A) 40 W bulb
B) 60 W bulb
C) Both will work
D) 20 W bulb
E) None of these
Correct Answer: A
Solution :
Resistance of 40 W bulb\[=\frac{240\times 240}{40}\] \[=1440\,\,\Omega \] Its safe current\[=\frac{240}{1440}=0.167\,A\] Resistance of 60 W bulb \[=\frac{240\times 240}{60}\] \[=960\,\Omega \] Its safe current\[=\frac{240}{960}=0.25\,A\] When connected in series to 420 V supply, then the current \[i=\frac{420}{1440+960}\] \[=\frac{420}{2400}\] \[=0.175\,A\] Thus, current is greater for 40 W bulb, so it will fuse.
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