A _____________ surface has the maximum thermal emissivity out of the following ?

Related Mcqs:

1. The thermal radiative flux from a surface of emissivity = 0.4 is 22.68 kW/m2. The approximate surface temperature (K) is (Stefan-Boltzmann constant = 5.67 × 10-8 W/m2.K4) ?

   A. 1000
   B. 727
   C. 800
   D. 1200

2. Out of 100 kcal/second of incident radiant energy on the surface of a thermally transparent body, 300 kcal/second is reflected back. If the transmissivity of the body is 0.25, the emissivity of the surface will be_________________?

   A. 0.35
   B. 0.45
   C. 0.55
   D. 0.85

3. The heat transfer by radiation from a mild steel surface is to be reduced by reducing the emissivity of the surface. This can be best achieved by__________________?

   A. Painting the surface black
   B. Painting the surface white (with aluminium paint)
   C. Giving the surface a mirror finish
   D. Roughening the surface

4. The inner wall of a furnace is at a temperature of 700°C. The composite wall is made of two substances, 10 and 20 cm thick with thermal conductivities of 0.05 and 0.1 W.m-1.°C- 1 respectively. The ambient air is at 30°C and the heat transfer co-efficient between the outer surface of wall and air is 20 W.m-2.°C-1. The rate of heat loss from the outer surface in W.m-2is__________________?

   A. 165.4
   B. 167.5
   C. 172.5
   D. 175

5. If the thermal conductivity of a wall material is independent of temperature, the steady state temperature distribution in the very large thin plane wall having steady, uniform surface temperature follows _____________ law?

   A. Parabolic
   B. Hyperbolic
   C. Linear
   D. Logarithmic

6. With increase in temperature, the total emissivity of conductors____________________?

   A. Increases
   B. Decreases
   C. Remain same
   D. Decreases linearly

7. The absorptivity of a body is equal to its emissivity _________________?

   A. At a particular temperature
   B. For circular bodies
   C. Under thermal equilibrium
   D. None of these

8. What is the emissivity of a black body ?

   A. 0
   B. 1
   C. 0.5
   D. 0.90

9. While the total emissivity of a perfect black body is unity, the same for a real body is__________________?

   A. 0
   B. 1
   C. > 1
   D. Between 0 and 1

10. A body is called grey if the monochromatic emissivity of the body is___________________?

    A. Zero
    B. Unity
    C. Same for all wavelengths
    D. Different for all wavelengths