mechanical engineering MCQs – Page 62

Practise multiple-choice questions from mechanical engineering for competitive examinations and subject revision. Review each marked answer and report any item that may be outdated or unclear.

3051. According of Kirchhoffs law______________________?
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A. emissive power and absorptivity are constant for all bodies

B. ratio of emissive power to absorptive power is maximum for perfectly black body

C. ratio of emissive power to absorptive power for all bodies is same and is equal to the emissive power of a perfectly black body

D. radiant heat is proportional to fourth power of absolute temperature

E. emissive power depends on temperature

3052. Which of the following has maximum value of thermal conductivity________________?
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A. brass

B. aluminium

C. lead

D. copper

E. steel

3053. Unit of thermal conductivity in S.I. units is__________________?
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A. B. and C. above

B. J/m K sec

C. J/m2 sec

D. A. and C. above

E. W/m K

3054. In convection heat transfer from hot flue gases to water tube, even though flow may be turbulent, a laminar flow region (boundary layer of film) exists close to the tube. The heat transfer through this film takes place by___________________?
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A. convection

B. radiation

C. conduction

D. both convection and conduction

E. none of the above

3055. Heat transfer in liquid and gases takes place by_____________________?
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A. conduction and convection

B. radiation

C. convection

D. convection and radiation

E. conduction

3056. Moisture would find its way into insulation by vapour pressure unless it is prevented by __________________ ?
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A. less thermal conductivity insulator

B. all of the above

C. high thickness of insulation

D. high vapour pressure

E. a vapour seal
3057. Thermal conductivity of wood depends on_______________?
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A. all of the above

B. density

C. none of the above

D. temperature

E. moisture

3058. Which of the following has least value of conductivity_________________?
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A. glass

B. air

C. plastic

D. rubber

E. water

3059. A grey body is one whose absorptivity_________________?
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A. varies with temperature

B. does not vary with temperature and. wavelength of the incident ray

C. varies with wavelength of the incident ray

D. is equal to its emissivity

E. none of the above

3060. Thermal conductivity of solid metals with rise in temperature normally___________________?
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A. increases

B. unpredictable

C. remains constant

D. decreases

E. may increase or decrease depending on temperature

3061. When heat is Transferred by molecular collision, it is referred to as heat transfer by __________________?
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A. convection and radiation

B. radiation

C. convection

D. scattering

E. conduction
3062. Heat transfer takes place as per ________________?
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A. first law of thermodynamic

B. zeroth law of thermodynamics

C. second law of the thermodynamics

D. Kirchoffs law E. Stefans law

3063. 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_________________?
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A. Krichoffs law

B. Black body law

C. Wien law

D. Stefans law

E. Plancks law

3064. In regenerator type heat exchanger, heat transfer takes place by___________________?
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A. a complete separation between hot and cold fluids

B. generation of heat again and again

C. direct mixing of hot and cold fluids

D. indirect transfer

E. flow of hot and cold fluids alternately over a surface
3065. The emissive power of a body depends upon its___________________?
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A. wave length

B. physical nature

C. all of the above

D. temperature

E. none of the above

3066. Total emissivity of polished silver compared to black body is___________________?
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A. more or less same

B. very much higher

C. higher

D. same

E. very much lower
3067. Which of the following property of air does not increase with rise in temperature ________________?
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A. thermal conductivity

B. density

C. thermal diffusivity

D. dynamic viscosity

E. kuiematic viscosity

3068. All radiations in a black body are___________________?
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A. reflected

B. transmitted

C. refracted

D. partly reflected and partly absorbed

E. absorbed
3069. The unit of overall coefficient of heat transfer is___________________?
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A. kacl/m hr C

B. kcal/hr C

C. kcal/m3 hr C

D. kcal/m2 hr C

E. kcal/m2

3070. The value of Prandtl number for air is about _______________________?
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A. 1.7

B. 10.5

C. 0.1

D. 0.7

E. 0.3

3071. Stefan Boltzmann law is applicable for heat transfer by __________________?
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A. conduction

B. convection

C. convection and radiation combined

D. radiation

E. conduction and radiation combined

3072. According to Stefan-Boltzmann law, ideal radiators emit radiant energy at a rate proportional to___________________?
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A. absolute temperature

B. fourth power of absolute temperature

C. cube of absolute temperature

D. square of temperature

E. fourth power of temperature

3073. According to Kirchoffs law, the ratio of emissive power to absorptivity for all bodies is equal to the emissive power of a _________________?
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A. grey body

B. brilliant white polished body

C. none of the above

D. black body

E. red hot body

3074. Depending on the radiating properties, a body will be opaque when _________________?
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A. p=l,x = 0anda = 0

B. x 0, a + p = 1

C. p = 0, x = 0 and a = 1

D. a=0,x + p= 1.

E. p = 0, x = 1 and a = 0

3075. If the temperature of a solid surface changes form 27C to 627C, then its emissive power changes in the ratio of___________________?
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A. 3

B. 6

C. 9

D. 27

E. 81
3076. The value of the wavelength for maximum emissive power is given by ___________________?
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A. Plancks law

B. Stefans law

C. Kirchhoffs law

D. Wiens law

E. Fouriers law

3077. According to Wiens law, the wavelength corresponding to maximum energy is proportion to ________________?
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A. absolute temperature (T)

B. I2

C. t

D. f

E. 1/r

3078. Two plates spaced 150 mm apart are maintained at 1000C and 70C. The heat transfer will take place mainly by ___________________?
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A. free convection

B. radiation and convection

C. convection

D. radiation

E. forced convection

3079. The ratio of the energy absorbed by the body to total energy falling on it is called_________________?
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A. absorptive power

B. absorptivity

C. none of the above

D. emissive power

E. emissivity

3080. Depending on the radiating properties, a body will be white when_________________?
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A. p=l,T = 0anda = 0

B. a = 0, x + p = 1.

C. p = 0, x = 1 and a = 0

D. p = 0, x = 0 and a = 1

E. x = 0, a + p = 1

3081. According to Stefans law, the total radiation from a black body per second per unit area is proportional to____________________?
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A. T2

B. l/T

C. absolute temperature

D. T5

E. t
3082. 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____________________?
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A. 0.75

B. 0.45

C. 0.6

D. 0.4

E. 0.55

3083. A perfect black body is one which_______________?
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A. transmits all heat radiations

B. abslprbs heat radiations of all wave lengths falling on it

C. fully opaque

D. reflects all heat

E. is black in colour

3084. In free con-vection heat transfer, Nusselt number is function of________________?
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A. Grashoff no and Prandtl no.

B. none of the above

C. Prandtl no. and Reynold no

D. Grashoff no. and Reynold no

E. Grashoff no., Prandtl no and Reynold no

3085. The total emissivity power is .defined as the total amount of radiation emitted by a black body per unit_______________?
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A. temperature

B. area and time

C. area

D. time

E. thickness

3086. The thermal diffusivities for gases are generally_____________________?
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A. less than those for liquids

B. more than those for liquids

C. same as for the liquids

D. dependent on the viscosity

E. more than those for solids

3087. A grey body is one whose absorptivity _________________?
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A. varies with the wave length of incident ray

B. there is no such criterion

C. varies with both

D. varies with temperature

E. does not vary with temperature and wave length of the incident ray
3088. The unit of Stefan Boltzmann constant is ____________________?
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A. watt/cm4 K

B. watt2/cm K4

C. watt/cm2 K4

D. watt/cm2 K2

E. watt/cm2 K

3089. Depending on the radiating properties, a body will be black when_________________?
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A. p = 0, x = 0 and a = 1

B. x = 0, a + p = 0

C. p= l,T = 0anda = 0

D. p = 0, x = 1 and a = 0

E. a = 0,x + p= 1.

F. re a = absorptivity, p == reflectivity, X = transmissivity

3090. Log mean temperature difference in case of counter flow compared to parallel flow will be________________?
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A. less

B. same

C. more

D. depends on other factors

E. none of the above

3091. The thermal diffusivities for solids are generally__________________?
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A. zerci

B. less than those for gases

C. more than those for liquids and gases

D. more or less same as for liquids and gases

E. jess than those for liquids

3092. Depending on the radiating properties, body will be transparent when________________?
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A. a = 0,x + p= 1

B. p = 0, T= l,anda = 0

C. p = 0, x = 0 and a = 1

D. p=l,x = 0,anda = 0

E. X = 0, a + p = 1

3093. Joule sec is the unit of_________________?
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A. universal gas constant

B. Plancks constant

C. none of the above

D. kinematic viscosity

E. thermal conductivity

3094. Absorptivity of a body will be equal to its emissivity___________________?
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A. at all temperatures

B. at one particular temperature

C. at critical temperature

D. for a polished body

E. when system is under thermal equi-librium
3095. Plancks law holds good for________________?
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A. all coloured bodies

B. polished bodies

C. none of the above

D. all of the above

E. black bodies
3096. The amount of radiation mainly depends on___________________?
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A. none of the above

B. temperature of body

C. all of the above

D. type of surface of body

E. nature of body

3097. Thermal diffusivity of a substance is __________________?
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A. inversely proportional to density of substance

B. inversely proportional to specific heat

C. directly proportional to thermal conductivity

D. all of the above

E. none of the above

3098. The energy distribution of an ideal reflector at higher temperatures is largely in the range of ___________________?
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A. longer wavelength

B. wavelength has nothing to do with it

C. none of the above

D. shorter wavelength

E. remains same at all wavelengths

3099. The ignition coil in diesel engines produces voltage of the order of________________?
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A. 20,000 volts

B. 6 to 12 volts

C. 80,000 volts

D. 240 volts

E. 1000 volts

3100. Method of governing used in petrol engine is________________?
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A. combined governing

B. hit and miss governing

C. partial governing

D. quantity governing

E. quality governing