Class 12 Physics: Thermodynamics & Entropy MCQs
Select an option to test your knowledge instantly with detailed step-by-step explanations.
Q1. A frictionless heat engine can be 100% efficient only if its exhaust temperature is:
Explanation:
Efficiency is calculated as $\eta = 1 - \frac{T_2}{T_1}$. For efficiency $\eta = 1$ (100%), the exhaust temperature $T_2$ must be absolute zero ($0\text{ K}$).
Q2. A refrigerator, with its door open. The temperature of the room will:
Explanation:
A refrigerator expels heat back into the room equal to the heat removed plus the electrical compressor work ($Q_{\text{rejected}} = Q_{\text{absorbed}} + W$). Leaving the door open means more energy is added as heat to the room, causing the overall temperature to rise.
Q3. Which device is not used in a diesel engine?
Explanation:
Diesel engines use compression ignition via a fuel injector rather than a spark plug. Spark plugs are exclusively used in petrol (gasoline) engines.
Q4. Which one of the following processes is irreversible?
Explanation:
A chemical explosion is a rapid, non-equilibrium process that dissipates energy and increases entropy permanently, making it completely irreversible. Quasi-static (slow) processes can be reversed.
Q5. According to 2nd law of thermodynamics, 100% conversion of heat into mechanical work is:
Explanation:
The Kelvin-Planck statement of the 2nd law states that no heat engine working in a cyclic process can convert heat completely into mechanical work without rejecting a portion of heat to a cold reservoir.
Q6. The change in entropy is given by:
Explanation:
Entropy change ($\Delta S$) is defined mathematically as the ratio of reversible heat added ($\Delta Q$) to the absolute temperature ($T$) at which it occurs: $\Delta S = \frac{\Delta Q}{T}$.
Q7. The net change in entropy of a system in a natural process is:
Explanation:
All natural or real processes are irreversible and spontaneous. According to the law of entropy, the total entropy of an isolated system always increases ($\Delta S > 0$, positive).
Q8. The efficiency of diesel engine is:
Explanation:
Diesel engines operate at higher compression ratios (around 14:1 to 22:1) compared to petrol engines (around 8:1 to 12:1), yielding higher thermodynamic efficiency (~35–40% vs ~25–30%).
Q9. Second law of thermodynamics states that:
Explanation:
In an isolated system, processes spontaneously evolve toward states of maximum disorder or thermodynamic equilibrium, meaning total entropy increases over time ($\Delta S_{\text{univ}} \ge 0$).
Q10. The process that violates the 2nd law of thermodynamics is:
Explanation:
The Clausius statement dictates that heat cannot flow spontaneously from a cooler body to a hotter body without external work input. Unassisted heat transfer from cold to hot violates the 2nd law.
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