• # question_answer An electric kettle takes 4 A current at 220 V. How much time will it take to boil 1 kg of water from temperature$\text{20 }\!\!{}^\circ\!\!\text{ C}$? The temperature of boiling water is$\text{100 }\!\!{}^\circ\!\!\text{ C}$. A)  6.3 min       B)  8.4 minC)  12.6 min     D)  4.2 min

Correct Answer: A

Solution :

Heat evolved due to Joule's effect is used up in boiling water. $VIt=ms\Delta t$or $t=\frac{ms\Delta t}{VI}$ We are given $I=4A,V=200\,volt,\,m=1\,kg,$ $\Delta t=(100-20){{\,}^{o}}C,s=4200\,J/kg{{\,}^{o}}C$ $\therefore$ $t=\frac{1\times 4200\times 80}{220\times 4}=6.3\,\min$

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