# Multiple choice question for engineering

## Set 1

1. When the surface temperature variation inside a solid are periodic in nature, the profile of temperature variation with time may assume

a) Triangular

b) Linear

c) Parabolic

d) Hyperbolic

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2. The surface temperature oscillates about the mean temperature level in accordance with the relation

a) α _{S,T} – α _{S,A} = 2 sin (2 π n T)

b) α _{S,T} – α _{S,A} = 5 sin (2 π n T)

c) α _{S,T} – α _{S,A} = sin (2 π n T)

d) α _{S,T} – α _{S,A} = 3 sin (2 π n T)

### View Answer

_{S,T}= t

_{ S,T}– t

_{M}.

3. The temperature variation of a thick brick wall during periodic heating or cooling follows a sinusoidal waveform. During a period of 24 hours, the surface temperature ranges from 25 degree Celsius to 75 degree Celsius. Workout the time lag of the temperature wave corresponding to a point located at 25 cm from the wall surface. Thermo-physical properties of the wall material are; thermal conductivity = 0.62 W/m K; specific heat = 450 J/kg K and density = 1620 kg/m^{3}

a) 3.980 hour

b) 6.245 hour

c) 2.648 hour

d) 3.850 hour

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^{ ½}where x = 0.25 m and n = frequency.

4. A single cylinder 2-stroke engine operates at 1500 rpm. Calculate the depth where the temperature wave due to variation in cylinder is damped to 1% of its surface value. For the cylinder material, thermal diffusivity = 0.042 m^{2}/hr

a) 0.1996 cm

b) 0.3887 cm

c) 0.2774 cm

d) 0.1775 cm

### View Answer

_{X,A }= α

_{S,A}exponential [-x (π n/α)

^{ ½}] where frequency = 1500 * 60.

5. The temperature distribution at a certain time instant through a 50 cm thick wall is prescribed by the relation

T = 300 – 500 x – 100 x^{2} + 140 x^{3}

Where temperature t is in degree Celsius and the distance x in meters has been measured from the hot surface. If thermal conductivity of the wall material is 20 k J/m hr degree, calculate the heat energy stored per unit area of the wall

a) 4100 k J/hr

b) 4200 k J/hr

c) 4300 k J/hr

d) 4400 k J/hr

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^{2}. Now heat storage rate = Q

_{IN }– Q

_{ OUT}= 10000 – 5900 = 4100 k J/hr.

6. A large plane wall, 40 cm thick and 8 m^{2} area, is heated from one side and temperature distribution at a certain time instant is approximately prescribed by the relation

T = 80 – 60 x +12 x^{2} + 25 x^{3} – 20 x^{4}

Where temperature t is in degree Celsius and the distance x in meters. Make calculations for heat energy stored in the wall in unit time.

For wall material:

Thermal conductivity = 6 W/m K and thermal diffusivity = 0.02 m^{2}/hr

a) 870.4 W

b) 345.6 W

c) 791.04 W

d) 238.5 W

### View Answer

_{IN }= – k A (d t/d x)

_{X = 0}= 2880 W and Q

_{OUT }= – k A (d t/d x)

_{X = 0.4}= 2088.96 W.

7. Consider the above problem, calculate rate of temperature change at 20 cm distance from the side being heated

a) 0.777 degree Celsius/hour

b) 0.888 degree Celsius/hour

c) 0.999 degree Celsius/hour

d) 0.666 degree Celsius/hour

### View Answer

^{2}t/d x

^{ 2}= 0.888 degree Celsius/hour.

8. At a certain time instant, the temperature distribution in a long cylindrical fire tube can be represented approximately by the relation

T = 650 + 800 r – 4250 r^{2}

Where temperature t is in degree Celsius and radius r is in meter. Find the rate of heat flow such that the tube measures: inside radius 25 cm, outside radius 40 cm and length 1.5 m.

For the tube material

K = 5.5 W/m K

α = 0.004 m^{2}/hr

a) 3.672 * 10 ^{8} W

b) 3.672 * 10 ^{2} W

c) 3.672 * 10 ^{5} W

d) – 3.672 * 10 ^{5} W

### View Answer

_{IN }– Q

_{ OUT}= – 3.672 * 10

^{5}W.

9. Consider he above problem, find the rate of change of temperature at the inside surface of the tube

a) – 35 degree Celsius/hour

b) – 45 degree Celsius/hour

c) – 55 degree Celsius/hour

d) – 65 degree Celsius/hour

### View Answer

^{2}t/d r

^{2}+ d t/r d r] = – 55 degree Celsius/hour.

10. Time lag is given by the formula

a) x/2 [1/ (α π n) ^{½}].

b) x/3 [1/ (α π n) ^{½}].

c) x/4 [1/ (α π n) ^{½}].

d) x/5 [1/ (α π n) ^{½}].

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## Set 2

1. The energy emitted by a black surface should not vary in accordance with

a) Wavelength

b) Temperature

c) Surface characteristics

d) Time

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2. In the given diagram let r be the length of the line of propagation between the radiating and the incident surfaces. What is the value of solid angle W?

a) A sin α

b) A cos α

c) 2A cos α

d) 2A cos α

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_{n}/r

^{2}.

3. Likewise the amount of emitted radiation is strongly influenced by the wavelength even if temperature of the body is

a) Constant

b) Increasing

c) Decreasing

d) It is not related with temperature

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4. A small body has a total emissive power of 4.5 kW/m^{2}. Determine the wavelength of emission maximum

a) 8.46 micron m

b) 7.46 micron m

c) 6.46 micron m

d) 5.46 micron m

### View Answer

_{max }t = 2.8908 * 10

^{-3}.

5. The sun emits maximum radiation of 0.52 micron meter. Assuming the sun to be a black body, Calculate the emissive ability of the sun’s surface at that temperature

a) 3.47 * 10 ^{7 }W/m^{2}

b) 4.47 * 10 ^{7 }W/m^{2}

c) 5.47 * 10 ^{7 }W/m^{2}

d) 6.47 * 10 ^{7 }W/m^{2}

### View Answer

_{b }t

^{4 }= 5.47 * 10

^{7 }W/m

^{2}.

6. The law governing the distribution of radiant energy over wavelength for a black body at fixed temperature is referred to as

a) Kirchhoff’s law

b) Planck’s law

c) Wein’s formula

d) Lambert’s law

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7. The Planck’s constant h has the dimensions equal to

a) M L ^{2 }T ^{-1}

b) M L T ^{-1}

c) M L T ^{-2}

d) M L T

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^{-34}.

8. Planck’s law is given by

a) (E) _{b }= 2 π c^{ 2 }h (Wavelength)^{ -5}/[c h/k (Wavelength) T] – 2

b) (E) _{b }= π c^{ 2 }h [exponential [c h/k (Wavelength) T] – 3].

c) (E) _{b }= 2 π c^{ 2 }h (Wavelength)^{ -5}/exponential [c h/k (Wavelength) T] – 1

d) (E) _{b }= 2 c^{ 2 }h (Wavelength)^{ -5}/exponential [c h/k (Wavelength) T] – 6

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9. A furnace emits radiation at 2000 K. Treating it as a black body radiation, calculate the monochromatic radiant flux density at 1 micron m wavelength

a) 5.81 * 10^{ 7 }W/m^{2}

b) 4.81 * 10^{ 7 }W/m^{2}

c) 3.81 * 10^{ 7 }W/m^{2}

d) 2.81 * 10^{ 7 }W/m^{2}

### View Answer

_{b }= C

_{1}(Wavelength)

^{-5}/exponential [C

_{2}/ (Wavelength) T] – 1.

10. A metal sphere of surface area 0.0225 m^{2} is in an evacuated enclosure whose walls are held at a very low temperature. Electric current is passed through resistors imbedded in the sphere causing electrical energy to be dissipated at the rate of 75 W. If the sphere surfaces temperature is measured to be 560 K, while in steady state, calculate emissivity of the sphere surface

a) 0.498

b) 0.598

c) 0.698

d) 0.798

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## Set 3

1. A small thermo-couple is positioned in a thermal boundary layer near a flat plate past which water flows at 30 degree Celsius and 0.15 m/s. The plate is heated to a surface temperature of 50 degree Celsius and at the location of the probe, the thickness is 15 mm. The probe is well-represented by

t – t _{S}/t _{INFINITY }– t _{S} = 1.5 (y/δ) – 0.5 (y/δ) ^{3}

Determine the heat transfer coefficient

a) 33.3 W/m^{2 }K

b) 43.3 W/m^{2 }K

c) 53.3 W/m^{2 }K

d) 63.3 W/m^{2 }K

### View Answer

_{INFINITY }– t

_{S}) = 63.3 W/m

^{2 }K.

2. Air at 25 degree Celsius approaches a 0.9 m long and 0.6 m wide flat plate with a velocity 4.5 m/s. Let the plate is heated to a surface temperature of 135 degree Celsius. Find local heat transfer coefficient from the leading edge at a distance of 0.5 m

a) 5.83 W/m^{2} K

b) 6. 83 W/m^{2} K

c) 7. 83 W/m^{2} K

d) 8. 83 W/m^{2} K

### View Answer

^{2}K.

3. Consider the above problem, find the total rate of heat transfer from the plate to the air

a) 316.78 W

b) 416.78 W

c) 516.78 W

d) 616.78 W

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4. A small thermo-couple is positioned in a thermal boundary layer near a flat plate past which water flows at 30 degree Celsius and 0.15 m/s. The plate is heated to a surface temperature of 50 degree Celsius and at the location of the probe, the thickness of thermal boundary layer is 15 mm. If the temperature profile as measured by the probe is well-represented by

t – t _{S}/t _{INFINITY }– t _{S} = 1.5 (y/δ _{t}) – 0.5 (y/δ _{t}) ^{3}

Determine the heat flux from plate to water

a) 266 W/m^{2}

b) 1266 W/m^{2}

c) 2266 W/m^{2}

d) 3266 W/m^{2}

### View Answer

_{INFINITY }– t

_{S}) d/d y [t – t

_{S}/t

_{INFINITY }– t

_{S}]

_{ Y = 0}. So, heat flux = 1266 W/m

^{2}.

5. Atmospheric air at 30 degree Celsius temperature and free stream velocity of 2.5 m/s flows along the length of a flat plate maintained at a uniform surface temperature of 90 degree Celsius. Let length = 100 cm, width = 50 cm and thickness = 2.5 cm. Thermal conductivity of the plate material is 25 W/m K, find heat lost by the plate

a) 155.88 W

b) 165.88 W

c) 175.88 W

d) 185.88 W

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6. Consider the above problem, find the temperature of bottom surface of the plate for steady state condition

a) 90.372 degree Celsius

b) 80.372 degree Celsius

c) 70.372 degree Celsius

d) 60.372 degree Celsius

### View Answer

_{S }– t

_{B})/δ.

7. Ambient air at 20 degree Celsius flows past a flat plate with a sharp leading edge at 3 m/s. The plate is heated uniformly throughout its entire length and is maintained at a surface temperature of 40 degree Celsius. Calculate the distance from the leading edge at which the flow in the boundary layer changes from laminar to turbulent conditions. Assume that transition occurs at a critical Reynolds number of 500000

a) 4.67 m

b) 3.67 m

c) 2.67 m

d) 1.67 m

### View Answer

_{ INFINITY}/v.

8. Ambient air at 20 degree Celsius flows past a flat plate with a sharp leading edge at 3 m/s. The plate is heated uniformly throughout its entire length and is maintained at a surface temperature of 40 degree Celsius. Calculate the thickness of the hydrodynamic boundary layer. Assume that transition occurs at a critical Reynolds number of 500000

a) 16.5 mm

b) 17.5 mm

c) 18.5 mm

d) 19.5 mm

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^{½}.

9. Ambient air at 20 degree Celsius flows past a flat plate with a sharp leading edge at 3 m/s. The plate is heated uniformly throughout its entire length and is maintained at a surface temperature of 40 degree Celsius. Calculate the thickness of the thermal boundary layer. Assume that transition occurs at a critical Reynolds number of 500000

a) 19.23 mm

b) 18.23 mm

c) 17.23 mm

d) 16.23 mm

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^{1/3}.

10. Ambient air at 20 degree Celsius flows past a flat plate with a sharp leading edge at 3 m/s. The plate is heated uniformly throughout its entire length and is maintained at a surface temperature of 40 degree Celsius. Calculate the local convective heat transfer coefficient. Assume that transition occurs at a critical Reynolds number of 500000

a) 4.519 k J/m^{2} hr degree

b) 5.519 k J/m^{2} hr degree

c) 6.519 k J/m^{2} hr degree

d) 7.519 k J/m^{2} hr degree

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## Set 4

1. A radiation shield should

a) Have high transmissivity

b) Absorb all the radiations

c) Have high reflexive power

d) Partly absorb and partly transmit the incident radiation

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2. Radiation shield are used between the emitting surfaces such that

a) To reduce overall heat transfer

b) To increase overall heat transfer

c) To increase density

d) To reduce density

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3. Which of the following can be used as a radiating shield?

a) Carbon

b) Thin sheets of aluminum

c) Iron

d) Gold

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4. Two large parallel planes with emissivity 0.4 are maintained at different temperatures and exchange heat only by radiation. What percentage change in net radiative heat transfer would occur if two equally large radiation shields with surface emissivity 0.04 are introduced in parallel to the plates?

a) 65.1%

b) 75.1%

c) 85.1%

d) 95.1%

### View Answer

_{12}= (F

_{g})

_{ 12}A

_{ 1 }σ

_{ b}(T

_{1}

^{4}– T

_{ 2}

^{4}) = 0.2 A

_{ 1 }σ

_{ b}(T

_{1}

^{4}– T

_{ 2}

^{4}) and when shields are used Q

_{12}= 0.0098 A

_{ 1 }σ

_{ b}(T

_{1}

^{4}– T

_{ 2}

^{4}).

5. Determine the net radiant heat exchange per m^{ 2} area for two infinite parallel plates held at temperature of 800 K and 500 K. Take emissivity as 0.6 for the hot plate and 0.4 for the cold plate

a) 6200 W/m^{2}

b) 7200 W/m^{2}

c) 8200 W/m^{2}

d) 9200 W/m^{2}

### View Answer

_{12}= (F

_{g})

_{ 12}A

_{ 1 }σ

_{ b}(T

_{1}

^{4}– T

_{ 2}

^{4}) and (F

_{g})

_{ 12}= 0.135. Therefore, Q

_{12}= 6200 W/m

^{2}.

6. Consider the above problem, what should be the emissivity of a polished aluminum shield placed between them if heat flow is to be reduced to 40 percent of its original value?

a) 0.337

b) 0.347

c) 0.357

d) 0.367

### View Answer

_{g})

_{ 12}= 1/E

_{1}+1/E

_{ 2}+2/E

_{3}– 2 = 7.936.

7. Consider radiative heat transfer between two large parallel planes of surface emissivities 0.8. How many thin radiation shields of emissivity 0.05 be placed between the surfaces is to reduce the radiation heat transfer by a factor of 75?

a) 1

b) 2

c) 3

d) 4

### View Answer

_{12})

_{ONE SHIELD }= A

_{ }σ

_{ b}(T

_{1}

^{4}– T

_{ 2}

^{4})/ 1/E

_{1}+1/E

_{ 2}+2/E

_{3}– 2 and 75 = (Q

_{12})

_{NO SHIELD }/ (Q

_{12})

_{N SHIELD}.

8. Two parallel square plates, each 4 m^{2} area, are large compared to a gap of 5 mm separating them. One plate has a temperature of 800 K and surface emissivity of 0.6, while the other has a temperature of 300 K and surface emissivity of 0.9. Find the net energy exchange by radiations between the plates

a) 61.176 k W

b) 51.176 k W

c) 41.176 k W

d) 31.176 k W

### View Answer

_{12}= (F

_{g})

_{ 12}A

_{ 1 }σ

_{ b}(T

_{1}

^{4}– T

_{ 2}

^{4}).

9. The furnace of a boiler is laid from fire clay brick with outside lagging from the plate steel, the distance between the two is quite small compared with the size of the furnace. The brick setting is at an average temperature of 365 K whilst the steel lagging is at 290 K. Calculate the radiant heat flux. Assume the following emissivity values

For brick = 0.85

For steel = 0.65

a) 352.9 W/m^{2}

b) 452.9 W/m^{2}

c) 552.9 W/m^{2}

d) 652.9 W/m^{2}

### View Answer

_{12}= (F

_{g})

_{ 12}A

_{ 1 }σ

_{ b}(T

_{1}

^{4}– T

_{ 2}

^{4}).

10. Consider the above problem, find the reduction in heat loss if a steel screen having an emissivity value of 0.6 on both sides is placed between the brick and steel setting

a) 5.56

b) 4.46

c) 3.36

d) 2.36

### View Answer

_{g})

_{ 12}= 0.247 and Q = 149.51 W/m

^{2}.

## Set 5

1. What is the dimension of coefficient of volumetric expansion?

a) α

b) α ^{1}

c) α ^{-2}

d) α ^{-1 }

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2. What is the dimension of convective film coefficient?

a) M T ^{-3} α ^{-1}

b) M T ^{-3} α ^{-2}

c) M T ^{-2} α ^{-1}

d) M T ^{-1} α ^{-1}

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^{2}hr degree. It is used in thermodynamics to calculate the heat and it is denoted by h.

3. What is the dimension of velocity?

a) L T ^{-2}

b) L T ^{1}

c) L T ^{-1}

d) L T

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4. What is the dimension of area?

a) M L ^{2}

b) L^{ 2}

c) L ^{1}

d) L^{ 3}

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^{2}. It is defined as a range of activity or an interest.

5. What is the dimension of volume?

a) L ^{2}

b) M L ^{3}

c) L

d) L ^{3}

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^{3}. It is defined as a region or part of a town, a country or the world.

6. What is the dimension of work?

a) M L ^{2} T^{ -2}

b) M L ^{2} T^{ -1}

c) M L ^{2} T^{ }

d) M L ^{1} T^{ -2}

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7. What is the dimension of temperature?

a) α ^{-2}

b) α ^{2}

c) α

d) M α

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8. What is the dimension of mass?

a) M

b) M L

c) L

d) M L T

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9. What is the dimension of length?

a) T

b) M

c) L

d) M L

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10. What is the dimension of time?

a) M T

b) T

c) M

d) M L