Question

Water in an electric kettle connected to a 220 V supply took 5 minutes to reach its boiling point. How long would it have taken if the supply voltage had fallen to 200 V?

Answer 1

P = `V^2/R`

Heated gained =`(V^2/R)xx t`

`(V_1^2/R)xx t_1` = `(V_2^2/R)xx t_2`

`t_2 = (V_1/V_2)^2 xx t_1`

 = `(220/200)^2 xx300 = 363s=6.05 min`