A 420-W immersion heater is placed in a pot containing 2.10 litres of water at . We have to find (a) the time required for bringing the water to boiling temperature, assuming that 77.0% of the available energy is absorbed by the water. We will calculate how much longer will it take for half the water to boil away.

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(a)

Specific heat capacity of water is

.

Therefore, the amount of heat required for boiling 2.10 litres of water from will be

Power of the immersion heater is given as

As only 77% of the available energy is absorbed by the water, the Joule heat absorbed by the water per second will be

Therefore, the amount of time required for boiling the given amount of water using the immersion heater will be

(b)

For answering the next part of the problem, we note that the heat of vaporisation of water is

Therefore, the amount of heat required for vaporisation of 1.05 litres of water will be

The additional time required for boiling away half the quantity of water will be