11

THERMODYNAMICS

MCQs with One Correct Answer (a) R (b) 2 R

R R
1. Which of the following is not a thermodynamical (c) (d)
3 4
function 5. Four curves A, B, C and D are drawn in the figure
(a) Enthalpy (b) Work done for a given amount of a gas. The curves which
(c) Gibb’s energy (d) Internal energy represent adiabatic and isothermal changes are
2. A gas can be taken from A to B via two different (a) C and D respectively
processes ACB and ADB. B
(b) D and C respectively P C
P A
D
C B (c) A and B respectively
(d) B and A respectively V
6. One mole of an ideal gas at an initial temperature
of T K does 6R joules of work adiabatically. If
A D the ratio of specific heats of this gas at constant
V pressure and at constant volume is 5/3, the final
When path ACB is used 60 J of heat flows into temperature of gas will be
the system and 30J of work is done by the (a) (T – 4) K (b) (T + 2.4) K
(c) (T – 2.4) K (d) (T + 4) K
system. If path ADB is used work done by the
7. A thermally insulted vessel contains 150 g of
system is 10 J. The heat Flow into the system in
water at 0°C. Then the air from the vessel is
path ADB is :
pumped out adiabatically. A fraction of water
(a) 40 J (b) 80 J turns into ice and the rest evaporates at 0°C
(c) 100 J (d) 20 J itself. The mass of evaporated water will be
3. Unit mass of a liquid with volume V 1 is closed to :
completely changed into a gas of volume V2 at a (Latent heat of vaporization of water = 2.10 ×
constant external pressure P and temperature T. 106 J kg–1 and Latent heat of Fusion of water
If the latent heat of evaporation for the given
= 3.36 × 105 J kg–1)
mass is L, then the increase in the internal energy
of the system is (a) 150 g (b) 20 g
(a) Zero (b) P(V2 – V1) (c) 130 g (d) 35 g
(c) L – P(V2 – V1) (d) L 8. Two Carnot engines A and B are operated in
4. The specific heat capacity of a monoatomic gas series. The engine A receives heat from the
for the process TV2 = constant is (where R is gas source at temperature T1 and rejects the heat to
constant) the sink at temperature T. The second engine B
 40 PHYSICS

receives the heat at temperature T and rejects to undergoes and adiabatic change until the
its sink at temperature T2. For what value of T temperature returns to the initial value. What is
the efficiencies of the two engines are equal? the final volume of the gas?
T1 + T2 T -T
(a) (b) 1 2 (a) 112.4 lit. (b) 115.2 lit
2 2
(c) T 1T 2 (d) T1T2 (c) 120 lit (d) 125 lit
15. The relation between U, P and V for an ideal gas
9. An ideal heat engine works between in an adiabatic process is given by relation U = a
temperatures T1 = 500 K and T2 = 375 K. If the + bP V. Find the value of adiabatic exponent (g)
engine absorbs 600J of heat from the source, of this gas.
then the amount of heat released to the sink is: b +1 b +1
(a) 450 J (b) 600 J (a) (b)
b a
(c) 45 J (d) 500 J a +1 a
10. In a Carnot engine, the temperature of reservoir (c) (d)
b a+b
is 927°C and that of sink is 27°C. If the work 16. A thermodynamic system undergoes cyclic
done by the engine when it transfers heat from process ABCDA as shown in fig. The work
reservoir to sink is 12.6 × 106J, the quantity of done by the system in the cycle is :
heat absorbed by the engine from the reservoir (a) P0V0
is P C B
(b) 2P0V0 3P0
(a) 16.8 × 106 J (b) 4 × 106 J 2P0
(c) 7.6 × 106 J (d) 4.2 × 106 J P0 V0
(c) P0 D
11. A reversible engine converts one-sixth of the 2 A
heat input into work. When the temperature of V0 2V0 V
(d) Zero
the sink is reduced by 62ºC, the efficiency of the
17. An ideal gas goes through
engine is doubled. The temperatures of the
a r eversible cycle V
source and sink are c
a®b®c®d has the V - T b
(a) 99ºC, 37ºC (b) 80ºC, 37ºC diagram shown below.
(c) 95ºC, 37ºC (d) 90ºC, 37ºC Process d®a and b®c are d
a
12. Adiabatic modulus of elasticity of gas is 2.1 × adiabatic. T
105 N/m2. What will be its iosthermal modulus The corresponding P - V diagram for the process
æC ö
is (all figures are schematic and not drawn to
p
of elasticity ? çç C = 1.4 ÷÷ scale) :
è v ø P P
(a) 1.8 × 105 N/m2 (b) 1.5 × 105 N/m2 d c a b
5
(c) 1.4 × 10 N/m 2 (d) 1.2 × 105 N/m2. (a) (b) c
a b d
13. In an adiabatic process, the pressure is increased
2 3 V V
by % . If g = , then the volume decreases by P P
a
3 2 d c b
nearly
(c) a b (d) d c
4 2
(a) % (b) % V V
9 3 18. An ideal monatomic gas with pressure P, volume
9 V and temperature T is expanded isothermally to
(c) 1% (d) % a volume 2V and a final pressure Pi. If the same
4 gas is expanded adiabatically to a volume 2V,
14. Two moles of helium gas (g = 5/3) are initially at P
temperature 27°C and occupy a volume of 20 the final pressure is Pa. The ratio a is
Pi
litres. The gas is first expanded at constant –1/3 1/3
(a) 2 (b) 2
pressure until the volume is doubled. Then, it
(c) 22/3 (d) 2–2/3