Thermodynamics

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Thermodynamics — solved practice questions

8 NEET Physics questions with step-by-step solutions. Attempt each, then reveal the answer.

1. Q1easy

   A Carnot engine operates between a source at 500 K and a sink at 300 K. Its efficiency is:

   • A30%
   • B40%
   • C50%
   • D60%
   Show answer & solution

   Correct answer: (B) 40%

   Carnot efficiency η = 1 − T2/T1 = 1 − 300/500 = 0.4, i.e. 40%.

2. Q2easy

   A heat engine absorbs 1000 J of heat from the source and does 300 J of useful work per cycle. Its efficiency is:

   • A10%
   • B20%
   • C30%
   • D70%
   Show answer & solution

   Correct answer: (C) 30%

   Efficiency η = W/Q1 = 300/1000 = 0.3 = 30%.

3. Q3easy

   In a thermodynamic process, 200 J of heat is supplied to a gas and it does 50 J of work on the surroundings. The change in internal energy of the gas is:

   • A50 J
   • B100 J
   • C250 J
   • D150 J
   Show answer & solution

   Correct answer: (D) 150 J

   First law: ΔU = Q − W = 200 − 50 = 150 J.

4. Q4medium

   For an ideal gas undergoing an isothermal process, which of the following is true?

   • AΔU = 0 and Q = W
   • BQ = 0
   • CW = 0
   • DΔU = Q
   Show answer & solution

   Correct answer: (A) ΔU = 0 and Q = W

   In an isothermal process temperature is constant, so ΔU = 0 (internal energy of an ideal gas depends only on temperature), and Q = W.

5. Q5medium

   The molar specific heat at constant volume of a monatomic ideal gas is (R = gas constant):

   • A(1/2)R
   • B(3/2)R
   • C(5/2)R
   • D(7/2)R
   Show answer & solution

   Correct answer: (B) (3/2)R

   For a monatomic ideal gas the internal energy is (3/2)RT per mole, so Cv = (3/2)R.

6. Q6medium

   The ratio of specific heats γ = Cp/Cv for an ideal diatomic gas (at ordinary temperatures, translational + rotational modes) is:

   • A1.33
   • B1.67
   • C1.40
   • D1.50
   Show answer & solution

   Correct answer: (C) 1.40

   For a diatomic gas Cv = (5/2)R and Cp = (7/2)R, so γ = 7/5 = 1.4.

7. Q7medium

   A refrigerator maintains its interior at 250 K while rejecting heat to surroundings at 300 K. Its maximum (ideal) coefficient of performance is:

   • A2
   • B3
   • C4
   • D5
   Show answer & solution

   Correct answer: (D) 5

   COP = T2/(T1 − T2) = 250/(300 − 250) = 250/50 = 5.

8. Q8medium

   For an adiabatic process on an ideal gas, which statement is correct?

   • AQ = 0, so ΔU = −W
   • BW = 0
   • CΔU = 0
   • DTemperature stays constant
   Show answer & solution

   Correct answer: (A) Q = 0, so ΔU = −W

   In an adiabatic process no heat is exchanged (Q = 0), so from the first law ΔU = −W; work is done at the expense of internal energy.

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