First slide

Thermal effect of current

Question

Three electric bulbs rated 40, 60 and 100 watts respectively are connected in parallel with 230 volts d.c. mains. The current (in amp) through the 60 W bulb and the effective resistance (in ohm) of the circuit will be respectively, approximately:

Moderate

A

0.23 A
B

329 $Ω$
C

0.26 A
D

265 $Ω$

Solution

$\begin{array}{l} R = \frac{v^{2}}{P} \\ \frac{1}{R_{eq}} = \frac{1}{V^{2}} (P_{1} + P_{2} + P_{3}) \\ R_{eq} = \frac{230 \times 230}{40 + 60 + 100} = \frac{230 \times 230}{200} = 265 Ω \end{array}$
$Current in 60 W bulb = \frac{P}{V} = \frac{60}{230} = 0 .26 amp$

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