Given quantity of water is boiled by an electric heater in 5 min. If supply voltage of heater reduces to half then time taken to boil the same quantity or water will be ...... min. (Assume the resistance of the beater remaining constant)

Options

A. 40
B. 20
C. 10
D. 2.5

Answer

B

Explanation

20
H = `I^(2) Rt = (V^(2))/(R^(2)) xx Rt = (V^(2))/(R) t `
In t =`(HR)/(V^(2)) `, HR remains same.
`therefore t prop (1)/(V^(2))`
`therefore (t_(2))/(t_(1)) = ((V_(1))/(V_(2)))^(2) `
= `((V)/((V)/(2)))^(2)` = 4
`therefore t_(2) = t_(1) xx 4 = 5 xx 4 ` = 20 min