# A water heater marked 2 kW, 220 V is connected to a 220 V supply line. How long will it take to heat 20 kg of water from 10Â°C to 30Â°C? Assume that all the heat is taken up by the water.

$\begin{array}{l} P=2 K W \\ Energy=P \times t=m s \Delta t \\ \Rightarrow t=\frac{m c \Delta t}{P}=\frac{20 \times 4200 \times(30-10)}{2 \times 10^{3}}=840 \sec =14 \mathrm{min} \end{array}$

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