Heat Transfer

71. Thermal conductivity of solid metals with rise in temperature normally

  1. increases
  2. decreases
  3. remains constant
  4. may increase or decrease depending on temperature
  5. unpredictable.

Correct answer: (B)
decreases

72. Thermal conductivity of water at 20°C is of the order of

  1. 0.1
  2. 0.23
  3. 0.42
  4. 0.51
  5. 0.64.

Correct answer: (D)
0.51

73. Thermal conductivity of water in general with rise in temperature

  1. increases
  2. decreases
  3. remains constant
  4. may increase or decrease depending on temperature
  5. none of the above.

Correct answer: (D)
may increase or decrease depending on temperature

74. Thermal conductivity of wood depends on

  1. moisture
  2. density
  3. temperature
  4. all of the above
  5. none of the above.

Correct answer: (D)
all of the above

75. Thermal diffusivity is

  1. a dimensionless parameter
  2. function of temperature
  3. used as mathematical model
  4. a physical property of the material
  5. useful in case of heat transfer by radiation.

Correct answer: (D)
a physical property of the material

76. Thermal diffusivity of a substance is

  1. directly proportional to thermal con¬ductivity
  2. inversely proportional to density of substance
  3. inversely proportional to specific heat
  4. all of the above
  5. none of the above.

Correct answer: (D)
all of the above

77. Thermal diffusivity of a substance is .

  1. proportional of thermal conductivity
  2. inversely proportional to k
  3. proportional to (k)
  4. inversely proportional to k2
  5. none of the above.

Correct answer: (A)
proportional of thermal conductivity

78. Total emissivity of polished silver compared to black body is

  1. same
  2. higher
  3. more or less same
  4. very much lower
  5. very much higher.

Correct answer: (D)
very much lower

79. Total heat is the heat required to

  1. change vapour into liquid
  2. change liquid into vapour
  3. increase the temperature of a liquid or vapour
  4. convert water into steam and superheat it
  5. convert saturated steam into dry steam.

Correct answer: (D)
convert water into steam and superheat it

80. Two balls of same material and finish have their diameters in the ratio of 2 : 1 and both are heated to same temperature and allowed to cool by radiation. Rate of cooling by big ball as compared to smaller one will be in the ratio of

  1. 01:01
  2. 02:01
  3. 01:02
  4. 04:01
  5. 01:04

Correct answer: (C)
01:02

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