A body cools down from 75°C to 70°C in 10 minutes. It will cool down from 70° C to 65° C in ______________ minutes?

Related Mcqs:

1. A cube, sphere & a thin circular plate (all having same mass and made of same material) are all heated to 300°C and allowed to cool in natural air. Which one will cool the slowest ?

   A. Cube
   B. Plate
   C. Sphere
   D. All will cool at the same rate

2. In a cooling tower, water becomes cool by________________?

   A. Loosing sensible heat
   B. Heat transfer to surroundings
   C. Vaporisation due to heat loss to air
   D. Loosing latent heat

3. A metal ball of radius 0.1 m at a uniform temperature of 90°C is left in air at 30°C. The density and the specific heat of the metal are 3000 kg/m3 and 0.4 kJ/kg.K respectively. The heat transfer co-efficient is 50 W/m2.K Neglecting the temperature gradients inside the ball, the time taken (in hours) for the ball to cool to 60°C is_________________?

   A. 555
   B. 55.5
   C. 0.55
   D. 0.15

4. The wavelength at which the maximum monochromatic emissive power occurs for a black body, is (where, T = absolute temperature of the black body)?

   A. αT
   B. α × 1/T
   C. α T4
   D. Independent of T

5. 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

6. 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

7. 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

8. A steel sphere of radius 0.1 m at 400°K is immersed in an oil at 300°K. If the centre of the sphere reaches 350°K in 20 minutes, how long will it take for a 0.05 m radius steel sphere to reach the same temperature (at the centre) under identical conditions? Assume that the conductive heat transfer co-efficient is infinitely large ?

   A. 5 minutes
   B. 10 minutes
   C. 20 minutes
   D. 40 minutes

9. Stefan’s block body radiation law can also be derived from ______________ law?

   A. Kirchoff’s
   B. Planck’s
   C. Fourier’s
   D. None of these

10. What is the absorptivity of a black body?

    A. 1
    B. 0
    C. 0.78
    D. 0.95