Q. An immersion heater is rated $836\ watt.$ It should heat 1 $litre$ of water from $10℃$ to $40℃$ in about
A) $200\ sec$
B) $150\ sec$
C) $836\ sec$
D) $418\ sec$
Explanation
(b) $W = JH \Rightarrow P \times t = J \times ms\ \mathrm{\Delta}\theta$
$\Rightarrow t = \frac{J \times m \times s\mathrm{\Delta}\theta}{P}\ \ \lbrack For\ water\ 1\ litre = 1\ kg\rbrack$
$\Rightarrow t = \frac{4.2 \times 1 \times 1000 \times (40 - 10)}{836} = 150\ sec$
Short Trick : use formula $t = \frac{4200 \times m \times \mathrm{\Delta}\theta}{P}$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.