First slide

Two litres of water (density = 1 g/ml) in an open lid insulated kettle is heated by an electric heater of power 1 kW. The heat is lost from the lid at the rate of 160 J/s. The time taken for heating water (specific heat capacity 4.2kJkg1K1) from 20°C to 75°C is ___seconds.


Mass of 2 litres of water = 2 kg. Heat energy needed to raise the temperature of2 kg of water from 20°C to 75°C is


If t is the time taken, heat energy supplied by the heater in time t is

Q1=( power × time )=1000 t joule 

Heat energy lost in time t is

Q2=160 t joule 

Heat energy available for heating water is

Q=Q1Q2=840 t

Equating Q=Q, we get t550s.

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