Question

An electric kettle has two coils. When one of them is switched on, the water in the kettle boils in 6 minutes. When the other coil is switched on, the water boils in 3 minutes If the two coils are connected in series 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 (D) 9 minutes

Explanation:

Heat produced = (V²/R)t i.e. H = {(V²t)/R}

hence R = {(V²t)/H}

Now (V²/H) = constant hence R ∝ t

R ∝ time.

Hence when connected in series time will increases.

R = R₁ + R₂ hence

t = t₁ + t₂

t = 3 + 6 = 9 minutes