Thermodynamics PYQs Set 1 (PDF)

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Mere Teacher · Previous Year Questions NEET Physics · Thermodynamics · Set 1 (Q1-30)
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1. $A body cool from 70^{\circ} C to 40^{\circ} C in 5 min . Calculate the time it takes to cool from 60^{\circ} C to 40^{\circ} C . The temperature of the surrounding is 20^{\circ} C .$ (1) $3.77 min$ (2) $3.56 min$ (3) $3.68 min$ (4) $3.89 min$ 2. $Find the difference in temperature between the water at the top and the bottom of 20 m high waterfall assuming 10 % of the energy of fall is spent in heating the water [Use specific heat capacity of water = 4000 J kg^{- 1} k^{- 1} and g = 10 m / s^{2}]$ (1) $0.002^{\circ} C$ (2) $0.004^{\circ} C$ (3) $0.005^{\circ} C$ (4) $0.006^{\circ} C$ 3. $If for a gas, \frac{R}{C _{V}} = 0.67, this gas is made up of molecules which are$ (1) $diatomic$ (2) $mixture of diatomic and polyatomic molecules$ (3) $monoatomic$ (4) $polyatomic$ 4. $A carnot engine has the same efficiency between 800 K to 500 K and x K to 600 K. The value of x is$ (1) $1000 K$ (2) $960 K$ (3) $846 K$ (4) $754 K$ 5. $The efficiency of Carnot's heat engine is 0.5 when the temperature of the source is T_{1} and that of sink is T_{2} . The efficiency of another Carnot's heat engine is also 0.5. The temperature of source and sink of the second engine are respectively$ (1) $2 T_{1}, 2 T_{2}$ (2) $2 T_{1}, \frac{T _{2}}{2}$ (3) $T_{1} + 5, T_{2} - 5$ (4) $T_{1} + 10, T_{2} - 10$ 6. $The work done in an adiabatic change in an ideal gas depends upon only:$ (1) $change in its temperature$ (2) $change in its volume$ (3) $change in its pressure$ (4) $change in its specific heat$ 7. $Let γ_{1} be the ratio of molar specific heat at constant pressure and molar specific heat at constant volume of a monoatomic gas and γ_{2} be the similar ratio of diatomic gas. Considering the diatomic gas molecule as a rigid rotator, the ratio, \frac{γ _{1}}{γ _{2}} is$ (1) $\frac{27}{35}$ (2) $\frac{35}{27}$ (3) $\frac{25}{21}$ (4) $\frac{21}{25}$ 8. $Which of the following statements is NOT true?$ (1) $Specific heat capacity is dependent on nature of substance but independent of its quantity.$ (2) $Specific heat capacity depends on temperature.$ (3) $Specific heat capacity of water doesn't vary with temperature.$ (4) $The SI unit of specific heat capacity is JK^{- 1} kg^{- 1} .$ 9. $The efficiency of a Carnot engine operating with reservoir temperature of 10 0^{o} Cand - 2 3^{o} C will be$ (1) $\frac{373 + 250}{373}$ (2) $\frac{373 - 250}{373}$ (3) $\frac{100 + 23}{100}$ (4) $\frac{100 - 23}{100}$ 10. $The coefficient of performance of a Carnot refrigerator working between 3 0^{o} C and 0^{o} C is$ (1) $10$ (2) $1$ (3) $9$ (4) $0$ 11. $The pressure and volume of an ideal gas are related as PV^{3/2} = K (Constant). The work done by gas when the gas is taken from state A (P_{1}, V_{1}, T_{1}) to state B (P_{2}, V_{2}, T_{2}) is:$ (1) $2 (P_{1} V_{1} - P_{2} V_{2})$ (2) $2 (P_{2} V_{2} - P_{1} V_{1})$ (3) $2 (P_{1} V_{1} - P_{2} V_{2})$ (4) $2 (P_{2} V_{2} - P_{1} V_{1})$ 12. $Even Carnot engine cannot give 100% efficiency because we cannot$ (1) $Prevent radiation$ (2) $Find ideal sources$ (3) $Reach absolute zero temperature$ (4) $Eliminate friction$ 13. $A Carnot engine operates between a source and a sink. The efficiency of the engine is 40 % and the temperature of the sink is 27^{\circ} C . If the efficiency is to be increased to 50 % then the temperature of the source must the increased by$ (1) $80 K$ (2) $120 K$ (3) $100 K$ (4) $160 K$ 14. $The change in internal energy when 20 g of a gas is heated from 25^{\circ} C to 35^{\circ} C at constant volume is (Specific heat capacity of the gas at constant volume = 0.2 cal g^{- 1} ° C^{- 1} and (J = 4200 J / kcal) .$ (1) $74 J$ (2) $336 J$ (3) $136 J$ (4) $168 J$ 15. $A diatomic gas, having C_{p} = \frac{7}{2} R and C_{v} = \frac{5}{2} R, is heated at constant pressure. The ratio dU : dQ : dW$ (1) $03 : 07 : 02$ (2) $05 : 07 : 02$ (3) $05 : 07 : 03$ (4) $03 : 05 : 02$ 16. $If Δ U and Δ W represent the increase in internal energy and work done by the system respectively in a thermodynamical process, which of the following is true?$ (1) $Δ U = - Δ W, in an adiabatic process$ (2) $Δ u = Δ W, in an isothermal process$ (3) $Δ u = Δ W, in an adiabatic process$ (4) $Δ u = - Δ W, in an isothermal process$ 17. $The efficiency of Carnot engine is 50% and temperature of sink is 500 K. If the temperature of source is kept constant and its efficiency is to be raised to 60%; then the required temperature of the sink will be:$ (1) $600 K$ (2) $500 K$ (3) $400 K$ (4) $100 K$ 18. $The surface of a black body is at a temperature 727^{\circ} C and its cross-section is 1 m^{2} . Heat radiated from this surface in one minute in joules is (Stefan's constant = 5.7 \times 10^{- 8} W / m^{2} / k^{4})$ (1) $34.2 \times 10^{5}$ (2) $2.5 \times 10^{5}$ (3) $3.42 \times 10^{5}$ (4) $2.5 \times 10^{6}$ 19. $Under an adiabatic process, the volume of an ideal gas gets doubled. Consequently the mean collision time between the gas molecules changes from τ_{1} to τ_{2} . If \frac{C _{p}}{C _{v}} = γ for this gas then a good estimate for \frac{τ _{2}}{τ _{1}} is given by$ (1) $(2)^{\frac{1 + γ}{2}}$ (2) $2$ (3) $(\frac{1}{2})^{γ}$ (4) $(\frac{1}{2})^{\frac{γ}{2} + 1}$ 20. $An ideal Carnot engine whose efficiency is 50 % receives heat at 500 K . If the efficiency is to be 60 %, then intake temperature for the same exhaust temperature is$ (1) $600 K$ (2) $625 K$ (3) $650 K$ (4) $700 K$ 21. $A heat engine has an efficiency of (\frac{1}{6}) . When the temperature of sink is reduced by 6 2^{0} C, its efficiency get doubled. The temperature of the source is:$ (1) $12 4^{0} C$ (2) $3 7^{0} C$ (3) $6 2^{0} C$ (4) $9 9^{0} C$ 22. $Molecules of an ideal gas are known to have three translational degrees of freedom and two rotational degrees of freedom. The gas is maintained at a temperature of T . The total internal energy, U of a mole of this gas, and the value of γ (= \frac{C _{p}}{C _{v}}) are given, respectively, by$ (1) $U=5RT and γ = \frac{7}{5}$ (2) $U = \frac{5}{2} RT and γ = \frac{6}{5}$ (3) $U=5RT and γ = \frac{6}{5}$ (4) $U = \frac{5}{2} RT and γ = \frac{7}{5}$ 23. $A gas is kept in a container having walls which are thermally non-conducting. Initially the gas has a volume of 800 cm^{3} and temperature 27^{\circ} C . The change in temperature when the gas is adiabatically compressed to 200 cm^{3} is: (Take γ = 1.5; γ is the ratio of specific heats at constant pressure and at constant volume)$ (1) $300 K$ (2) $600 K$ (3) $327 K$ (4) $522 K$ 24. $A number of Carnot engines are operated at identical cold reservoir temperatures (T_{L}) . However, their hot reservoir temperatures are kept different. A graph of the efficiency of the engines versus hot reservoir temperature (T_{H}) is plotted. The correct graphical representation is$ (1) (2) (3) (4) 25. $A monatomic gas at 630 K expands adiabatically to 27 times its initial volume. The final temperature of the gas is$ (1) $30 K$ (2) $130 K$ (3) $170 K$ (4) $70 K$ 26. $An ideal gas undergoes an adiabatic process. If the pressure of the gas is reduced by 0.1 %, then the volume is changed by (Given, (γ = \frac{C _{p}}{C _{v}} = 5/3)$ (1) $0.1 %$ (2) $0.05 %$ (3) $0.06 %$ (4) $- 0.05 %$ 27. $When an iron rod is heated, the variation of colour from dull red to white can be explained by$ (1) $Boltzmann Law$ (2) $Newton's Law of cooling$ (3) $Stefan's Law of radiation$ (4) $Wien's displacement Law.$ 28. $For an ideal gas at a temperature of 27^{\circ} C and at constant pressure, the coefficient of volume expansion is nearly$ (1) $33 \times 10^{- 5} K^{- 1}$ (2) $22 \times 10^{- 4} K^{- 1}$ (3) $37 \times 10^{- 5} K^{- 1}$ (4) $33 \times 10^{- 4} K^{- 1}$ 29. $The temperature of 100 g of water is to be raised from 24^{\circ} C to 90^{\circ} C by adding steam at 100^{\circ} C to it. The mass of the steam required in this process is (latent heat of steam is 540 cal g^{- 1})$ (1) $2 g$ (2) $4 g$ (3) $10 g$ (4) $12 g$ 30. $If the volume of a block of metal changes by 0.12 % when heated through 20^{\circ} C, then find its coefficient of linear expansion.$ (1) $4 \times 10^{- 5} ° C^{- 1}$ (2) $4 \times 10^{- 4} ° C^{- 1}$ (3) $2 \times 10^{- 5} ° C^{- 1}$ (4) $2 \times 10^{- 4} ° C^{- 1}$