In a heat engine, which quantity represents heat absorbed from the high-temperature reservoir?

• A. Q_H
• B. Q_C
• C. W
• D. ΔU

Answer: (A)

Heat absorbed from the high-temperature reservoir is the energy flow into the engine as heat, labeled Q_H. In a heat engine, energy from the hot source goes in as Q_H, and the engine converts part of that energy into work while the rest is rejected as heat to the cold reservoir (Q_C). The first-law relation for a cycle shows Q_H = W + Q_C, meaning the work output comes from the portion of Q_H not dumped as Q_C. The change in internal energy, ΔU, tracks the engine’s internal energy; over a complete cycle, this returns to its starting value, so ΔU is zero for the cycle. So Q_H is the quantity representing heat absorbed from the high-temperature reservoir.