Q.1 Why is the average velocity of the molecules in a gas zero
but the average of the square of velocities is not zero?
Ans. The molecules of the gas moves in random direction. We assume that the same number of molecules move in both directions, so the average of each component velocity is zero. But the average of the squares of the velocities of the molecules include square of negative velocity and so cannot be zero.
Ans. The molecules of the gas moves in random direction. We assume that the same number of molecules move in both directions, so the average of each component velocity is zero. But the average of the squares of the velocities of the molecules include square of negative velocity and so cannot be zero.
Q.2 Why does the pressure of a gas in a car tyre increase when it is driven through some distance?
Ans. In driving, the car tyre gets hot due to force of friction. This heat goes inside the tyre and increases translational kinetic energy. So increase of KEtrans. makes pressure increase.
Q.3 A system undergoes from state P1 V1 to state P2 V2 as shown in the fig. What will be the change in internal energy?
Ans. The change in internal energy (∆U) will be zero. In the figure the graph is isotherm. It means temperature remain constant. So ∆U = 0
Q.4 Variation of volume by pressure is given in the fig. A gas is taken along the paths ABCDA, ABCA and A to A. What will be the change in internal energy?
Ans. In the figure, all three paths returns to the initial state, so there is no change in internal energy.
Q.5 Specific heat of a gas at constant pressure is greater than specific heat at constant volume. Why?
Ans. Specific heat at constant pressure (Cp) is greater than Cv , because a part of heat is used in doing work as gas expands against constant pressure.
Q.6 Give an example of a process in which no heat is transferred to or
from the system but the temperature of the system changes.
Ans. In adiabatic expansion of a gas, the work is done by the system and so the temperature decreases.
Ans. In adiabatic expansion of a gas, the work is done by the system and so the temperature decreases.
Q.7 Is it possible to convert internal energy into mechanical
energy?Explain with example.
Ans. Yes. In adiabatic expansion of a gas internal energy converts into mechanical energy or work.
Q.8 Is it possible to construct a heat engine that will not expel heat into the atmosphere?
Ans. No. It is not possible. Because according to 2nd law of thermodynamics it is not possible to construct an engine without a sink or cold body to reject a part of heat to it, the atmosphere (or cold body).
Ans. Yes. In adiabatic expansion of a gas internal energy converts into mechanical energy or work.
Q.8 Is it possible to construct a heat engine that will not expel heat into the atmosphere?
Ans. No. It is not possible. Because according to 2nd law of thermodynamics it is not possible to construct an engine without a sink or cold body to reject a part of heat to it, the atmosphere (or cold body).
Q.9 A thermos flask containing milk as a system is shaken rapidly. Does
the temperature of milk rise?
Ans. Yes. As K.E. of the molecules increases due to rapid shaking, so the temperature of the milk rises.
Q.10 What happens to the temperature of the room, when a air conditioner is left running on a table in the middle of the room?
Ans. The temperature of the room will not reduce, because heat absorbs from the room is expelled in the same room. Rather the temperature will rise due to work done by the compressor will change into heat.
Ans. Yes. As K.E. of the molecules increases due to rapid shaking, so the temperature of the milk rises.
Q.10 What happens to the temperature of the room, when a air conditioner is left running on a table in the middle of the room?
Ans. The temperature of the room will not reduce, because heat absorbs from the room is expelled in the same room. Rather the temperature will rise due to work done by the compressor will change into heat.
Q.11 Can the mechanical energy be converted completely into heat energy?
If so give an example.
Ans. Yes. When brakes of a speeding car applied, due to frictional force, all its mechanical energy (kinetic energy) is converted into heat. ii) In a adiabatic compression, work done on the gas, increased the internal energy, i.e. converting mechanical energy (work) into heat energy ( ∆U).
Ans. Yes. When brakes of a speeding car applied, due to frictional force, all its mechanical energy (kinetic energy) is converted into heat. ii) In a adiabatic compression, work done on the gas, increased the internal energy, i.e. converting mechanical energy (work) into heat energy ( ∆U).
Q.12 Does entropy of a system increases or decreases due to friction?
Ans. The entropy of the system increases, due to friction. As work done against friction changes into heat and this irreversible process increases its entropy.
Q.13 Give an example of a natural process that involves an increase in entropy.
Ans. i) Melting of ice into water: The heat Q transferred to the ice at absolute temperature from the surroundings. ∆S = Q / T Since heat is added, Q is +ve and entropy increases. ii) Free expansion: In a free expansion of a gas in a chamber, which is irreversible process. Here the gas molecules confined to one half of a box are permitted to fill the entire box, which is irreversible process.
Q.14 An adiabatic change is the one in which
a. No heat is added to or taken out of a system
b. No change of temperature takes place
c. Boyle’s law is applicable d. Pressure and volume remains constant
Ans. Correct answer is (a) No heat is added to or taken out of a system in the adiabatic change.
a. No heat is added to or taken out of a system
b. No change of temperature takes place
c. Boyle’s law is applicable d. Pressure and volume remains constant
Ans. Correct answer is (a) No heat is added to or taken out of a system in the adiabatic change.
Q.15 Which one of the following process is irreversible?
a. Slow compressions of an elastic spring
b. Slow evaporation of a substance in an isolated vessel
c. Slow compression of a gas d. A chemical explosion
Ans. Correct answer is (d) a chemical explosion is irreversible process.
Q.16 An ideal reversible heat engine has
a. 100 % efficiency b. Highest efficiency
c. An efficiency, which depends on the nature of working substance
d. None of these.
Ans. Correct answer is (b), an ideal reversible heat engine has highest
efficiency. From the knowledge of 2nd law of TD, a heat engine
cannot have 100 % efficiency and is independent of the working substance.
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