Heat and Mass Transfer MCQ Objective Question and Answers Part 3

(a) radiant heat is proportional to fourth power of absolute temperature
(b) emissive power depends on temperature
(c) emissive power and absorptivity are constant for all bodies
(d) ratio of emissive power to absorptive power is maximum for perfectly black body
(e) ratio of emissive power to absorptive power for all bodies is same and is equal to the emissive power of a perfectly black body.
Ans: e

63. All radiations in a black body are
(a) reflected
(b) refracted
(c) transmitted      
(d) absorbed
(e) partly reflected and partly absorbed.
Ans: d

64. According to Kirchoff’s law, the ratio of emissive power to absorptivity for all bodies is equal to the emissive power of a
(a) grey body
(b) brilliant white polished body
(c) red hot body  
(d) black body
(e) none of the above.
Ans: d

65. The concept of overall coefficient of heat transfer is used in case of heat transfer by
(a) conduction    
(b) convection
(c) radiation
(d) conduction and convection
(e) convection and radiation.
Ans: d

66. The unit of overall coefficient of heat transfer is
(a) kcal/m2
(b) kcal/hr °C
(c) kcal/m2 hr °C
(4) kacl/m hr °C  
(e) kcal/m3 hr °C.
Ans: c

68. Joule sec is the unit of
(a) universal gas constant
(b) kinematic viscosity
(c) thermal conductivity
(d) Planck’s constant
(e) none of the above.
Ans: d

69. The value of Prandtl number for air is about
(a) 0.1
(b) 0.3
(c) 0.7
(d) 1.7
(e) 10.5.
Ans: c

70. The value of the wavelength for maximum emissive power is given by —
(a) Wien’s law
(b) Planck’s law
(c) Stefan’s law
(d) Fourier’s law
(e) Kirchhoff’s law.
Ans: a

72. Log mean temperature difference in case of counter flow compared to parallel flow will be
(a) same
(b) more
(c) less
(d) depends on other factors
(e) none of the above.
Ans: b

73. The energy distribution of an ideal reflector at higher temperatures is largely in the range of
(a) shorter wavelength
(b) longer wavelength
(c) remains same at all wavelengths
(d) wavelength has nothing to do with it
(e) none of the above.
Ans: a

74. Total emissivity of polished silver compared to black body is
(a) same
(b) higher
(c) more or less same
(d) very much lower
(e) very much higher.
Ans: d

75. According to Stefan-Boltzmann law, ideal radiators emit radiant energy at a rate proportional to
(a) absolute temperature
(b) square of temperature
(c) fourth power of absolute temperature
(d) fourth power of temperature
(e) cube of absolute temperature.
Ans: c

76. Which of the following property of air does not increase with rise in temperature
(a) thermal conductivity
(b) thermal diffusivity
(c) density
(d) dynamic viscosity
(e) kinematic viscosity.
Ans: c

77. The unit of Stefan Boltzmann constant is
(a) watt/cm2 °K    
(b) watt/cm2°K

 Ans: c

83. Thermal diffusivity of a substance is
(a) directly proportional to thermal conductivity
(b) inversely  proportional  to density  of substance
(c) inversely proportional to specific heat
(d) all of the above
(e) none of the above.
Ans: d

85. 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) Krichoff’s law
(b) Stefan’s law
(c) Wien’ law      
(d) Planck’s law
(e) Black body law.
Ans: a

86. According to Stefan’s law, the total radiation from a black body per second per unit area is proportional to
(a) absolute temperature
(b) T2
(c) T5
(d) t
(e) l/T.
Ans: d

87. According to Wien’s law, the wavelength corresponding to maximum energy is proportion to
(a) absolute temperature (T)
(b) I2
(c)  f
(d)  t                      
(e)  1/r.
Ans: a

88. Depending on the radiating properties, a body will be white when
(a) p = 0, x = 0 and a = 1
(b) p=l,T = 0anda = 0
(c) p = 0, x = 1 and a = 0
(d) x = 0, a + p = 1
(e) a = 0, x + p = 1.
where a = absorptivity, p = reflectivity, x = transmissivity
Ans: b

89. Depending on the radiating properties, a body will be black when
(a) p = 0, x = 0 and a = 1
(b) p= l,T = 0anda = 0
(c) p = 0, x = 1 and a = 0
(d) x = 0, a + p = 0
(e) a = 0,x + p= 1.
where a = absorptivity, p == reflectivity, X = transmissivity.
Ans: a

90. Depending on the radiating properties, a body will be opaque when
(a) p = 0, x = 0 and a = 1
(b) p=l,x = 0anda = 0
(c) p = 0, x = 1 and a = 0
(d) x – 0, a + p = 1
(e) a=0,x + p= 1.
where a = absorptivity, p = reflectivity, X = transmissivity.
Ans: d

91. The total emissivity power is .defined as the total amount of radiation emitted by a black body per unit
(a) temperature    
(b) thickness
(c) area
(d) time
(e) area and time.
Ans: d

92. The ratio of the energy absorbed by the body to total energy falling on it is called
(a) absorptive power
(b) emissive power
(c) absorptivity    
(d) emissivity
(e)     none of the above.
Ans: a

93. 40% of incident radiant energy on the surface of a thermally transparent body is reflected back. If the transmissivity of the body be 0.15, then the emissivity of surface is
(a) 0.45
(b) 0.55
(c) 0.40
(d) 0.75
(e) 0.60.
Ans: a

94. The amount of radiation mainly depends on
(a) nature of body
(b) temperature of body
(c) type of surface of body
(d) all of the above
(e) none of the above.
Ans: d

95. The emissive power of a body depends upon its
(a) temperature
(b) wave length
(c) physical nature
(d) all of the above
(e) none of the above.
 Ans: d

96. Two plates spaced 150 mm apart are maintained at 1000°C and 70°C. The heat transfer will take place mainly by
(a) convection      
(b) free convection
(c) forced convection
(d) radiation
(e) radiation and convection.
Ans: d

97. Absorptivity of a body will be equal to its emissivity
(a) at all temperatures
(b) at one particular temperature
(c) when system is under thermal equi-librium
(d) at critical temperature
(e) for a polished body.
Ans: c

98. In regenerator type heat exchanger, heat transfer takes place by
(a) direct mixing of hot and cold fluids
(b) a complete separation between hot and cold fluids
(c) flow of hot and cold fluids alternately over a surface
(d) generation of heat again and again
(e) indirect transfer.
Ans: c

99. A perfect black body is one which
(a) is black in colour
(b) reflects all heat
(c) transmits all heat radiations
(d) absorbs heat radiations of all wave lengths falling on it
(e) fully opaque.
Ans: d

100. Planck’s law holds good for
(a) black bodies
(b) polished bodies
(c) all coloured bodies
(d) all of the above
(e) none of the above.
Ans: a

101. If the temperature of a solid surface changes form 27°C to 627°C, then its emissive power changes in the ratio of
(a) 3
(b) 6
(c) 9
(d) 27
(e) 81.
Ans: e

102. Depending on the radiating properties, body will be transparent when
(a) p = 0, x = 0 and a = 1
(b) p=l,x = 0,anda = 0
(c) p = 0, T= l,anda = 0
(d) X = 0, a + p = 1
(e)     a = 0,x + p= 1.
Ans: c

103.   A grey body is one whose absorptivity
(a) varies with temperature
(b) varies with the wave length of incident ray
(c) varies with both
(d) does not vary with temperature and wave length of the incident ray
(e)  there is no such criterion.
Ans: d