Question

Two litre of water at inital temperature of ${27}^{o}C$ is heated by a heater of power $1kW$. If the lid of kettle is opened, then heat is lost at the constant rate of $160J/s$. Find, the time required to raise the temperature of water at ${77}^{o}C$ with the lid open (specific of water $4.2kJ/kg$)

• A. $5min$ $40s$
• B. $14min$ $20s$
• C. $8min$ $20s$
• D. $1min$ $10s$

Answer 1

The correct option is C $8min$ $20s$

Rate of heat gain $1000-160=840J/s$

$\therefore$ Required time $=\frac{2\times 4.2\times {10}^3\times (77-27)}{840}=500s=8min20s$