The minimum number of identical bulbs of ...

CBSE Class 10 Science 2025 Solved Paper All Sets

Question : 14 of 20

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The minimum number of identical bulbs of rating 4 V; 6 W, that can work safely with desired brightness, when connected in series with a 240 V mains supply is :

Solution:

R = \frac{V^2}{P} = \frac{4 \times 4}{6} = \frac{8}{3} \Omega

I_{\max} = \frac{V}{R} = \frac{4 \times 3}{8} = \frac{3}{2} \text{A}

Now,

I_{\max} = \frac{V}{nR}

\frac{3}{2} = \frac{240 \times 3}{n \times 8}

n = 60

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