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Relationship between Voltage and Current

  1. Aug 26, 2011 #1
    P = VI . Now say I want to transfer power over long distance, i put it through the the transformer increase the voltage to decrease loss over the line, this is where I am confused.

    So when u increase the voltage through the transformer, you decrease the current. Voltage is the potential difference between two points. Now if there is a more potential difference then before between 2 points shouldn't there be more electrons going through the wire. And thus increase the resistance in the wires and give more loss. Also what doesn't make sense is that the Current decreases as voltage goes up. But then if u look at the equation V = IR, if resistance is constant then as voltage goes up then current goes up?

    This is contradicting each other, I am confused.
  2. jcsd
  3. Aug 26, 2011 #2
    It depends what you are holding constant. If you hold power constant (which is virtually never the case), then V and I are inversely proportional. If you hold resistance constant (which is much more often the case, including power lines), then they are directly proportional.

    In the case of power transmission, the power lines' resistances are almost constant, so much so that for learning purposes you can consider them that way.
  4. Aug 26, 2011 #3
    Well I guess I am mixing up the resistance of the wire to the load. So there will be some power loss through the wires but why does increasing the voltage decrease the loss? Because I thought the more voltage u have the more current you have through a circuit. But I guess I can't compare it to a power grid circuit where the load is always changing. This is so hard to picture it in my head. This is also an alternating current so the voltage is swinging and the electron movement is also increasing and decreasing. Now it all makes sense. So increasing the Voltage RMS value through a transformer will all increase the amount of electrons flowing at peak value. Damn it I am still confused.
  5. Aug 26, 2011 #4
    The power loss in the line is I2Rline....

    so the idea is to keep current low to reduce lines losses.....
  6. Aug 26, 2011 #5
  7. Aug 26, 2011 #6
    The losses incurred in the t-line are as follows.

    Insulation loss = V2Gline....

    Conduction loss = I2Rline....

    For typical t-lines, the conduction loss is many orders of magnitude greater than the insulation loss. By stepping the voltage up with transformers, the current is stepped down. The insulation loss increases but it is tiny to begin with. The conduction loss goes down greatly. That is why the power companies transmit the power at high voltage with low current. The losses are lower this way.

    Some day, high temperature superconductors may be a reality & cheap. That might change things, but for now, this is how it's done.

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