Kinetic Theory of Gases
Questions for Short Answer
Questions for Short Answer
1. When we place a gas cylinder on a van and the van moves, does the kinetic energy of the molecules increase?
Answer: If we consider the mechanical kinetic energy, then the kinetic energy of the gas increases because the gas has a mass. But the kinetic energy of molecules which is the measure of the internal energy does not increase.
2. While the gas from a cooking gas cylinder is used, the pressure does not fall appreciably until the last few minutes. Why?
Answer: The cooking gas is filled at very high pressure in the cylinder. At this pressure, the gas liquefies. On the liquified surface of the gas, there is a saturated vapor of the gas. As soon as some gas is used in cooking, the saturation balance is disturbed and some of the liquid converts to gas to maintain the balance. Thus the pressure of the gas does not appreciably drop until there is liquid gas in the cylinder.
3. Do you expect the gas in a cooking gas cylinder to obey the ideal gas equation?
Answer: No. The density of the gas is very high here, so it will not obey the ideal gas equation.
4. Can we define the temperature of a vacuum? The temperature of a single molecule?
Answer: No. The temperature of a vacuum can not be defined, because the temperature of a gas is proportional to the square of the rms speed of its molecules and hence to the total translational kinetic energy of its molecules. Since no molecules in the vacuum hence no temperature defined.
The temperature of a single molecule can not be defined but for a bunch of molecules.
5. Comment on the following statement. The temperature of all molecules in a sample of a gas is the same.
Answer: The temperature is defined for a given mass of the substance and it is proportional to the total translational kinetic energy of its molecules. The temperature of each molecule is not defined.
6. Consider a gas of neutrons. Do you expect it to behave much better as an ideal gas as compared to hydrogen gas at the same temperature and pressure?
Answer: Though it is hardly possible to have a gas of neutrons and very difficult to find a container that can hold it if we assume it to be possible then the answer will be yes. Because in the assumptions of the ideal gas the size of a molecule is much smaller than the average separation between them, neutrons have much smaller size than a hydrogen molecule. Also, neutrons do not exert electrostatic force on each other.
7. A gas is kept in a rigid cubical container. If a load of 10 kg is put on the top of the container, does the pressure increase?
Answer: No. Since the cubical container is rigid, the load will not be transferred to the gas and the pressure will not increase.
8. If it were possible for a gas in a container to reach the temperature of 0 K, its pressure would be zero. Would the molecules not collide with the walls? Would they not transfer momentum to the walls?
Answer: At the temperature of 0 K, the kinetic energy of the molecules will be zero and hence they will not have velocities and momentum. So the molecules will not collide with the walls and not transfer momentum to the walls.
9. It is said that the assumptions of the kinetic theory are good for gases having low densities. Suppose a container is so evacuated that only one molecule is left in it. Which of the assumptions of kinetic theory will not be valid for such a situation? Can we assign a temperature to this gas?
Answer: The assumption "when gas is left for sufficient time, it comes to a steady-state. The density and distribution of molecules with different velocities are independent of position, direction and time" may be justified if the number of molecules is very large. In the given condition this assumption will not be valid. This gas can not be assigned a temperature.
10. A gas is kept in an enclosure. The pressure of the gas is reduced by pumping out some gas. Will the temperature of the gas decrease by Charles' law?
Answer: The condition in Charles' law is that volume and the mass of the gas should be constant. In the given condition the mass of the gas in the enclosure is not constant. Hence the Charles' law can not be applied.
11. Explain why cooking is faster in a pressure cooker.
Answer: In the open cooking the water-salt mixture boils just above 100°C but in a pressure cooker the pressure is about 4 times higher than atmospheric pressure and due to this the water boils around 140°C. Since the cooking in the pressure cooker is at higher temperature, the cooking is faster.
12. If the molecules were not allowed to collide among themselves, would you expect more evaporation or less evaporation?
Answer: The molecules move randomly and they can only be prevented from colliding among themselves if they have zero velocities. In this case, the evaporation will be less.
13. Is it possible to boil water at room temperature, say 30°C? If we touch a flask containing water boiling at this temperature, will it be hot?
Answer: A liquid boils when the saturated vapor pressure above its surface equals the external pressure. By reducing the external pressure over the water surface, it can be made to boil even at 30°C.
The hotness is related to the temperature not with the boiling of water. So it will not be hot when touched at 30°C.
14. When you come out of a river after a dip, you feel cold. Explain.
Answer: After a dip in the river the whole body is wet and the evaporation from the wet body surface starts. In the evaporation the molecules having higher kinetic energy escapes. It leaves the average kinetic energy of the left behind molecules reduced and hence the temperature of the wet body surface is reduced. That is why the feeling of coldness comes.
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Links to the Chapters
Links to the Chapters
CHAPTER- 23 - Heat and Temperature
Questions for Short Answer
OBJECTIVE - I
OBJECTIVE - II
EXERCISES - Q-1 TO Q-10
EXERCISES - Q-11 TO Q-20
EXERCISES - Q-21 TO Q-34
CHAPTER- 21 - Speed of Light
CHAPTER- 20 - Dispersion and Spectra
CHAPTER- 19 - Optical Instruments
CHAPTER- 18 - Geometrical Optics
CHAPTER- 17 - Light Waves
CHAPTER- 16 - Sound Waves
CHAPTER- 15 - Wave Motion and Waves on a String
CHAPTER- 14 - Fluid Mechanics
CHAPTER- 13 - Fluid Mechanics
CHAPTER- 12 - Simple Harmonic Motion
CHAPTER- 11 - Gravitation
CHAPTER- 10 - Rotational Mechanics
CHAPTER- 9 - Center of Mass, Linear Momentum, Collision
OBJECTIVE - I
OBJECTIVE - II
EXERCISES - Q-1 TO Q-10
EXERCISES - Q-11 TO Q-20
EXERCISES - Q-21 TO Q-34
CHAPTER- 21 - Speed of Light
CHAPTER- 20 - Dispersion and Spectra
CHAPTER- 19 - Optical Instruments
CHAPTER- 18 - Geometrical Optics
CHAPTER- 17 - Light Waves
CHAPTER- 16 - Sound Waves
CHAPTER- 15 - Wave Motion and Waves on a String
CHAPTER- 14 - Fluid Mechanics
CHAPTER- 13 - Fluid Mechanics
CHAPTER- 12 - Simple Harmonic Motion
CHAPTER- 11 - Gravitation
CHAPTER- 10 - Rotational Mechanics
CHAPTER- 9 - Center of Mass, Linear Momentum, Collision
CHAPTER- 8 - Work and Energy
Click here for → Question for Short Answers
Click here for → OBJECTIVE-I
Click here for → OBJECTIVE-II
Click here for → Exercises (1-10)
Click here for → Question for Short Answers
Click here for → OBJECTIVE-I
Click here for → OBJECTIVE-II
Click here for → Exercises (1-10)
Click here for → Exercises (11-20)
Click here for → Exercises (21-30)
Click here for → Exercises (31-42)
Click here for → Exercise(43-54)
Click here for → Exercises (21-30)
Click here for → Exercises (31-42)
Click here for → Exercise(43-54)
CHAPTER- 7 - Circular Motion
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Click here for → OBJECTIVE-I
Click here for → OBJECTIVE-II
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Click here for → EXERCISES (11-20)
Click here for → EXERCISES (21-30)
CHAPTER- 6 - Friction
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Click here for → OBJECTIVE-I
Click here for → OBJECTIVE-II
Click here for → EXERCISES (1-10)
Click here for → EXERCISES (11-20)
Click here for → EXERCISES (21-30)
CHAPTER- 6 - Friction
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Click here for → Questions for Short Answer
Click here for → OBJECTIVE-I
Click here for → Friction - OBJECTIVE-II
Click here for → EXERCISES (1-10)
Click here for → Exercises (11-20)
Click here for → EXERCISES (21-31)
Click here for → OBJECTIVE-I
Click here for → Friction - OBJECTIVE-II
Click here for → EXERCISES (1-10)
Click here for → Exercises (11-20)
Click here for → EXERCISES (21-31)
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CHAPTER- 5 - Newton's Laws of Motion
Click here for → QUESTIONS FOR SHORT ANSWER
Click here for → QUESTIONS FOR SHORT ANSWER
Click here for→ Newton's laws of motion - Objective - I
Click here for → Newton's Laws of Motion - Objective -II
Click here for → Newton's Laws of Motion-Exercises(Q. No. 1 to 12)
Click here for→ Newton's laws of motion - Objective - I
Click here for → Newton's Laws of Motion - Objective -II
Click here for → Newton's Laws of Motion-Exercises(Q. No. 1 to 12)
Click here for→Newton's Laws of Motion,Exercises(Q.No. 13 to 27)
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CHAPTER- 4 - The Forces
The Forces-
"Questions for short Answers"
Click here for "The Forces" - OBJECTIVE-I
Click here for "The Forces" - OBJECTIVE-II
Click here for "The Forces" - Exercises
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CHAPTER- 3 - Kinematics - Rest and Motion
Click here for "Questions for short Answers"
Click here for "OBJECTIVE-I"
Click here for EXERCISES (Question number 1 to 10)
Click here for EXERCISES (Question number 11 to 20)
Click here for EXERCISES (Question number 21 to 30)
Click here for EXERCISES (Question number 31 to 40)
Click here for EXERCISES (Question number 41 to 52)
CHAPTER- 2 - "Physics and Mathematics"
CHAPTER- 2 - "Physics and Mathematics"
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