Heat Transfer

61. Sensible heat is the heat required to

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

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62. tefan Boltzmann law is applicable for heat transfer by

1. conduction
2. convection
3. radiation
4. conduction and radiation combined
5. convection and radiation combined.

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63. Temperature of steam at around 540°C can be measured by

1. thermometer
2. radiatiouv pyrometer
3. thermistor
4. thermocouple
5. thermopile.

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64. The amount of heat flow through a body by conduction is

1. directly proportional to the surface area of the body
2. directly proportional to the temperature difference on the two faces of the body
3. dependent upon the material of the body
4. inversely proportional to the thickness of the body
5. all of the above.

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65. The concept of overall coefficient of heat transfer is used in heat transfer problems of

1. conduction
2. convection
3. radiation
4. all the three combined
5. conduction and comte_ction.

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66. The insulation ability of an insulator with the presence of moisture would

1. increase
2. decrease
3. remain unaffected
4. may increase/decrease depending on temperature and thickness of insulation
5. none of the above.

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67. The rate of energy emission from unit surface area through unit solid angle, along a normal to the surface, is known as

1. emissivity
2. transmissivity
3. reflectivity
4. intensity of radiation
5. absorptivity.

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68. The ratio of heat flow Q1/Q2 from two walls of same thickness having their thermal conductivities as ATj - 2K2 will be

1. I
2. 0.5
3. 2
4. 0.25
5. 4

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69. The time constant of a thermocouple is

1. the time taken to attain the final temperature to be measured
2. the time taken to attain 50% of the value of initial temperature difference
3. the time taken to attain 63.2% of the value of initial temperature difference
4. determined by the time taken to reach 100°C from 0°C
5. none of the above.

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70. Thermal conductivity of a material may be defined as the

1. quantity of heat flowing in one second through one cm cube of material when opposite faces are maintained at a temperature difference of 1°C
2. 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
3. heat conducted in unit time across unit area through unit thickness when a temperature difference of unity is maintained between opposite faces
4. all of the above
5. none of the above.

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