First slide

Electrical power

Question

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

Moderate

A

3 min 20 sec
B

5 min
C

7 min 30 sec
D

2 min 30 sec

Solution

In parallel $\frac{1}{t_{p}} = \frac{1}{t_{1}} + \frac{1}{t_{2}} \Rightarrow t_{p} = \frac{t_{1} t_{2}}{t_{1} + t_{2}}$
$= \frac{5 \times 10}{5 + 10} = \frac{50}{15} = 3 .33 \min = 3 \min 20 \sec$

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