Question

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

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

Answer 1

Answer: (D)

8 min

Answer 2

By using the formula $t_{p} = \frac{t_{1}t_{2}}{t_{1} + t_{2}}$

(as we discussed in theory)

⇒ $t_{p} = \frac{10 \times 40}{(10 + 40)} = 8min$

Note : In this question if coils are connected in series then the time taken by the same quantity of water to boil will be t_s = t₁ + t₂ = 10 + 40 = 50 min