First slide

Electrical power

Question

If a 30 V 90 w bulb is to be worked on a 120 V line, a resistance of how many ohms should be connected in series with the bulb?

Moderate

A

10 ohm
B

20 ohm
C

30 ohm
D

40 ohm

Solution

Suppose resistance R is connected in series with bulb. Current through the bulb, $i = \frac{90}{30} = 3 A$
Hence for resistance V = iR
$\Rightarrow 90 = 3 \times R \Rightarrow = 30 Ω$

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