Question

An electric kettle takes 4 A current at $220V$. How much time will it take to boil $1kg$ of water from temperature $20^{\circ }C?$ The temperature of boiling water is $100^{\circ }C$.

• A. 12.6 min
• B. 4.2 min
• C. 6.3 min
• D. 8.4 min

Answer 1

Answer: (C)

Heat taken by water when its temperature changes from $20^{\circ }C$ to $100^{\circ }C$ is
$H_1=ms\left({\theta }_2-{\theta }_1\right)=1000\times 1\times (100-20)$ cal
$=1000\times 80\times 4.2J$
Heat produced in time $t$ due to current in resistor is
$H_2=VIt=220\times 4tJ$
As per question, $220\times 4\times t=1000\times 80\times 4.2$
or $t=\frac{1000\times 80\times 4.2}{220\times 4}=382s=6.3$ min