SATHEE: Current Electricity - Result Question 78

Current Electricity - Result Question 78

83. If $25 W, 220 V$ and $100 W, 220 V$ bulbs are connected in series across a $440 V$ line, then

[2001]

(a) only $25 W$ bulb will fuse

(b) only $100 W$ bulb will fuse

(d) none of these

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Answer:

Correct Answer: 83. (a)

Solution:

(a) As for an electric appliance $R = (V_{s}^{2} / W)$ , so for same specified voltage $V_{s}$

$\frac{R_{25}}{R_{100}} = \frac{100}{25} = 4$

i.e, $R_{25} = 4 R$ with $R_{100} = R$

Now in series potential divides in proportion to resistance. So, $V_{1} = \frac{R_{1}}{(R_{1} + R_{2})} V$

i.e., $V_{25} = \frac{4}{5} \times 440 = 352 V$

and $V_{2} = \frac{R_{2}}{(R_{1} + R_{2})} V$

i.e., $V_{100} = \frac{1}{5} \times 440 = 88 V$

From this, it is clear that voltage across $100 W$ bulb $(= 88 V)$ is lesser than specified $(220 V)$ while across $25 W$ bulb $(= 352 V)$ is greater than specified $(220 V)$ , so, $25 W$ bulb will fuse.

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Current Electricity - Result Question 78

83. If 25W,220V and 100W,220V bulbs are connected in series across a 440V line, then

Answer:

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83. If $25 W, 220 V$ and $100 W, 220 V$ bulbs are connected in series across a $440 V$ line, then