An electric heater of resistance $10 Ω$ connected to 220V power supply is immersed in water of mass 1kg. How long the heater has to be switched on to increase its temperature from 30⁰C to 60⁰C. (Specific heat of water $= 4200 {JKg}^{-1} C^{-1}$ )

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16s

42s

26s

20s

answer is B.

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Detailed Solution

$\begin{array}{l} Q = ms △ T = I^{2} Rt = \frac{v^{2} t}{R} \Rightarrow t = \frac{ms △ TR}{v^{2}} \\ \Rightarrow t = \frac{1 \times 4200 \times 30 \times 10}{220 \times 220} = 26 .03 s \end{array}$

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