Question

An electric kettle (rated accurately at 2.5 kW) is used to heat 3 kg of water from 15 C to boiling point. It takes 9.5 min. Then the amount of heat that has been lost is

• A. $3.5\times {10}^{5}J$
• B. $3.7\times {10}^{8}J$
• C. $3.5\times {10}^{4}J$
• D. $7\times {10}^{8}J$

Answer 1

The correct option is A $3.5\times {10}^{5}J$

Energy consumed by electric kettle in 9.5 min $=Pt=(2.5\times {10}^3)\times (9.5\times 60)=14.25\times {10}^{5}J$

Energy usefully consumed $=ms\Delta T=3\times (4.2\times {10}^3)\times (100-15)=10.71\times {10}^5$ where $s=4.2J/g^{o}C=$ specific heat of water and boiling point temp$={100}^{o}C$

Heat lost $=14.25\times {10}^5-10.71\times {10}^5=3.54\times {10}^5$