A heater boils a kettle of water in 20 minutes. The heater coil is stretched to double the length and connected to the same source. The time to boil water now is?

Dear Student,
Please find below the solution to the asked query:

If length is doubled then the resistance id doubled. And the heater is connected to the fixed voltage source. So in this case heat output is given by V2R, so the heat output will be half if resistance is doubled. So the heater will take 40 minute to boil the water. 

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