### 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

- grey body
- brilliant white polished body
- red hot body
- black body
- 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

- absolute temperature
- square of temperature
- fourth power of absolute temperature
- fourth power of temperature
- 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

- absolute temperature
- T2
- T5
- t
- l/T.

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

- absolute temperature (T)
- I2
- f
- t
- 1/r.

Correct answer: (A)

absolute temperature (T)

**15. **Cork is a good insulator because it has

- free electrons
- atoms colliding frequency
- low density
- porous body
- all of the above.

Correct answer: (D)

porous body

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

- p = 0, x = 0 and a = 1
- p=l,x = 0,anda = 0
- p = 0, T= l,anda = 0
- X = 0, a + p = 1
- 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

- higher
- lower
- same
- depends upon the shape of body
- none of the above.

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

- p = 0, x = 0 and a = 1
- p= l,T = 0anda = 0
- p = 0, x = 1 and a = 0
- x = 0, a + p = 0
- 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

- p = 0, x = 0 and a = 1
- p=l,x = 0anda = 0
- p = 0, x = 1 and a = 0
- x - 0, a + p = 1
- 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

- p = 0, x = 0 and a = 1
- p=l,T = 0anda = 0
- p = 0, x = 1 and a = 0
- x = 0, a + p = 1
- a = 0, x + p = 1. where a = absorptivity, p = reflectivity, x = transmissivity

Correct answer: (B)

p=l,T = 0anda = 0