Question

A portable electric water heater transfers 255 watts (W) of power to 5.5 L of water, where 1 W=1J/s. A portable electric water heater transfers 255 watts (W) of power to 5.5 L of water, where 1 W=1J/s.

Answer 1

$T_1=25\degree C;T_2=42\degree C;m=1000\times 5.5 \times 10^{-3}=$ $5.5 \ kg$

$C$ for water$=4.184 \times 10^3 J/Kg$

Energy required to raise this temperature$=$ $mC(T_2-T_1)=5.5\times 4.184 \times 10^3\times (42-25)=$ $391.204\times 10^3 J$

Power given$=255 J/sec$

Time required$=\frac{Energy}{Power}=\frac{391.204\times 10^3}{255}=$ $154.133 \ sec=\frac{154.133}{60}=2.585 \ min$