1 Given that the rating of an electric kettle is "220V , 1500w" and its and its
effciency is 80%. It takes 12 minutes to heat up n kg of the water ftom
20℃ to boiling point.
(a) The effciency is usually lower than 100%. Explain briefly.
(b) Find n (Specific heat capacity of water = 4200Jkg-10C-1)
physics
2011-10-22 12:07 am
回答 (1)
2011-10-22 12:25 am
✔ 最佳答案
(a) There is energy spent on heating up the kettle itself. Hence, the energy transferred to the water is always less than the energy produced by the kettle, leading to an efficiency lower than 100%.(b) Energy absorbed by water
= n.(4200).(100-20) J = 336,000n J
Energy given to water by the kettle
= 1500 x (12 x 60) x (80/100) J = 864 000 J
Hence, 336000n = 864000
i.e. n = 2.57 kg
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