# Engineering Physics Questions and Answers – Kinetic Theory of Gases

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This set of Engineering Physics Multiple Choice Questions & Answers (MCQs) focuses on “Kinetic Theory of Gases”.

1. On which factor does the average kinetic energy of gas molecules depend?
a) Nature of the gas
b) Temperature
c) Volume
d) Mass

Explanation: The average kinetic energy of a gas molecule depends only on the absolute temperature of the gas and is directly proportional to it.

2. What is the average velocity of the molecules of an ideal gas?
a) Infinity
b) Constant
c) Unstable
d) Zero

Explanation: The average velocity of the molecules of an ideal gas is zero because the molecules possess all sorts of velocities in all possible directions so their vector sum and hence the average is zero.

3. The ration of the speed of sound in nitrogen gas to that in helium gas, at 300 K is?
a) √(2/7)
b) √(1/7)
c) √3/5
d) √6/5

Explanation: v=$$\sqrt{\frac{γRT}{M}}$$
$$\frac{v(N_2)}{v(He)} = \sqrt{\frac{γ_{N_2}}{γ_{He}} \times \frac{M_{He}}{M_{N_2}}}=\sqrt{\frac{\frac{7}{5}}{\frac{5}{3}}×\frac{4}{28}}=\frac{\sqrt{3}}{5}$$
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4. Cooking gas containers are kept in a lorry moving with uniform speed. The temperature of the gas molecules inside will ___________
a) Increase
b) Decrease
c) Remain the same
d) Decreases for some, while the increase for others

Explanation: The centre of mass of the gas molecules moves with uniform speed along with the lorry. As there is no change in relative motion, the translational kinetic energy and hence the temperature of the gas molecules will remain the same.

5. At Boyle’s temperature?
a) Joule’s effect is positive
b) Van der Waal’s equation becomes zero
c) Gases obey Boyle’s law
d) Water solidifies

Explanation: At Boyle’s temperature, a gas obeys Boyle’s law to a high degree of accuracy. Above and below this temperature, the gas deviates from Boyle’s law.

6. For Boyle’s law to hold good, the gas should be ___________
a) Perfect and of constant mass and temperature
b) Real and of constant mass and temperature
c) Perfect and at constant temperature but variable mass
d) Real and at constant temperature but variable mass

Explanation: Boyle’s law is valid for a constant mass of a perfect gas at a given temperature.

7. A gas behaves as an ideal gas at ___________
a) Low pressure and high temperature
b) Low pressure and low temperature
c) High pressure and low temperature
d) High pressure and high temperature

Explanation: A gas behaves as an ideal gas at low pressure and high temperature when the attractive forces between the molecules become negligible.

8. The temperature of gas is held constant, while its volume is decreased. The pressure exerted by the gas on the wall of the container increases, because of its molecules ___________
a) Strike the walls with higher velocities
b) Strike the walls with large farce
c) Strike the walls more frequently
d) Are in contact with the walls for a shorter time

Explanation: When the volume is decreased, the gas molecules strike the walls more frequently.

9. A bulb contains one mole of hydrogen mixed with one mole of oxygen at temperature T. The ratio of rms values of velocity of hydrogen molecules to that of oxygen molecules is?
a) 1:16
b) 1:4
c) 4:1
d) 16:1

Explanation: vrms=√(RT/m)
For constant temperature,
(vrms (H2))/(vrms (O2))=√(M(O2)/M(H2))=√(32/2)=4:1.

10. Statement: The root mean square and most probable speeds of the molecules in a gas is symmetrical.
Reason: The Maxwell distribution for the speed to molecules in a gas is symmetrical.
a) Both statement and reason are true and the reason is the correct explanation of the statement
b) Both statement and reason are true but the reason is not a correct explanation of the statement
c) Statement it true but the reason is false
d) Both statement and reason are false

Explanation: Both the statement and reason are false. The two speeds are different from each other. Also, the Maxwell distribution for the speed to molecules in a gas is symmetrical.

11. The degree of freedom of a triatomic gas is?
a) 1
b) 2
c) 6
d) 8

Explanation: A triatomic gas molecule has 3 degrees of freedom due to translator motion and 3 degrees of freedom due to rotator motion.

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