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Tuesday, 20 January 2015

F.Sc ICS Notes: Physics XI: Chapter 11 Heat and Thermodynamics Exercise Short Questions:

FSc ICS Notes: Physics XI: Chapter 11 Heat and Thermodynamics Exercise Short Questions:


Question 11.1 Why is the average velocity of the molecules in a gas zero but the average of the square of velocities is not zero?
Answer 11.1 According to the assumption of kinetic molecular theory of gasses, there is a large number of molecules in gas which are not in the state of rest. These molecules move in all directions. It means that direction with some velocity is equal to the number of molecules moving in opposite direction with the same velocity the number of molecules moving in one. So sum of their velocities will be zero but the average of the square of the velocities will not be zero i.e.,
                                                                 (-v2) = <-v2>


Question 11.2 Why does the pressure of a gas in a car Tyre increase when it is driven through some distance?
Answer 11.2 When a car is driven on road through some distance. There is a force of friction between the Tyre and the road. Due to this force of friction, the Tyre heats up and the gas inside the Tyre. Work done by the car is converted into heat which raises the temperature of the gas in the Tyre. This increases the kinetic energy of the molecules. Since pressure is directly proportional to the temperature. i..e,
                                                                  P ∝ <K.E>


Question 11.3 A system undergoes from state P1 V1 to state P2 V2 as shown in the fig. What will be the change in internal energy?
Answer 11.3 As internal energy depends on temperature. In this case the temperature is constant. So internal energy is also constant. Hence there will be no change in internal energy.


Question 11.4 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?
Answer 11.4 There will be no change in the internal energy in all three cases because the system returns to its initial state.


Question 11.5 Specific heat of a gas at constant pressure is greater than specific heat at constant volume. Why?
Answer 11.5 When a gas is heated at constant volume, heat supplied only increase the internal energy i.e., temperature because the piston is fixed so no external work is done to expand the gas i.e., W = 0, the total heat supplied is used to increase the internal energy and temperature of the gas. But when a gas is heated at constant pressure, the heat supplied is used in two ways i.e., some heat is used to do external work to expand the gas and the remaining heat is used to increase the internal energy.
Thus more heat is required at constant pressure than at constant volume. So the specific heat at constant pressure is greater than specific heat at constant volume.
                                             Cp > Cv



Question 11.6 Give an example of a process in which no heat is transferred to or from the system but the temperature of the system changes.
Answer 11.6 In adiabatic expansion or compression, no heat is transferred to or from the system but the temperature of the system changes. During adiabatic expansion temperature of the system falls while during adiabatic compression temperature of the system rises.


Question 11.7 Is it possible to convert internal energy into mechanical energy?Explain with example.
Answer 11.7 Yes, in adiabatic expansion, internal energy is changed into mechanical energy.
According to 1st law of thermodynamics.
                                                 Q = ΔU + W
In adiabatic process Q = 0
Then 0 = ΔU + W
                                               W = -ΔU
Thus the internal energy decreases because some internal energy is converted into mechanical energy.
Exp: In petrol engine, hot gases expand and the piston moves so internal energy is converted into work.


Question 11.8 Is it possible to construct a heat engine that will not expel heat into the atmosphere?
Answer 11.8  No it is impossible to construct a heat engine that will not expel heat into the atmosphere. According to second law of thermodynamics. "No heat engine operating continuously in a cycle can convert all heat supplied into work"


Question 11.9 A thermos flask containing milk as a system is shaken rapidly. Does the temperature of milk rise?
Answer 11.9 When the milk is shaken rapidly, kinetic energy of the milk molecules increases which causes the increase in temperature and the internal energy of the molecules of milk. When we are shaking, some work is done on it which converts into K.E of the molecules of milk so the temperature of milk increases.


11.10 What happens to the temperature of the room, when a air conditioner is left running on a table in the middle of the room?
Answer 11.10 Temperature of the room increases, as heat absorbed from the room is expelled in the same room. Also work done by the compressor is changed into heat which is expelled in the same room.


Question 11.11 Can the mechanical energy be converted completely into heat energy? If so give an example.
Answer 11.11 Yes, mechanical energy can be converted into heat energy. When work is done in compressing the gas by adiabatic process, the increase in internal energy of the gas is equal to the work done according to 1st law of thermodynamics. Fox exp: If we rub our hands, the whole mechanical energy is converted into heat energy.


Question 11.12 Does entropy of a system increases or decreases due to friction?
Answer 11.12 Entropy of a system increases,as work done due to friction is changed into heat and this heat goes into surrounding and becomes useless. According to law of increase of entropy, entropy increases for irreversible process.


Question 11.13 Give an example of a natural process that involves an increase in entropy.
Answer 11.13  When ice is melted due to high temperature of surroundings. The heat transferred to ice from surroundings is positive. Since ΔS = ΔQ / T. As ΔS is positive thus the entropy of this natural process increases.


Question 11.14 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
Answer 11.14  (a) is correct because in an adiabatic process, no heat enters or leaves the system.


Question 11.15 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
Answer 11.15 (d) is correct because a chemical explosion cannot be reversed. It is a irreversible process.


Question 11.16 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.
Answer 11.16 (b) is correct because according to 2nd law of thermodynamics, the efficiency of an ideal heat engine cannot be 100%. It has highest efficiency.

Written By: Asad Hussain.

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