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Power voltage current?

  1. Aug 3, 2008 #1
    hello, I'm new here =D
    dunno if this is the right section to ask this
    I'm really confused about power voltage and current

    according to ohm's law, V = I x R
    therefore if we increase voltage, the current will increase

    but according to the power formula, P = V x I
    they say increasing voltage to deliver electricity to houses, will decrease the current therefore decreasing the power loss along the way
    as the resistance across the wire will not change (at least not significant) therefore it still follow the ohm's law

    how can this be possible?

    thanks
     
  2. jcsd
  3. Aug 3, 2008 #2

    rock.freak667

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    Homework Helper

    From Ohm's law and P=VI

    Powerloss,P=I2R. Usually they transfer electricity at high voltage/low current. Low current=> Low powerloss in a given resistance.
    High Current=> High powerloss in a given resistance.

    It's more feasible to transfer it at high voltage/low current. It doesn't mean that because V is high will mean that I will be low, power companies could just transfer it at high voltage/high current. But that won't be good.
     
  4. Aug 3, 2008 #3
    hmm that means that the power company can control both the voltage and the current transferred?

    that does make sense =D

    thanks
     
  5. Aug 3, 2008 #4

    rock.freak667

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    Homework Helper

    I'm not too sure of the extent to which they can control it, I think it's only high voltage/high current OR low voltage/high current they can transfer it in. Ignore the high voltage/high current part.

    You could also check the formula for the ideal transformer where

    [tex]\frac{V_s}{V_p}=\frac{I_i}{I_s}[/tex]

    Where Ip=Current in the primary circuit
    Is= Current in secondary circuit.
    Vp=Voltage in the primary circuit
    etc.

    The ratio of Vs/Vp is constant such that if Vs is high it means that Is would be low and so forth
     
  6. Aug 3, 2008 #5

    russ_watters

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    Staff: Mentor

    You're just mixing scenarios, that's all. They are different equations for different situations.

    If you have a light and you vary the voltage (assuming you don't burn it out), you can use both equations and you'll find that you get more current and therefore more power as the voltage increases.

    But the power company doesn't want to vary the voltage at your house, they want to keep it at 120, but design a system that loses less along the way. So the power is fixed as a matter of practicality. So when they design the transmission lines and can choose the voltage they want, they pick a higher voltage to lower the amperage and decrease the IR2 losses. But that doesn't mean the power increases: at your house, you still get 120V.
     
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