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Question
Water in an electric kettle connected to a 220 V supply took 5 minutes to reach its boiling point. How long will it take if the supply voltage falls to 200 V?
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Solution
P = `"V"^2/"R"`
Heat gained = `("V"^2)/"R" xx "t"`
`("V"_1^2/"R") xx "t"_1 = ("V"_2^2/"R") xx "t"_2`
t2 = `("V"_1/"V"_2)^2 xx "t"_1`
`= (220/200)^2 xx 300 = 363 "s"`
= 6.05 min
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