a. It would cost a man Rs. 3.50 to buy 1.0 kW h of electrical energy from the Main Electricity Board. His generator has a maximum power of 2.0 kW. The generator produces energy at this maximum power for 3 hours.Calculate how much it would cost to buy the same amount of energy from the Main Electricity Board.

b. A student boils water in an electric kettle for 20 minutes. Using the same mains supply he wants to reduce the boiling time of water. To do so should he increase or decrease the length of the heating element? Justify your answer.


a. E = P X T

SO, E = 3X2 = 6 kWh (1 mark)

Cost of buying electricity from the main electricity board = 6 x 3.50 =

Rs. 21.0

b. To reduce the boiling time using the same mains supply, the rate of heat production should be large. We know that P= V2/R. Since V is constant, R should be decreased. Since R is directly proportional to l so length should be decreased.

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Maninder Singh

CA Maninder Singh is a Chartered Accountant for the past 14 years and a teacher from the past 18 years. He teaches Science, Economics, Accounting and English at Teachoo