An electric kettle has two thermal coils. When a current is passed in one of them, water in the kettle boils in 6 minutes and when current is passed in other one, the water boils in 8 minutes.
If both the coils are joined in parallel, then how long will it take water to boil ?
Answer & explanation
Correct answer: option 2
Let Power of coils are P1 and P2 respectively then
$R_1= \frac{V^2}{P_1} \text{ and } R_2 = \frac{V^2}{P_2} \text{ respectively.}$
$ t_1 = \frac{H}{P_1}$
$ t_2 = \frac{H}{P_2}$
In Parallel Combination$ P = P_1 + P_2 $
$ t_p = \frac{H}{P} = \frac{H}{P_1 + P_2} = \frac{t_1t_2}{t_1+t_2} =\frac{6\times 8}{6+8}= 3.43min.$