Question

A real heat engine has an efficiency of $40\%$. The work output of the engine is $100\text{kJ}$ per cycle. How much energy is extracted from the high temperature reservoir per cycle ?

• A. $40\text{kJ}$
• B. $250\text{kJ}$
• C. $400\text{kJ}$
• D. Can be calculated only if the engine is a Carnot engine.

Answer 1

The correct option is B $250\text{kJ}$

Given,
Efficiency of heat engine $\left(\eta \right)=40\%$
Work output $\left(W\right)=100\text{kJ}$
Using the formula,
Efficiency $\left(\eta \right)=\frac{\text{Work output}\left(W\right)}{\text{Heat input}\left(Q\right)}$
$0.4=\frac{100}{\text{Heat input}\left(Q\right)}$
$\therefore$ Heat extracted from high temperature reservoir per cycle
$Q=\frac{100}{0.4}=250\text{kJ}$
Thus, option (b) is the correct answer.