61. The total emissivity power is .defined as the total amount of radiation emitted by a black body per unit
temperature
thickness
area
time
area and time.
Correct answer: (D) time
62. The unit of overall coefficient of heat transfer is
kcal/m2
kcal/hr °C
kcal/m2 hr °C
kacl/m hr °C
kcal/m3 hr °C.
Correct answer: (C) kcal/m2 hr °C
63. The unit of Stefan Boltzmann constant is
watt/cm2 °K
watt/cm4 °K
watt2/cm °K4
watt/cm2 °K4
watt/cm2 °K2.
Correct answer: (D) watt/cm2 °K4
64. The value of Prandtl number for air is about
0.1
0.3
0.7
1.7
10.5.
Correct answer: (C) 0.7
65. The value of the wavelength for maximum emissive power is given by —
Wien's law
Planck's law
Stefan's law
Fourier's law
Kirchhoff's law.
Correct answer: (A) Wien's law
66. Thermal conductivity of a material may be defined as the
quantity of heat flowing in one second through one cm cube of material when opposite faces ^re maintained at a temperature difference of 1°C
quantity of heat flowing in one second through a slab of the material of area one cm square, thickness 1 cm when its faces differ in temperature by 1°C
heat conducted in unit time across unit area through unit thickness when a temperature difference of unity is maintained between opposite faces
all of the above
none of the above.
Correct answer: (D) all of the above
67. Thermal conductivity of air at room temperature in kcal/m hr °C is of the order of
0.002
0.02
0.01
0.1
0.5.
Correct answer: (B) 0.02
68. Thermal conductivity of air with rise in temperature
increases
decreases
remains constant
may increase or decrease depending on temperature
none of the above.
Correct answer: (A) increases
69. Thermal conductivity of glass-wool varies from sample to sample because of variation in
composition
density
porosity
structure
all of the above.
Correct answer: (E) all of the above.
70. Thermal conductivity of non-metallic amorphous solids with decrease in temperature