Question

An electric kettle has two heating coils. When one of the coils is connected to an $AC$ source, the water in the kettle boils in $10$ min. When the other coil is used, the water boils in $40$ min. If both the coils are connected in parallel, the time taken by the same quantity of water to boil will be

• A. $25min$
• B. $15min$
• C. $8min$
• D. $4min$

Answer 1

Answer: (C)

We know when kettle are connected in parallel then time taken
$T_{total}=\frac{T_{1} T_{2}}{T_{1} + T_{2}}$
if connected in series
$T_{total}=T_{1}+T_{2}$
Here $T_{1}$ and $T_{2}$ are time taken by kettle seperately boil the same amount of water
so. in parallel
$T_{total}=\frac{T_{1} T_{2}}{T_{1} + T_{2}}=\frac{\left(\right. 10 \left.\right) \left(\right. 40 \left.\right)}{40 + 10}=8min$