Q. An electric kettle has two heating coils. When one coil is used, water in the kettle boils in 5 minutes, while when second coil is used, same water boils in 10 minutes. If the two coils, connected in parallel are used simultaneously, the same water will boil in time
A) $3\min{20\ sec}$
B) $5\ min$
C) $7\min{30\ sec}$
D) $2\min{30\ sec}$
Explanation
(a) In parallel $\frac{1}{t_{p}} = \frac{1}{t_{1}} + \frac{1}{t_{2}} \Rightarrow t_{p} - \frac{t_{1}t_{2}}{t_{1} + t_{2}}$
$= \frac{5 \times 10}{5 + 10} = \frac{50}{15} = 3.33\ \min = 3\ min.\ 20sec$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.