Water boils in an electric kettle in 15 minutes after switching on. If the length of the heating wire is decreased to `2//3` of its initial value, then the same amount of water will with the supply voltage in

Answer

`10 minutes `

Explanation

`H = (V^(2)t_(1))/(R_(1)) = (V^(2)t_(2))/(R_(2)) or (R_(1))/(R_(2)) = (l_(1))/(l_(2))` ….(i)
But ` R prop 1, so (R_(1))/(R_(2)) = (l_(1))/("l_(2))`….(ii)
Now `l_(2) = (2//3) l_(1)`
From (i) and (ii)
`(l_(1))/(l_(2)) = (r_(1))/(r_(2)) or (3)/(2) = (15)/(t_(2)) or l_(2) = 10 minutes `