Q. Two electric bulbs have ratings respectively of 25 W, 220 V and 100 W, 220 V. If the bulbs are connected in series with a supply of 440, which bulb will fuse?
A) 25 W bulb
B) 100 W bulb
C) Both of these
D) None of these
Explanation
(a) Resistance of 25 W bulb $\ \ \ = \frac{220\ \times 220}{25} = 1936\ \ \Omega$
It’s safe current $\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ = \frac{220}{1936} = 0.11\ A$
Resistance of 100 W bulb $= \frac{220\ \times \ 220}{100} = 484\ \Omega$
It’s safe current $= \frac{220}{484} = 0.48\ A$
when connected in series to 440 V supply, then the current
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ i = \frac{440}{(1936\ + 484)} = 0.18\ A$
Thus, current is greater for 25 W bulb, so it will fuse.
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.