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Thermodynamics — Questions and Answers

Thermodynamics is the highest-weightage subject in mechanical engineering objective papers and the one that recruiters use to check conceptual clarity. The laws themselves are few; what is tested is whether you can apply them — sign conventions for work and heat, the difference between reversible and irreversible processes, and where entropy actually comes from.

8 solved questionsMechanical EngineeringFree · no signup

Thermodynamics— Concepts, Formulas & Shortcuts

  • Zeroth law: two bodies in thermal equilibrium with a third are in equilibrium with each other — the basis of temperature measurement.
  • First law: energy is conserved; for a closed system dQ = dU + dW.
  • Second law: entropy of an isolated system never decreases; no engine can convert all heat into work.
  • Third law: entropy of a perfect crystal approaches zero as temperature approaches absolute zero.
  • Carnot efficiency η = 1 − T_cold/T_hot, with temperatures in kelvin — the maximum possible for any engine between those reservoirs.
  • For an ideal gas, PV = mRT; cp − cv = R and γ = cp/cv (about 1.4 for air).

Thermodynamics Practice Questions with Answers

Attempt each question first, then open the explanation. All 8 questions below are free to read and require no signup.

  1. Q1.The first law of thermodynamics is a statement of:

    Easy
    • AConservation of mass
    • BConservation of energy
    • CIncrease of entropy
    • DThermal equilibrium
    +Show Answer & Explanation

    Answer: B. Conservation of energy

    Explanation: The first law states that energy can be neither created nor destroyed, only converted: dQ = dU + dW.

    Thermodynamics question 1 of 8

  2. Q2.The efficiency of a Carnot engine operating between 800 K and 400 K is:

    Moderate
    • A25%
    • B50%
    • C60%
    • D75%
    +Show Answer & Explanation

    Answer: B. 50%

    Explanation: η = 1 − T_c/T_h = 1 − 400/800 = 0.5 = 50%.

    Thermodynamics question 2 of 8

  3. Q3.Which law forms the basis of temperature measurement?

    Easy
    • AZeroth law
    • BFirst law
    • CSecond law
    • DThird law
    +Show Answer & Explanation

    Answer: A. Zeroth law

    Explanation: The zeroth law establishes transitivity of thermal equilibrium, which is what makes a thermometer meaningful.

    Thermodynamics question 3 of 8

  4. Q4.In an adiabatic process:

    Easy
    • ATemperature is constant
    • BPressure is constant
    • CNo heat is transferred
    • DVolume is constant
    +Show Answer & Explanation

    Answer: C. No heat is transferred

    Explanation: Adiabatic means Q = 0; the system exchanges no heat with its surroundings, so all work changes internal energy.

    Thermodynamics question 4 of 8

  5. Q5.For an ideal gas, cp − cv equals:

    Moderate
    • A0
    • BR
    • Cγ
    • DR/γ
    +Show Answer & Explanation

    Answer: B. R

    Explanation: Mayer's relation gives cp − cv = R for an ideal gas.

    Thermodynamics question 5 of 8

  6. Q6.Entropy of an isolated system during an irreversible process:

    Moderate
    • ADecreases
    • BRemains constant
    • CIncreases
    • DMay increase or decrease
    +Show Answer & Explanation

    Answer: C. Increases

    Explanation: The second law requires entropy generation to be positive for any irreversible process in an isolated system.

    Thermodynamics question 6 of 8

  7. Q7.A process in which temperature remains constant is called:

    Easy
    • AIsobaric
    • BIsochoric
    • CIsothermal
    • DAdiabatic
    +Show Answer & Explanation

    Answer: C. Isothermal

    Explanation: Isothermal = constant temperature; isobaric is constant pressure and isochoric constant volume.

    Thermodynamics question 7 of 8

  8. Q8.The Carnot cycle consists of:

    Moderate
    • ATwo isothermal and two adiabatic processes
    • BTwo isobaric and two isochoric processes
    • CFour adiabatic processes
    • DTwo isothermal and two isochoric processes
    +Show Answer & Explanation

    Answer: A. Two isothermal and two adiabatic processes

    Explanation: The Carnot cycle uses two reversible isothermal processes (heat addition and rejection) joined by two reversible adiabatic processes.

    Thermodynamics question 8 of 8

Thermodynamics — Frequently Asked Questions

What does the second law of thermodynamics say in practical terms?+

That no heat engine can be 100% efficient and that heat flows spontaneously only from hot to cold. Every real process generates entropy, which sets an upper limit — the Carnot efficiency — on any engine.

Why is the Carnot cycle important if it cannot be built?+

It is the benchmark. Its efficiency depends only on the two reservoir temperatures, so it tells you the best any real engine operating between them could possibly achieve.

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