Question

An electric kettle has two coils. When one coil is connected to the ac mains, the water in the kettle boils in $10minutes$. When the other coil is used, the same quantity of water takes $15minutes$ to boil. The time taken by the same quantity of water to boil if the two coils are connected in parallel is

• A. 6 min
• B. 12 min
• C. 18 min
• D. 24 min

Answer 1

Answer: (A)

6 min

$Q=\frac{E^2\times 10}{R_1}Q=\frac{E^2\times 15}{R_2}$

When connected in parallel

$\Rightarrow Q=E^2[{\frac{1}{R}}_1+\frac{1}{R_2}]\times t$

$Q=[\frac{Q}{10}+\frac{Q}{15}]\times t$

$t=\frac{10\times 15}{25}=6min$