Q.12.    Several electric bulbs designed to be used on a 220 V electric supply line, are rated 10 W. How many lamps can be connected in parallel with each other across the two wires of 220 V line if the maximum allowable current is 5 A?

 

Answers (1)

Given: V=220V and P=10W

R=\frac{V^2}{P}=\frac{220^2}{10}=4840\Omega

Let x be the number of bulbs.

I = 5A              and      V=220V

R=\frac{V}{I}=\frac{220}{5}=44\Omega

For x bulbs of resistance 4680 Ohm, are connected in parallel to obtain 44 Ohm equivalent resistance.

Thus,

         \frac{1}{R}=\frac{1}{4840}+\frac{1}{4840}+.......................to\, \, x\, \, times

\Rightarrow \frac{1}{44}=\frac{x}{4840}

\Rightarrow x=\frac{4840}{44}=110

Hence, 110 bulbs  of 4840 Ohm are connected in parallel to obtain 44 Ohm

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