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JEE Main 2024 (8 Apr, Shift 2) · Electrical Power & Heating Effect

Electrical Power & Heating Effect question from JEE Main 2024 (8 Apr, Shift 2)

Q16JEE Main 2024 · 8 APR · SHIFT 2Medium

Electrical Power & Heating Effect

Water boils in an electric kettle in 20 minutes after being switched on. Using the same main supply, the length of the heating element should be _________ to __________ times of its initial length if the water is to be boiled in 15 minutes.

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Answer: (c)

Heat needed is fixed: $H = \frac{V^2}{R}t$ , so with the same $V$ , $t \propto R \propto L$ .
$\frac{L'}{L} = \frac{t'}{t} = \frac{15}{20} = \frac{3}{4}$
So the length must be decreased to $\frac{3}{4}$ of its initial value: answer (c).

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