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 4.2kJkg−1K−1) 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 isQ=2×4.2×103×55=4.62×105JIf t is the time taken, heat energy supplied by the heater in time t isQ1=( power × time )=1000 t joule Heat energy lost in time t isQ2=160 t joule Heat energy available for heating water isQ=Q1−Q2=840 tEquating Q=Q′, we get t≃550s.