Download Aptitude Crack
img not found img not found

Heat Transfer - Engineering

Q1:

A hollow sphere and a solid sphere of the same material and equal radii are heated to the same temperature. In this case,

A the cooling rate will be the same for the two spheres and hence the two spheres will have equal temperatures at any instant.

B both the spheres will emit equal amount of radiation per unit time in the beginning.

C both will absorb equal amount of radiation from the surrounding in the beginning.

D both(b) & (c).

ANS:D - both(b) & (c).

The correct statement is: both (b) & (c). Here's why:

  1. Both the spheres will emit equal amount of radiation per unit time in the beginning: According to Stefan-Boltzmann law, the rate of radiation emitted by a surface is proportional to the fourth power of its absolute temperature (assuming blackbody radiation). Since both spheres are at the same temperature, they will initially emit radiation at the same rate.
  2. Both will absorb equal amount of radiation from the surrounding in the beginning: Similarly, according to Kirchhoff's law of thermal radiation, the emissivity of a surface at a particular wavelength and temperature is equal to its absorptivity at the same wavelength and temperature. Since the spheres are of the same material and are at the same temperature, they will initially absorb radiation from the surroundings at the same rate.
Therefore, both statements (b) and (c) are correct, and the correct choice is both (b) & (c).