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 a gas are always in a random motion. The number of molecules moving in a certain direction are equal to the number of molecules moving in opposite direction with same velocity. Thus their average velocity is zero. But the average of square of velocities is not zero because square of negative values becomes positive.”
Q # 2. Why does the pressure of gas in a car tyre increase when it is driven through some distance?
Ans. When a car is driven through some distance the pressure of gas in the tyre increases .This is due to friction between road and tyre. As a result the velocity of the gas molecules increases and they start colliding with the walls of container with higher velocities. Thus pressure of the tyre increases.
Q # 3. Specific heat of a gas at constant pressure is greater than specific heat at constant volume. Why?
Ans.When a gas is heated at constant pressure, some part of the heat is used in moving the piston and the remaining heat is used to increase the internal energy of the gas.
Whereas,
when the gas is heated at constant volume, whole of the heat is used to increase
the internal energy of the gas.
Hence
for equal rise of temperature for both the cases, more heat has to be supplied
for constant pressure than for constant volume.
Q # 4. 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.An
adiabatic process is the one in which no heat enters or leaves the system but
the temperature of the system changes.
For
example, in adiabatic expansion, the temperature of the system falls at the
cost of its own internal energy. Whereas, in adiabatic compression, the
temperature of the system increases without
the transfer of heat to the system.
Q # 5. Is it possible to convert internal energy into mechanical energy? Explain with an example.
Ans.Yes
it is possible to convert internal energy into mechanical energy. In adiabatic
expansion, the system expands and moves the piston upward at the cost of its
own internal energy.
Q # 6. Is it possible to construct a heat engine that will not expel heat into the atmosphere?
Ans. No
it is not possible. According to Kelvin’s statement of 2nd law of
thermodynamics, it is impossible to construct a heat engine without a sink. It
means that the heat engine must expel the remaining heat into the sink or
atmosphere.
Q # 7. A thermos flask containing milk as a system is shaken rapidly. Does the temperature of milk rise?
Ans.When
milk is shaken rapidly in a thermos flask, the K.E. of the molecules increases.
As a result, its temperature rises a little.
Q # 8. What happens to the temperature of the room, when an air conditioner is left running on the table in the middle of the room?
Ans. As
the air conditioner is running on a table n the middle of the room, it absorbs
and rejects the heat in the same room. So there will be no change in the
temperature of the room.
Q # 9. Can the mechanical energy be converted completely into heat energy? If so, give an example.
Ans.Yes
mechanical energy can be converted into heat energy. In adiabatic compression,
when the piston of the cylinder is pushed downwards, the temperature of the gas
increases.
Q # 10. Does entropy of a system increases or decreases due to friction?
Ans.Since heat increases due to friction, so according to the relation , $ \Delta \mathbf{S}=\frac{\Delta \mathbf{Q}}{T}$ , entropy also increases, as it varies directly with the heat.
Q # 11. Give an example of a natural process that involves an increase in entropy.
Ans.When ice melts by taking heat from surroundings, then according to the relation $ \Delta \mathbf{S}=\frac{\Delta \mathbf{Q}}{T}$, entropy also increases, as it varies directly with the heat.
Q # 12. An adiabatic change is one
in which:
a)
No heat is
added to or taken out of a system
Ans. (a)
No heat is added to or taken out of a system
Q # 13. Which one of the following
process is irreversible?
a)
Slow
compression of an elastic spring
Ans. (d)
a chemical explosion
Q # 14. An ideal reversible heat
engine has
a)
100% efficiency
Ans. (b)
Highest efficiency
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