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Homework Help: Utility companies AC instead of DC?

  1. Jul 24, 2005 #1
    Utility companies AC instead of DC??

    Why do utility companies transmit electric power via AC instead of DC???
  2. jcsd
  3. Jul 24, 2005 #2


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    One reason is so that they can transmit the power at high voltages through the power lines. This leads to less line loss. Then they can use transformers, which work with AC but not DC, to step down the voltage for consumer usage.
  4. Jul 24, 2005 #3


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    Edison originally advocated purely D.C. power with every home having a bank of batteries, that they would take to a central location to have recharged, to run their lights and so on. Westinghouse advocated A.C. which could be generated in a central location and then sent by wire to individual homes. Edison maintained that running the high voltage into homes would be too dangerous. Westinghouse argued that requiring people to transport wet-cell batteries (like automobile batteries) to have them recharged was dangerous and difficult. Once Westinghouse was able to transform A.C. from high to low voltage so that the high voltage did not have to go into the home, he won.
    (Although if electric cars catch on, Edison's method will become popular also.)
  5. Jul 24, 2005 #4


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    Last edited: Jul 24, 2005
  6. Jul 24, 2005 #5
    Are there any countries that use primarily DC?
    I'm sure this is probably not true but this guy I used to work with told me that he thought the big reason we use AC over DC is money. He said things about how for DC the one thing that you really need is just resistors for changing things like voltage. For AC you need all sorts of things like transformers and what not for stepping down/up voltage. He thought it was simply a matter of money, if you need transformers than you need people to make them and you see where this all leads.
    I can't say, based on my own knowledge whether this is true or not. Maybe someone else here can?
  7. Jul 24, 2005 #6


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    Almost certainly not. Different countries use different voltages and frequencies, but all use AC.

    Almost all engineering decisions boil down to money at the most basic level. Indeed, if (all aspects considered) DC was cheaper than AC, then I'm sure that's what we'd be using. But the cost in terms of equipment (generating equipment, transformers, substations etc) would be insignificant compared with the cost of the massive resistive losses which would accompany a purely DC system. DC just doesn't make sense, practically or economically, for widespread power transmission networks.
  8. Jul 24, 2005 #7


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    If you want to transmit DC, then you will need a much thicker power line. DC povides low voltage/high current. If you convert P=IV, then to handle more Amps you need a thicker power line.

    For AC, you can transmit very little current. To compensate this a very high voltage is used- example 200KV. But since the current is small, it can all be transmitted in one small cable.

    It's much simpler to transmit using thinner power lines.
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