MadSci Network: Physics

Re: should the resistance in a heater be high or low

Date: Sat Apr 18 07:26:03 1998
Posted By: Lawrence Skarin, Faculty, Electrical Engineering, Monroe Community College
Area of science: Physics
ID: 892829481.Ph

Thank you for your question. The power formula you quote is v^2/R. With Ohm's Law, this can also be written as i^2*R or v*i. Regardless, the expression yields the power in watts being dissipated as heat in a resistance. Nichrome is a high-resistance wire often used in toasters. If you were to replace the nichrome with a low-resistance wire like copper, the toaster would draw too much current and get too hot -- blowing fuses or popping breakers, and probably melting itself in the process. You correctly figured that out with your v^2/R formula. So the high-resistance wire is used not because it's a more efficient heat producer. It's used because it limits the current to a safe value and can stand the heat. Transmission lines not following the same power rules? Au contraire! They do. But the wire for transmission lines -- its job is to carry electric power -- not change it into heat like toaster filaments. So the trick there is to transform the voltage up so the current goes down. Then i^2R shows you you do your best with low resistance. This saves as many watts as possible for your load centers. To see how well this works, think of the recent horrendous ice storm that knocked down power lines in Quebec and eastern Ontario. The transmission lines were not even warm enough to keep ice from forming. The resulting weight collapsed many towers. This was devastating in a province so dependent on electricity. Thanks again for your question. Larry Skarin

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