One litre of water in a light thin-walled vessel is heated up under atmospheric pressure by an electric heater with unknown power rating. Initially the temperature of the water is 20°C. After the temperature becomes 60°C, it stops increasing, while the heater is still on. As the heater is unable to boil water, it is turned off. During the first 20 seconds the water becomes 2 degrees cooler.
Estimate the power output of the heater.
(a) 8.40 W.
(b) 420 W.
(c) 1260 W.
(d) 8400 W.