JEE MAIN - Physics (2022 - 27th June Morning Shift - No. 20)
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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Answer
975
Explanation
$${R_b} = {{{{(25)}^2}} \over 5} = 125\,\Omega $$
$${I_{rms}} = \sqrt {{5 \over {125}}} = {1 \over 5}A$$
$$ \Rightarrow {{220} \over {R + 125}} = {1 \over 5}$$
$$ \Rightarrow R = 1100 - 125$$
$$ = 975\,\Omega $$
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