First slide

Electrical power

Question

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

Moderate

A

9 watts
B

90 watts
C

10 watts
D

100 watts

Solution

We know that $P = V^{2} / R$
When resistances are connected in series, then R = 3R
When resistances are connected in parellel, then
$\begin{array}{l} R_{2} = R / 3 \\ ∴ 10 = \frac{V^{2}}{3 R} and P = \frac{V^{2}}{(R / 3)} = \frac{3 V^{2}}{R} \end{array}$
or $\frac{P}{10} = 9 or P = 90 watt$

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