Question

The time taken by an electric heater to rise the temperature of $100cc$ of water through ${10}^{o}C$ is $7s$. If there is no loss in energy, power of that motor is ($J=4.2J/cal$)

• A. $420W$
• B. $42W$
• C. $4.2W$
• D. $600W$

Answer 1

The correct option is D $600W$

$\text{100cc of water has mass of 100 gram}$

$\Delta t={10}^{0}C$ , specific heat for water is $1cal/gm$ so heat in calorie for heating 100 gm water by temperature $\Delta t$ is $Q=ms\Delta t=100\times 1\times 10=1000cal$

the heat in Joule is $1000\times 4.2=4200J$

Time taken is 7 second so $Power=Q/t=4200/7=600W$