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Published on: 27/09/2019
Thermodynamics
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1.
Anoop who is the student of class VIII went to a village in Rajasthan with his elder brother a science graduate. It was a month of June. He realised that during the day time, it was extreme hot but during the night it was too cold. He asked his elder brother the reason behind it. Anoop did not feel like it at his residence in Delhi. His brother explained him about Newton's cooling law that in desert places, the sand becomes too hot during day time and according to Newton's Law of cooling "The rate of heating is equal to rate of cooling". Anoop understood this reason very well and became happy.
(i) What qualities, Anoop possess?
(ii) The climate of a harbour town is more temperate than that of a town in a desert at the same altitude. Why
2.
Calculate the efficiency of a Carnot's engine working between steam point and ice point.
3.
Why can't the efficiency of an internal combustion engine be raised beyond a limit?
4.
A car tyre contains air at a pressure of 4 atm and its temperature is 270C. The tyre suddenly bursts. Calculate the resulting temperature. (⋎ = 1.4)
5.
The volume of an ideal gas is V at a pressure P. On increasing the pressure by ΔP, the change in volume of the gas is (ΔV1) under isothermal conditions and (ΔV2) under adiabatic conditions. Is ΔV1 > ΔV2 or vice-versa and why?
6.
One mole of an ideal gas requires 207 J heat to raise the temperature by 10 K when heated at constant pressure. Find the amount of heat required to heat the same gas to raise the temperature by same 10 K under constant volume conditions. Given R = 8.3 J mol-1, K-1.
7.
Two Carnot engines A and B are operated in series. The first one A receives heat at 900 K and rejects it to a reservoir at temperature T. and rejects heat to a reservoir at 400 K. Calculate temperature T when efficiencies of both A and B are equal.
8.
What is a heat engine? What is the best way to increase efficiency of a heat engine? Is it possible to design a thermal engine that has 100% efficiency?
9.
An ideal refrigerator runs between - 23o C and 27o C
(i) Also, find heat extracted from cold body.
(ii) Find coefficient of performance of the refrigerator.
10.
In changing the state of a gas adiabatically from an equilibrium state A to another equilibrium state B, an amount of work equal to 22.3 J is done on the system. If the gas is taken from state A to B via a process in which the net heat absorbed by the system is 9.35 cal, how much is the net work done by the system in the latter case ? (Take 1 cal = 4.19 J)
11.
Explain why
(a) Two bodies at different temperatures T1 and T2 if brought in thermal contact do not necessarily settle to the mean temperature (T1 + T2 )/2.
(b) The coolant in a chemical or a nuclear plant (i.e., the liquid used to prevent the different parts of a plant from getting too hot) should have high specific heat.
(c) Air pressure in a car tyre increases during driving. (d) The climate of a harbour town is more temperate than that of a town in a desert at the same latitude.
12.
A monoatomic ideal gas\((\gamma =\frac { 5 }{ 3 } )\)initialy at 170C is suddenly compressed to one-eight of its original volume. Find the final temperature after compression.
1.
(i) Anoop possesses the qualities like having scientific attitude, awareness, intelligence and keen observer.
(ii) The relative humidity in a harbour town is more than that in a town in a desert. Hence the climate of a harbour town is more temperate than that of a town in a desert.
2.
Here, steam point,
T1 = 100 0C = 100 + 273 = 373 K
ice point, T2 = 0°C = 0 + 273 = 273 K
As \(η=1-{T_2\over T_1}\)
\(∴\ \ η=1-{273\over 373}={100\over 373}\)
\(={100\over 373}\times100\%=26.81\%\)
3.
To increase the efficiency [= 1 - (1/ρ)⋎-1], the compression ratio p has to be increased. ρ cannot be made greater than 10, because then the cylinder of the engine will have to be made very thick and heavy which will be unsuitable for lighter vehicles. Secondly, during the adiabatic compression, the temperature of the air-petrol mixture will be high enough to cause explosion.
4.
Here P1 = 4 atm, P2 = 1 atrn, T1 - 27°C = 300 K and ⋎ = 1.4
The sudden burst of tyre is an adiabatic process, in which
\(P_1^1-\ ^γT^γ_2 = \ ^γT^γ_2\)
\(T_2=T_1\left( P_1\over P_2\right)^{1-⋎\over ⋎}= T_1\left(P_2\over P_1\right)^{⋎-1\over ⋎}\)
\(=300\left(4\over 1\right)^{1.4-1\over 1.4}=201.9\)
= 202K or -710C.
5.
Under isothermal conditions, \(K_i={ΔP\over ΔV_1/V}=P\)
under adiabatic condition, \(K_a={ΔP\over ΔV_2/V}=γP\)
Dividing (ii) by (i), we get
\({ΔV_1\over ΔV_2}=⋎.\ As\ ⋎>1\)
(ΔV1) > (ΔV2)
6.
Here heat required to raise temperature of 1 mole of gas through 10 K under constant pressure conditions ΔQ 207 J
\(∴\ \ C_p={ΔQ\over μ.ΔT}={207\over 1\times10}=20.7J\ mol^{-1}K^{-1}\)
∴ Cv = Cp - R = 20.7 - 8.3 = 12.4 J mol-1 K-1
∴ Amount of heat required to raise the temperature of gas through 10 K under constant volume condition:
ΔQ' = μ.Cp.ΔT = 1 x 12.4 x 10 = 124 J mol-1 K-1
7.
(i) Efficiency of A = efficiency of B
\({ \eta }_{ A }={ \eta }_{ B }\)
\(\\ \Rightarrow 1-\frac { T }{ 900 } =1-\frac { 400 }{ T } \)
\(\\ \Rightarrow { T }^{ 2 }=900\times400\)
\(\\ T=600K\)
(i) Let the first engine take Q1 heat as input at temperature,
T1 = 800 K
and gives out heat Q2 at temperature T0 The second engine receive Q2 as input and give is out heat Q3 at temperature T3 = 300 K to the sink.
Work done by first (A) engine = work done by second (B) engine.
Thus, Q1 - Q2 = Q2 - Q3
Dividing both sides by Q1
\(1-\frac { { Q }_{ 2 } }{ { Q }_{ 1 } } =\frac { { Q }_{ 2 } }{ { Q }_{ 1 } } -\frac { { Q }_{ 3 } }{ { Q }_{ 1 } } \)
\(\\ \Rightarrow 1-T/{ T }_{ 1 }=\frac { { Q }_{ 2 } }{ { Q }_{ 1 } } (1-\frac { { Q }_{ 3 } }{ { Q }_{ 2 } } )\)
\(\\ \Rightarrow -T/{ T }_{ 1 }=\frac { T }{ { T }_{ 1 } } (1-{ T }_{ 3 }/T)\)
\(\\ \Rightarrow { T }_{ 1 }/T-1=1-\frac { { T }_{ 3 } }{ T }\)
\( \Rightarrow \frac { { T }_{ 1 } }{ T } +\frac { { T }_{ 3 } }{ T } =2\)
\(\\ \Rightarrow \frac { 1 }{ T } ({ T }_{ 1 }+{ T }_{ 3 })=2\)
\(\Rightarrow T=\frac { { T }_{ 1 }+{ T }_{ 3 } }{ 2 } \)
\(\\ \Rightarrow T=\frac { 900+400 }{ 2 } =650K\)
8.
A heat engine is a device (or a combination) which converts heat into work.
Its efficiency, \(\eta =\frac { Work\ output }{ Heat\ input } \)
\(\eta =1-\frac { { T }_{ 2 } }{ { T }_{ 1 } } \)
Where, T2 = temperature of sink
T1 = temperature of source.
From above expression, we can see that for 100% efficiency, T2 = 0
It is impossible to design a thermal engine that has 100% efficiency because it is not possible to have a sink with kelvin temperature.
9.
(i) Q2 = 5J
(ii) Coefficient of performance
\(\beta =\frac { { T }_{ 2 } }{ { { T }_{ 1 }-{ T }_{ 2 } } } =\frac { 250 }{ 300-250 } =5\)
10.
Given, work done (W) = - 22.3 J
Work done is taken negative as work is done on the system.
In an adiabatic change, \(\Delta \)Q = 0
Using first law of thermodynamics
\(\Delta \).U = \(\Delta \)Q - W = 0 -(- 22.3) = 22.3J
For another process between states A and B,
Heat absorbed (\(\Delta \)Q) = + 9.35 cal
= + (9.35 x 4.19) J = + 39.18 J
Change in internal energy between two states via different paths are equal.
\(\because \) \(\Delta \)U = 22.3 J
\(\therefore \) From first law of thermodynamics,
\(\Delta \)U = \(\Delta \)Q - W
or W = \(\Delta \)Q -\(\Delta \)U
= 39.18 - 22.3
= 16.88J \(\approx \)16.9J
11.
(a) When two bodies at different temperatures T1 and T2 are brought in thermal contact, heat flows from the body at the higher temperature to the body at the lower temperature till equilibrium is achieved, i.e., the temperatures of both the bodies become equal. The equilibrium temperature is equal to the mean temperature (T1 + T2)/2 only when the thermal capacities of both the bodies are equal.
(b) The coolant in a chemical or nuclear plant should have a high specific heat. This is because higher the specific heat of the coolant, higher is its heat-absorbing capacity and vice versa. Hence, a liquid having a high specific heat is the best coolant to be used in a nuclear or chemical plant. This would prevent different parts of the plant from getting too hot.
(c) When a car is in motion, the air temperature inside the car increases because of the motion of the air molecules. According to Charles’ law, the temperature is directly proportional to pressure. Hence, if the temperature inside a tyre increases, then the air pressure in it will also increase.
(d) A harbour town has a more temperate climate (i.e., without the extremes of heat or cold) than a town located in a desert at the same latitude. This is because the relative humidity in a harbour town is more than it is in a desert town.
12.
8870C
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