# Engineering Physics Questions and Answers – Absorption and Emission

This set of Engineering Physics Multiple Choice Questions & Answers (MCQs) focuses on “Absorption and Emission”.

1. At which temperature, the centigrade and Fahrenheit scales are equally?
a) 40°
b) -40°
c) 37°
d) -80°

Explanation: C/5=(F-32)/9
C/5=(C-32)/9
4C=-160
C=F=-40°.

2. The volume of a gas at 20°C is 100cm3 at normal pressure. If it is heated to 100°C, its volume becomes 125cm3 at the same pressure, then volume coefficients of the gas (at normal pressure) is?
a) 0.0033/°C
b) 0.0025/°C
c) 0.0030/°C
d) 0.0021/°C

Explanation: Vt=V0 (1+γt)
V100/V20 =(1+100γ)/(1+20γ )=125/100=3/4
γ=1/300=0.0033/°C.

3. Density of substance at 0°C is g/cc and 100°C, its density is 9.7g/cc. The coefficient of linear expansion of the substance is?
a) 10-1
b) 10-2
c) 10-3
d) 10-4

Explanation: γ=(ρ0t)/(ρ0×t)=(10-9.7)/(10×100)=3×10-4
α=γ/3=10-4°C-1.

4. The coefficient of volumetric expansion of mercury is 18×10-5/°C. The thermometer bulb has a volume of 10-6 m3 and cross-section of the stem is 0.002cm2. Assuming that bulb is filled with mercury at 0°C, the increase in length of the mercury column at 100°C will be ___________
a) 9cm
b) 9mm
c) 18cm
d) 18mm

Explanation: Let h be the length of mercury column at 100°C. Then
Expansion in mercury = Volume of stem
γV∆T=Ah
h=γV∆T/A=(18×10-5×10-6×100)/(0.002×10-4) m
h=9×10-2 m
h=9cm.

5. Amount of heat required to raise the temperature of a body through 1K is called ___________
a) Specific heat
b) Thermal capacity
c) Water equivalent
d) Entropy

Explanation: The amount of heat required to raise the temperature of a body through 1K is called its thermal capacity.
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6. If the temperature of a rod is increased by 10°C, its length increases by 1%. What is the percentage change in volume of the body of the same material for 10°C increase in temperature?
a) 9%
b) 1%
c) 5%
d) 3%

Explanation: ∆I/I×100=100α∆T
∆V/V×100=100γ∆T=100×3α×∆T
Hence for the same rise of temperature, an increase in volume is 3 times the increase in length.

7. In order that the heat flows from one part of a solid to another part, what is required?
a) Uniform density
d) Uniform temperature

Explanation: As heat flows from higher temperature to lower temperature, so a temperature gradient is required.

8. A body A of mass 0.5kg and specific heat 0.85 is at a temperature of 60°C. Another body B of mass 0.3kg and specific heat 0.9 is at a temperature of 90°C. When they are connected to a conducting rod, heat will flow from ___________
a) A to B
b) Heat can’t flow
c) B to A
d) First A to B and then B to A

Explanation: Heat always flows from a body at higher temperature to a body at a lower temperature.

9. Two rods having thermal conductivity in the ratio of 5:3 having equal lengths and equal cross-sectional area are joined end to end. If the temperature of the free end of the first rod is 100°C and free end of the second rod is 20°C, then the temperature of the junction is?
a) 50°C
b) 70°C
c) 60°C
d) 90°C

Explanation: Let T be the temperature of the junction. In the steady state,
Q1/t=Q2/t
(K1 A(100-T))/d=(K2 A(T-T2))/d
5(100-T)=3(T-20)
8T=560 or T=70°C.

10. Mud houses are cooler in summer and warmer in winter because ___________
a) Mud is a good conductor of heat
b) Mud is a superconductor of heat
c) Mud is a bad conductor of heat
d) Mud can adapt according to temperature

Explanation: Mud is an ad conductor of heat. It does not allow heat to come in from outside in summer and it does not allow heat to go out from a house in winter.

11. A perfectly black body is one whose ___________
a) Absorptive power is 1
b) Absorptive power is 0
c) Emissive power is 1
d) Absorptive power is 0.5

Explanation: A black body is one which absorbs the entire radiation incident upon it. Its absorptive power is unity.

12. A black body radiates energy at the rate of 1×105 J/(sm)2 at a temperature of 227°C. The temperature to which it must be heated so that it radiates energy at the rate of 1×109 J/(sm)2.
a) 5000 K
b) 5000°C
c) 500 K
d) 500°C

Explanation: From Stefan’s law,
T2=(E2/E1)(1/4) T1=10×500=5000K.

13. A black body at 200 K is found to emit maximum energy at a wavelength 14μm. When its temperature is raised to 1000K, then wavelength at which maximum energy emitted it?
a) 14mm
b) 7μm
c) 2.8μm
d) 28mm

Explanation: By Wien’s law,
200×14=100×T
T=2.8μm.

14. The triple point of water is ___________
a) 273.16°C
b) 273.16K
c) 273.16°F
d) 0.15K

Explanation: Triple point of water corresponds to a temperature of 273.16K and a pressure of 0.46cm of Hg.

15. During constant temperature, we feel colder on the day when the humidity is ___________
a) 85%
b) 40%
c) 60%
d) 25%

Explanation: If the relative humidity is low, we feel colder because of rapid evaporation of sweat from our body.

Sanfoundry Global Education & Learning Series – Engineering Physics.

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