Question

An electric kettle has two coils. When one of these is switched on the water in the kettle boils in $6min$. When the other coil is switched on, the water boils in $3min$. If the two coils are connected in parallel, the time taken to boil water in the kettle is:

• A. 3 minutes
• B. 6 minutes
• C. 2 minutes
• D. 9 minutes

Answer 1

The correct option is C 2 minutes

$Q=\frac{E^2\times 6}{R_1}$

$Q=\frac{E^2\times 3}{R_2}$

$\Rightarrow 2R_2=R_1$

When connected in Parallel

$R_{eff}=\frac{2R_2}{3}$

$\Rightarrow Q=\frac{3E^2\times t}{2R_2}$

$\frac{E^2\times 3}{R_2}=\frac{3E^2\times t}{2R_2}$

$\Rightarrow t=2min$