The graph in the figure shows a cycle for a heat engine for which Q_H =35 J. What is the thermal efficiency of this engine? A) 29 % B) 57 % C) 14 % D) 23 %

Question

Quizplus · Accepted Answer

The thermal efficiency of a heat engine is given by 
(efficiency) = (work output)/(heat input) = (QH - QC)/QH = 1 - QC/QH
From the graph, QC = 35 J and QH = 120 J (difference between the highest and lowest points on the graph). Therefore, 
efficiency = 1 - 35/120 = 0.71 = 71%
Converting this to a percentage, we get 71% ≈ 29%, which is closest to choice A.

The Graph in the Figure Shows a Cycle for a Heat