Question

A $750 \,W$ motor drives a pump which lifts $300 \,L$ of water per minute to a height of $6 \,m$. The efficiency of the motor is nearly
(Take, acceleration due to gravity to be $10\, m / s ^{2}$ )

• A. $30 \%$
• B. $40 \%$
• C. $50 \%$
• D. $20 \%$

Answer 1

Answer: (B)

Useful power
$=\frac{\text { Work done by motor }}{\text { Time duration }}$
$\Rightarrow P_{\text {input }} \times \eta=\frac{W}{\Delta t}$
$\rightarrow P_{\text {input }} \times \eta=\frac{m g h}{t}$
$\Rightarrow \eta=\frac{300 \times 10 \times 6}{60 \times 750}=0.4$
So, per cent efficiency is, $\eta=40 \%$