Heat And Mass Transfer - Study Mode

[#71] The total radiation from a black body per second per unit area is ________ fourth power of the absolute temperature. This statement is known as Stefan Boltzmann law.
Correct Answer

(B) Directly proportional to

Explanation

Solution: The total radiation from a black body per second per unit area is Directly proportional to fourth power of the absolute temperature. This statement is known as Stefan Boltzmann law.

[#72] The use of heat exchangers is made in
Correct Answer

(D) All of the above

Explanation

Solution: The use of heat exchangers is made in
Radiators in automobile,
Condensers and boilers in steam plants,
Condensers and evaporators in refrigeration and air conditioning units

[#73] The heat transfer from a hot body to a cold body is directly proportional to the surface area and difference of temperatures between the two bodies. This statement is called
Correct Answer

(B) Newton's law of cooling

Explanation

Solution: The heat transfer from a hot body to a cold body is directly proportional to the surface area and difference of temperatures between the two bodies. This statement is called Newton's law of cooling

[#74] Which of the following statement is wrong?
Correct Answer

(C) The thermal conductivity of solid metals increases with rise in temperature

Explanation

Solution: For metals, the thermal conductivity is mainly a function of the motion of free electrons. As the temperature increases, the molecular vibrations increase (in turn decreasing the mean free path of molecules). So, they obstruct the flow of free electrons, thus reducing the conductivity

[#75] When α is absorptivity, ρ is reflectivity and τ is transmissivity, then for a diathermanous body,
Correct Answer

(C) α = 0, ρ = 0 and τ = 1

Explanation

Solution: Diathermanous body is a body which transmits all the incident radiation without absorbing or reflecting. (A body with transmitivity 1, reflectivity 0 and absorptivity 0) Air, up to certain extent can be considered as the diathermanous body because, it transmits all the radiation without hearing itself.