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Homework Help: Electricity- Power dissipation formula question

  1. Jun 20, 2009 #1
    1. The problem statement, all variables and given/known data
    while surfing the internet i stumbled upon the following question:
    Why ac(alternating current) is used to power houses instead of DC?

    Answer:The answer lies in the transmission lines for the electricity. The wires have a defined resistance, and when current passes through them there is a power loss given by
    W (power) = I (current) squared x R (resistance)

    Clearly, lowering the current reduces the power loss considerably. The power transmitted is given by
    W = V (voltage) x I, so we could multiply the voltage by 100, reduce the current by 100, transmit the same power, and reduce the loss in the wires by 10000.
    This high voltage is desperately unsafe in the domestic situation, so it must be reduced. Reducing DC is very difficult, so it is much easier to use AC.

    2. Relevant equations
    How can one lower the current without lowering the voltages and, at the same time,without changing the resistance of the wires?
  2. jcsd
  3. Jun 20, 2009 #2


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    Homework Helper

    Please go through the principle of step up and step down transformer.
  4. Jun 20, 2009 #3


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    Science Advisor

    Power must be conserved so that power in must equal power out (taking into account losses). Since power, in its simplest form, equals current multiplied by voltage you can readily see what must necessarily happen to the voltage when the current is reduced.

    There's your answer.
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