# Heat and Mass Transfer – Set 03

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## Q1. Wien’s law states that the wave length corresponding to __ is proportional to the absolute temperature.

(A) Minimum energy
(B) Maximum energy
(C) Both (A) and (B)
(D) None of these

(A) Conduction
(B) Convection
(D) Scattering

## Q3. Thermal conductivity of wood depends on

(A) Moisture
(B) Density
(C) Temperature
(D) All of the above

Answer: (D) All of the above

## Q4. The thermal diffusivities for gases are generally

(A) More than those for liquids
(B) Less than those for liquids
(C) More than those for solids
(D) Dependent on the viscosity

Answer: (A) More than those for liquids

## Q5. Thermal conductivity of solid metals with rise in temperature normally

(A) Increases
(B) Decreases
(C) Remain constant
(D) May increase or decrease depending on temperature

(A) Liquids
(B) Energy
(C) Temperature
(D) Entropy

## Q7. The time constant of a thermocouple is

(A) The time taken to attain the final temperature to be measured
(B) The time taken to attain 50% of the value of initial temperature difference
(C) The time taken to attain 63.2% of the value of initial temperature difference
(D) Determined by the time taken to reach 100°C from 0°C

Answer:(C) The time taken to attain 63.2% of the value of initial temperature difference

(A) Aluminium
(B) Steel
(C) Brass
(D) Copper

## Q9. Heat conducted through per unit area and unit thick face per unit time when temperature difference between opposite faces is unity, is called

(A) Thermal resistance
(B) Thermal coefficient
(D) Thermal conductivity

## Q10. Fourier’s law of heat conduction is valid for

(A) One dimensional cases only
(B) Two dimensional cases only
(C) Three dimensional cases only
(D) Regular surfaces having non-uniform temperature gradients

Answer: (A) One dimensional cases only

## Q11. The energy distribution of an ideal reflector at higher temperatures is largely in the range of

(A) Shorter wavelength
(B) Longer wavelength
(C) Remain same at all wavelengths
(D) Wavelength has nothing to do with it

## Q12. The ratio of the emissive power and absorptive power of all bodies is the same and is equal to the emissive power of a perfectly black body. This statement is known as

(A) Kirchoff’s law
(B) Stefan’s law
(C) Wien’ law
(D) Planck’s law

(A) Iron
(C) Concrete
(D) Wood

(A) 20°C
(B) 40°C
(C) 60°C
(D) 66.7°C

## Q15. The process of heat transfer from one particle of the body to another is called conduction, when the particles of the body

(A) Move actually
(B) Do not move actually
(C) Affect the intervening medium
(D) Does not affect the intervening medium

Answer: (B) Do not move actually

## Q16. In free convection heat transfer transition from laminar to turbulent flow is governed by the critical value of the

(A) Reynold’s number
(B) Grashoff’s number
(C) Reynold’s number, Grashoff’s number
(D) Prandtl number, Grashoff’s number

Answer: (D) Prandtl number, Grashoff’s number

## Q17. The thickness of thermal and hydrodynamic boundary layer is equal if Prandtl number is

(A) Equal to one
(B) Greater than one
(C) Less than one
(D) Equal to Nusselt number

## Q18. The natural convection air cooled condensers are used in

(A) Domestic refrigerators
(B) Water coolers
(C) Room air conditioners
(D) All of these

## Q19. The logarithmic mean temperature difference ™ is given by (where Δt1 and Δt2 are temperature differences between the hot and cold fluids at entrance and exit)

(A) tm = (Δt1 – Δt2)/ loge (Δt1/Δt2)
(B) tm = loge (Δt1/Δt2)/ (Δt1 – Δt2)
(C) tm = tm = (Δt1 – Δt2) loge (Δt1/Δt2)
(D) tm = loge (Δt1 – Δt2)/ Δt1/Δt2

Answer: (A) tm = (Δt1 – Δt2)/ loge (Δt1/Δt2)

## Q20. The value of the wave length for maximum emissive power is given by

(A) Kirchhoff’s law
(B) Stefan’s law
(C) Wine’s law
(D) Planck’s law