Question

If a heat engine does 100 kJ of work per cycle while releasing 400 kJ of heat, what is its efficiency?

• A. 20%
• B. 25%
• C. 80%
• D. 400%

Answer 1

The correct option is A 20%

The efficiency of a heat engine is the ratio of the work done per cycle $W$ to the heat absorbed from the high-temperature reservoir $Q_h$. We can use the relationship between $W$, $Q_h$ & $Q_c$($W$ = $Q_h$ −$Q_c$ ) to express the efficiency of the heat engine in terms of $Q_c$ and $W$.

$\epsilon =\frac{W}{Q_h}=\frac{W}{W+Q_c}=\frac{1}{1+\frac{Q_c}{W}}=\frac{1}{1+\frac{100}{400}}=0.2=20$