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Electric Power
Appears in
Concepts tested here
- Resistance from power rating
- Resistance inversely proportional to power
All Questions
2004 AIPMT 1 question
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In India electricity is supplied for domestic use at 220 V. It is supplied at 110 V in USA. If the resistance of a 60W bulb for use in India is R, The resistance of a 60W bulb for use in USA will be :-Use power $P = \dfrac{V^2}{R}$, so $R = \dfrac{V^2}{P}$.
For India: $R = \dfrac{220^2}{60}$
For USA: $R' = \dfrac{110^2}{60}$
$\dfrac{R'}{R} = \dfrac{110^2}{220^2} = \left(\dfrac{1}{2}\right)^2 = \dfrac{1}{4}$
$\Rightarrow R' = \dfrac{R}{4}$
So the 60 W bulb meant for the USA has resistance $\dfrac{R}{4}$.
2000 AIPMT 1 question
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Two bulbs of (40 W, 200 V) and (100 W, 200 V). Then correct relation for their resistance :$P = \dfrac{V^2}{R}$; for the same rated voltage, $P \propto \dfrac{1}{R}$.
$R_{40} = \dfrac{200^2}{40} = 1000\ \Omega$ and $R_{100} = \dfrac{200^2}{100} = 400\ \Omega$
So the bulb of lower power has the higher resistance: $R_{40} \gt R_{100}$.