Question

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

• A. 90 watts
• B. 80 watts
• C. 70 watts
• D. 75 watts

Answer 1

Answer: (A)

90 watts

When connected in series,
$[R_{eq}=R+R+R]$
$[R_{eq}=3\times R]$
Power dissipation will be,
$P=\frac{V^2}{R_{eq}}$
i.e. $P=\frac{V^2}{\left(3R\right)}$

Given,$[P=10]$
Now, when they are connected in parallel
$\frac{1}{R_{eq}}=\frac{1}{R}+\frac{1}{R}+\frac{1}{R}$
$R_{eq}=R/3$

$[P=V^2/R_{eq}]$

$P=3V^2/R$

From above, $[P=90]$