JEE Main · 2019 · 11 Jan 2019 · Shift 1 · Current Electricity

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Two equal resistances when connected in series to a battery, consume electric power of 60 W . If these resistances are now connected in parallel combination to the same battery, the electric power consumed will be.

(A)60 W
(B)240 W✓ Correct
(C)120 W
(D)30 W

Answer: (B)

Solution

With a fixed battery voltage

V

and two equal resistances

R

, the series combination has resistance

2 R

and dissipates

P = \frac{V ^{2}}{2 R} = 60 W

, which fixes

\frac{V ^{2}}{R} = 120 W

. In parallel the resistance is

R /2

, so the power is

P^{'} = \frac{V ^{2}}{R /2} = 2 \frac{V ^{2}}{R} = 240 W

. Equivalently, the parallel power is four times the series power.

JEE Main · 2019 · 11 Jan 2019 · Shift 1 · Single correct option

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