# Physics VSQs for Class 11 with Answers Chapter 12 Thermodynamics

Very Short Answer Type Questions [1 Mark Each]

Q.1. Is the heat supplied to a system always equal to the increase in its internal energy?

Answer Answer: From 1st law of thermodynamics, dQ = dU + dW
• If dW = 0, then dQ = dU. i.e. if heat is supplied at constant volume so that the work done is zero then the heat supplied will be equal to the increase in the internal energy of the system.
• Q.2. If a drop of water falls on a very hot iron, it takes long time to evaporate. Explain.

Answer Answer: When a drop of water falls on a very hot iron, it gets separated from the iron by a thin layer of water vapour which is a bad conductor of heat. So, heat is conducted from the iron to the water drop very slowly so that it evaporates in a long time.

Q.3. Can temperature of a body be raised without heating it? Is the converse true?

Answer Answer: Yes, in adiabatic compression. Converse is also true, e.g. when a solid melts, temperature does not change, though heat is increased.

Q.4. Can water be made to boil without heating?

Answer Answer: Yes, by reducing pressure on water, boiling point can be reduced, even at room temperature.

Q.5. Which molecules, ice at 0°C or water have greater potential energy and why?

Answer Answer: Potential energy of water molecule at 0°C is more, because heat spent in melting is used in increasing the potential energy.

Q.6. When an ideal gas is compressed at a constant temperature, will its internal energy increase or decrease?

Answer Answer: No effect since internal energy of gas depends only on temperature of gas.

Q.7. Does the internal energy of an ideal gas change in an isothermal process? In an adiabatic process?

Answer Answer: In isothermal process, T = constant, dT = 0 ⇒ dU = 0.
In an adiabatic process, Q = constant, dQ = 0
⇒ dU = – dW ≠ 0

Q.8. A sample of an ideal gas in a cylinder is compressed adiabatically to one-third of its volume. Will the final pressure be more or less than 3 times the initial pressure?

Answer Answer: Change in pressure would be more than 3 times the initial pressure.

Q.9. If on giving 40 joule of heat to a system, work done on the system is 10 joule, what will be the change in internal energy of the system?

Answer Answer: Internal energy of the system would increase by (40 + 10) = 50 joule.

Q.10. In a thermodynamical process, 300 joule of heat is supplied to a gas and 200 joule of work is done by the gas. What is the change in internal energy of the system?

Answer Answer: Internal energy of the system would increase by (300 – 200) = 100 joule.

Q.11. In summer, when the valve of a bicycle tube is removed, the escaping air appears cold, why?

Answer Answer: This occurs due to adiabatic expansion of the air of the tube of the bicycle.

Q.12. When air of the atmosphere rise up, it cools why?

Answer Answer: When air rises up, it expands due to decrease in the atmospheric pressure. Therefore, temperature falls.

Q.13. Do the internal energy of ideal gas depend upon volume ?

Answer Answer: No, for an ideal gas the internal energy depends only on temperature i.e. U = f (T).

Q.14. Explain why it is impossible to design a heat engine with 100% efficiency ?

Either T2 = 0 or T1 = ¥ both of which cannot be attained. Hence it is impossible to design heat engine with 100% efficiency.

Q.15. What thermodynamical variable is defined by (a) Zeroth law (b) First law ?

Answer (a) Temperature T (b) Internal energy U

Q.16. If an inflated tyre bursts, the air escaping out is cooled. Why ?

Q.17. An engine working under isothermal conditions can produce no useful work. Explain.

Q.18. What does second law of thermodynamics imply about the efficiency of a heat engine ?

Answer According to second law of thermodynamics, the efficiency of a heat engine cannot be equal to 1.

Q.19. Which thermodynamic variables are involved in an indicator diagram ?

Answer Indicator diagram is a P-V graph, hence pressure P and volume V are involved in it.

Q.20. Is Carnot’s cycle a cyclic process ?

Answer Yes, in carnot’s cycle the working substance regains its original state.

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