Q. A coil takes 15 min to boil a certain amount of water; another coil takes 20 min for the same process. Time taken to boil the same amount of water when both coils are connected in series
A) 5 min
B) 8.6 min
C) 35 min
D) 30 min
Explanation
(c) From Joule’s law,
$\ \ \ \ \ \ \ \ H = \frac{V^{2}t}{RJ}\ cal$
Where $V$ is potential difference, $t$ the time and $R$ the resistance.
$\ \ \ \ \ \ \ \ R = R_{1} + R_{2}$
Let $\frac{t}{R} = \frac{HJ}{V^{2}} = \ constant = k$
$\Rightarrow R_{1} = kt_{1}\ and\ R_{2} = kt_{2}$
$\therefore\ kt = kt_{1} + kt_{2}$
Or $t = 15 + 20$
$\ \ \ \ \ \ \ t = 35\ \min$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.