Heat Short Questions(MCQs) & Answers

Questions

41 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

View Answer

Answer: black body

42 The unit of overall coefficient of heat transfer is

A kcal/m2

B kcal/hr °C

C kcal/m2 hr °C

D kacl/m hr °C

View Answer

Answer: kcal/m2 hr °C

43 Joule sec is the unit of

A universal gas constant

B kinematic viscosity

C thermal conductivity

D Planck's constant

View Answer

Answer: Planck's constant

44 The value of Prandtl number for air is about

A 0.1

B 0.3

C 0.7

D 1.7

View Answer

Answer: 0.7

45 Total emissivity of polished silver compared to black body is

A same

B higher

C more or less same

D very much lower

View Answer

Answer: very much lower

46 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

View Answer

Answer: fourth power of absolute temperature

47 The unit of Stefan Boltzmann constant is

A watt/cm2 °K

B watt/cm4 °K

C watt2/cm °K4

D watt/cm2 °K4

View Answer

Answer: watt/cm2 °K4

48 In free con-vection heat transfer, Nusselt number is function of

A Grashoff no. and Reynold no.

B Grashoff no. and Prandtl no.

C Prandtl no. and Reynold no.

D Grashoff no., Prandtl no. and Reynold no.

View Answer

Answer: Grashoff no. and Prandtl no.

49 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

View Answer

Answer: more than those for liquids

50 Thermal diffusivity of a substance is

A directly proportional to thermal con¬ductivity

B inversely proportional to density of substance

C inversely proportional to specific heat

D all of the above

View Answer

Answer: all of the above

51 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

View Answer

Answer: p=l,T = 0anda = 0

52 Depending on the radiating properties, a body will be opaque when

A p = 0, x = 0 and a = 1

B p = 0, x = 1 and a = 0

C x - 0, a + p = 1

D a=0,x + p= 1

View Answer

Answer: x - 0, a + p = 1

53 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

View Answer

Answer: time

54 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

View Answer

Answer: absorptive power

55 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

View Answer

Answer: all of the above

56 The emissive power of a body depends upon its

A temperature

B wave length

C physical nature

D all of the above

View Answer

Answer: all of the above

57 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

View Answer

Answer: when system is under thermal equi-librium

58 Planck's law holds good for

A black bodies

B polished bodies

C all coloured bodies

D all of the above

View Answer

Answer: black bodies

59 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 81

View Answer

Answer: 81

60 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

View Answer

Answer: p = 0, T= l,anda = 0

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