Question

An electric kettle (rated accurately at \[2.5{\rm{ }}kW\]) is used to heat \[3kg\] of water from \[15^\circ C\] 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

Hint: When the kettle is turned on, a high amount of electric current in amperes flows through the coil, or the 'heating element', This resistance turns electrical energy into heat as it passes through the coil. The heat brings the water inside it to boiling point.

Step by step answer: Inside each electric Kettle there is a metal coil. Electrical energy/high current travels through the coil, turning into heat and warming the cold water inside it.Some electric kettles use a thermostat as its main component, which helps them to switch off when the water reaches the right temperature and when the required heat is achieved.
Given: Electric kettle works on power 2.5 kW and used to heat 3 kg of water from 15 to boiling point. At time 9.5 min
Energy consumed by electric kettle in time \[9.5min = Pt = (2.5 \times ) \times {10^3} \times \left( {9.5 \times 60} \right) = {\rm{ }}14.25 \times {10^5}J\]
Energy usage consumed = \[ms\Delta T = 3 \times (4.2 \times {10^3}) \times \left( {100 - 15} \right) = 10.71 \times \;\;{10^5}\]
where \[s = 4.2J/g^\circ C\] = specific heat of water and boiling point temp of 100 degree.
Heat lost = \[14.25 \times {10^5} - 10.71 \times {10^5} = 3.54 \times {10^5}\]
Therefore, the Maximum amount of Heat which is lost given by \[3.5 \times {10^5}J\]

Which is Option A. \[3.5 \times {10^5}J\].

Note: If you were not as lucky, the kettle element might burn out all the components, blow a fuse, or even start a fire. Thankfully, virtually all modern days kettles switch themselves off automatically using thermostats in it(mechanical, electrical, or electronic devices that respond to changes in temperature)