How Long Will it Take to Raise the Temperature? Please help me!?

Question

A heating coil inside an electric kettle delivers 2.8 kW of electrical power to the water in the kettle. How long will it take to raise the temperature of 0.50 kg of water from 29.0°C to 72.0°C?

Answer 1

the total change in energy is = mass x heat capacity x change in temp.
.5 kg x 4.187 kJ/KgK x (72-29)K
that give the number of kiloJoules needed. the time is related by
power = energy / time where 1W = 1J/sec
time = energy (determined) / power (given)
remember to keep the units correct (kJ vs J, W vs kW))

Answer 2

As 1J/s = 1 Watt, then...
2.8 kW = 2.8kJ/s

Heat absorbed by the water: -
= 0.5 kg x 4.184kJ/kg/°C x 43°C ΔT
= 90 kJ
90kJ ÷ 2.8Kj/s = 32 seconds.