# Thermodynamics – Physics 10

## Thermodynamics – Physics 10

1.First law of thermodynamics is consequence of conservation of

(a)work

(b)energy

(c)heat

(d)all of these

2.At 27° C a gas is compressed suddenly such that its pressure becomes (1/8) of original pressure. Final temperature will be (γ= 5/3)

(a)420 K

(b)300K

(c)– 142°C

(d)327°C

3.A thermodynamic process is shown in the figure.The pressures and volumes corresponding to some points in the figure are POVABCDPA= 3 ×104 PaVA= 2 × 10-3 m3PB = 8 × 104 PaVD= 5× 10–3 m3.In process AB, 600 J of heat is added to the system and in process BC, 200 J of heat is added to the system. The change in internal energy of the system in process ACwould be

(a)560 J

(b)800 J

(c)600 J

(d)640 J

4.For hydrogen gas, Cp – Cv= a and for oxygen gas, Cp – Cv = b, so the relation between a and b is given by

(a)a= 16 b

(b)16 b= a

(c)a = 4 b

(d)a= b

5.If for a gas, R/Cv=0.67, the gas is made up of molecules which are

(a)diatomic

(b)a mixture of diatomic and polyatomic molecules

(c)monoatomic

(d)polyatomic

6.A thermodynamic system is taken from state A to B alongACBand is brought back to A along BDA as shown in the PVdiagram. The net work done during the complete cycle is given by the area

(a)P1ACBP2P1

(b)ACBB’A’A

(c)ACBDA

(d)ADBB’A’A

7.An ideal gasAand a real gas Bhave their volumes increased from V to 2Vunder isothermal conditions. The increase in internal energy

(a)will be same in both Aand B

(b)will be zero in both the gases

(c)of B will be more than that of A

(d)of A will be more than that of B

8.110 joules of heat is added to a gaseous system, whose internal energy is 40J; then the amount of external work done is

(a)150 J

(b)70 J

(c)110 J

(d)40 J

9.Which of the following is not thermodynamical function?

(a)Enthalpy

(b)Work done

(c)Gibb’s energy

(d)Internal energy

10.An ideal Carnot engine, whose efficiency is 40%receives heat at 500 K. If it sefficiency is 50%, then the intake temperature for the same exhaust temperature is

(a)600 K

(b)700 K

(c)800 K

(d)900 K

11.An ideal gas undergoing adiabatic change has the following pressure-temperature relationship[1996]
(a)Pγ-¹Tγ=constant

(b)PγTγ-¹=constant

(c)PγTl-γ=constant

(d)Pl-γTγ=constant

12.A diatomic gas initially at 18ºC is compressed adiabatically to one eighth of its original volume.The temperature after compression will be

(a)18ºC

(b)668.4ºK

(c)395.4ºC

(d)144ºC

13.A sample of gas expands from volume V1to V2.The amount of work done by the gas is greatest,when the expansion is

(a)adiabatic

(b)isobaric

(c)isothermal

(d)equal in all cases

14.The efficiency of aCarnot engine operating between the temperatures of 100ºC and –23ºCwill be

(a)100+23/100

(b)100-23/100

(c)373+250/373

(d)373-250/373

15.If the ratio of specific heat of a gas at constant pressure to that at constant volume is γ, the change in internal energy of a mass of gas, when the volume changes from V to 2Vat constant pressure P, is

(a)R/γ-1

(b)PV

(c)PV/(γ-1)

(d)γPV(γ-1)

16.We consider a thermodynamic system. If ΔU represents the increase in its internal energy andW the work done by the system, which of the following statements is true?

(a)ΔU= – Win an adiabatic process

(b)ΔU= Win an isothermal process

(c)ΔU= –Win an isothermal process

(d)ΔU= Win an adiabatic process

17.An ideal gas at 27ºC is compressed adiabatically to8/27 of its original volume. The rise in temperature is γ=5/3

(a)475ºC

(b)402ºC

(c)275ºC

(d)175ºC

18.A reversible engine converts one-sixth of the heat input into work. When the temperature of the sink is reduced by 62ºC, the efficiency of the engine is doubled. The temperatures of the source and sink are

(a)99ºC, 37ºC

(b)80ºC, 37ºC

(c)95ºC, 37ºC

(d)90ºC, 37ºC

19.If γ be the ratio of specific heats of a perfect gas, the number of degrees of freedom of a molecule of the gas is

(a)25/2(γ-1)

(b)3(γ-1)/2(γ-1)

(c)2/γ-1

(d))9/2(γ-1)

20.The temperature of source and sink of a heat engine are 127ºC and 27ºC respectively. An inventor claims its efficiency to be 26%, then:

(a)it is impossible

(b)it is possible with high probability

(c)it is possible with low probability

(d)data are insufficient

21.Agas at 27ºC temperature and 30 atmospheric pressure is allowed to expand to the atmospheric pressure. If the volume becomes 10 times its initial volume, then the final temperature becomes

(a)100ºC

(b)173ºC

(c)273ºC

(d)–173ºC

22.ACarnot engine whose efficiency is 50% has an exhaust temperature of 500 K. If the efficiency is to be 60% with the same intake temperature, the exhaust temperature must be (in K)

(a)800

(b)200

(c)400

(d)600

23.An ideal gas heat engine operates in aCarnotcycle between 227ºC and 127ºC. It absorbs 6 kcal at the higher temperature. The amount of heat(in kcal) converted into work is equal to

(a)1.2

(b)4.8

(c)3.5

(d)1.6

24.One mole of an ideal gas at an initial temperature of T K does 6R joules of work adiabatically. If the ratio of specific heats of this gas at constant pressure and at constant volume is 5/3, the final temperature of gas will be

(a)(T– 4) K

(b)(T+ 2.4)K

(c)(T– 2.4) K

(d)(T + 4) K

25.Which of the following processes is rever-sible?

(a)Transfer of heat by conduction

(b)Transfer of heat by radiation

(c)Isothermal compression

(d)Electrical heating of a nichrome wire

26.An ideal gas heat engine operates in Carnot cycle between 227°C and 127°C. It absorbs 6 ×104cals of heat at higher temperature. Amount of heat converted to work is

(a)4.8 × 10²+²cals

(b)6 × 10²+²cals

(c)2.4 × 10²+²cals

(d)1.2 ×10²+² cals

27.A Carnot engine whose sink is at 300 K has an efficiency of 40%. By how much should the temperature of source be increased so as to increase, its efficiency by 50% of original efficiency ?

(a)325 K

(b)250 K

(c)380 K

(d)275 K

28.The molar specific heat at constant pressure of an ideal gas is (7/2) R. The ratio of specific heat at constant pressure to that at constant volume is

(a)8/7

(b)5/7

(c)9/7

(d)7/5

29.An engine has an efficiency of 1/6. When the temperature of the sink is reduced by 62°C, its efficiency is doubled. Temperature of the source is

(a)37°C

(b)62°C

(c)99°C

(d)124°C

30.If Q, E and W denote respectively the heat added, change in internal energy and the work done ina closed cyclic process, then:

(a)W = 0

(b)Q= W= 0

(c)E = 0

(d)Q = 0

31.In thermodynamic processes which of the following statements is not true?

(a)In an isochoric process pressure remains constant

(b)In an isothermal process the temperature remains constant

(c)In an adiabatic process PVγ= constant

(d)In an adiabatic process the system is insulated from the surroundings

32.The internal energy change in a system that has absorbed 2 kcals of heat and done 500 J of work is:

(a)6400 J

(b)5400 J

(c)7900 J

(d)8900 J

33.If Uand Wrepresent the increase in internal energy and work done by the system respectively in a thermodynamical process, which of the following is true?

(a)ΔU=-ΔW, in an adiabatic process

(b)ΔU=ΔW, in an is othermal process

(c)ΔU=ΔW, in an adiabatic process

(d)ΔU=-ΔW, in an is othermal process

34.During an isothermal expansion, a confined ideal gas does –150 J of work against its surroundings. This implies that

(a)150 J heat has been removed from the gas

(b)300 J of heat has been added to the gas

(c)no heat is transferred because the processis isothermal

(d)150 J of heat has been added to the gas

35.When 1 kg of ice at 0°C melts to water at 0°C, the resulting change in its entropy, taking latent heat of ice to be 80 cal/°C, is

(a)273 cal/K

(b)8 × 104 cal/K

(c)80 cal/K

(d)293 cal/K

36.A mass of diatomic gas (γ= 1.4) at a pressure of2 atmospheres is compressed adiabatically so that its temperature rises from 27°C to 927°C.The pressure of the gas in final state is

(a)28 atm

(b)68.7 atm

(c)256 atm

(d)8 atm

37.During an adiabatic process, the pressure of a gas is found to be proportional to the cube of its temperature. The ratio of pvCCfor the gas is

(a)2

(b)53

(c)32

(d)4

38.Which of the following relations does not give the equation of an adiabatic process, where terms have their usual meaning?

(a)PγT¹-γ= constant

(b)P¹-γTγ= constant

(c)PVγ= constant

(d)TV¹-γ= constant

39.Two Carnot engines A and B are operated in series. The engine A receives heat from the source at temperature T1 and rejects the heat to the sink at temperature T. The second engine B receives the heat at temperature T and rejects to its sink at temperature T2. For what value of Tthe efficiencies of the two engines are equal?

(a)T1+T2/2

(b)T1-T2/2

(c)T1T2

(d)√T1T2

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