Discussion
Two electric bulbs marked 25W ? 220V and 100W ? 220V are connected in series to a 440Vsupply. Which of the bulbs will fuse?
Answer: A
The 25 W bulb will fuse.
Resistance from the rating: \[R = \frac{V^2}{P} \Rightarrow R_{25} = \frac{220^2}{25} = 1936\ \Omega,\quad R_{100} = \frac{220^2}{100} = 484\ \Omega\]
In series they carry the same current, and the voltage divides in proportion to resistance, so the 25 W bulb takes four-fifths of 440 V - about 352 V, far above its rating. In series, the lower-wattage bulb always gets the larger share.
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