Question

The efficiency of a cyclic process is limited by:

• A. The first law of thermodynamics
• B. The zeroth law of thermodynamics
• C. The second law of thermodynamics
• D. The conservation of momentum

Hint:

The second law of thermodynamics limits how much heat can be converted into work—no heat engine can be 100\% efficient due to inevitable losses.

Answer 1

The Correct Option is C

Step 1: The second law of thermodynamics governs the direction of heat transfer and the efficiency limits of heat engines operating in a cyclic manner. It asserts that no cyclic process can convert all the absorbed heat into work.
Step 2: For a heat engine, the efficiency \( \eta \) is fundamentally limited by the Carnot efficiency: \[ \eta = 1 - \frac{T_C}{T_H} \] where \( T_H \) and \( T_C \) are the absolute temperatures of the heat source and sink, respectively.
Step 3: This shows that 100% efficiency is impossible unless \( T_C = 0 \, \text{K} \), which is unattainable. Hence, the second law places a theoretical upper limit on the efficiency of all cyclic thermal processes.
Why the other options are incorrect:

• (A) The first law addresses energy conservation but not efficiency limits.
• (B) The zeroth law concerns thermal equilibrium, not process efficiency.
• (D) Conservation of momentum is a mechanical concept and unrelated to thermodynamic efficiency.