A 220 V, 50 Hz AC source is connected to a 25 V, 5 W lamp and an additional resistance R in series (as shown in figure)

to run the lamp at its peak brightness, then the value of R (in ohm) will be__________.

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Vishal Baghel | Contributor-Level 10

6 months ago

for lamp, $P = \frac{v^{2}}{R}$

$\Rightarrow R = \frac{25 * 25}{5} = 125 Ω$

$l_{m a x} = \frac{v}{R} = \frac{25}{125} = \frac{1}{5} A$

$\Rightarrow I_{m a x} = \frac{220}{125 + R} = \frac{1}{5}$

$\Rightarrow 1100 = 125 + R$

R = 975 $Ω$

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A 220 V, 50 Hz AC source is connected to a 25 V, 5 W lamp and an additional resistance R in series (as shown in figure) to run the lamp at its peak brightness, then the value of R (in ohm) will be__________.

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