Question

Three equal resistors connected across a source of e.m.f. together dissipate 10 watt of power. What will be the power dissipated in watts if the same resistors are connected in parallel across the same source of e.m.f ?

• A. 10
• B. $\frac{10}{3}$
• C. 30
• D. 90

Answer 1

The correct option is D 90

In series, Equivalent resistance $=$ $3R$
$Power=\frac{V^2}{3R}\Rightarrow 10=\frac{V^2}{3R}\Rightarrow V^2=30R$

In parallel, $\frac{1}{R^{\prime }}=\frac{1}{R}+\frac{1}{R}+\frac{1}{R}=\frac{3}{R}$

$\therefore$ Equivalent resistance $R^{\prime }=\frac{R}{3}$

$\therefore$ Power $=\frac{V^2}{R^{\prime }}=\frac{30R}{R/3}=90W$