correct answer: (D)
porous body

16. Depending on the radiating properties, body will be transparent when

  1. p = 0, x = 0 and a = 1
  2. p=l,x = 0,anda = 0
  3. p = 0, T= l,anda = 0
  4. X = 0, a + p = 1
  5. a = 0,x + p= 1.

Correct answer: (C)
p = 0, T= l,anda = 0

17. Emissivity of a white polished body in comparison to a black body is

  1. higher
  2. lower
  3. same
  4. depends upon the shape of body
  5. none of the above.

Correct answer: (B)
lower

18. epending on the radiating properties, a body will be black when

  1. p = 0, x = 0 and a = 1
  2. p= l,T = 0anda = 0
  3. p = 0, x = 1 and a = 0
  4. x = 0, a + p = 0
  5. a = 0,x + p= 1. where a = absorptivity, p == reflectivity, X = transmissivity.

Correct answer: (A)
p = 0, x = 0 and a = 1

19. epending on the radiating properties, a body will be opaque when

  1. p = 0, x = 0 and a = 1
  2. p=l,x = 0anda = 0
  3. p = 0, x = 1 and a = 0
  4. x - 0, a + p = 1
  5. a=0,x + p= 1. where a = absorptivity, p = reflectivity, X = transmissivity.

Correct answer: (D)
x - 0, a + p = 1

20. epending on the radiating properties, a body will be white when

  1. p = 0, x = 0 and a = 1
  2. p=l,T = 0anda = 0
  3. p = 0, x = 1 and a = 0
  4. x = 0, a + p = 1
  5. a = 0, x + p = 1. where a = absorptivity, p = reflectivity, x = transmissivity

Correct answer: (B)
p=l,T = 0anda = 0

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