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Electric potential question

  1. Mar 13, 2010 #1
    hi, in the formula ChangeInPotential = Current X Resistance (formula for change in potential across a resistor) , if the resistor has two ends (point A and point B), how do I know wether it is the change in potential from point A to point B or from point B to point A
     
  2. jcsd
  3. Mar 13, 2010 #2

    Doc Al

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    You'll need to be told the orientation of the voltage source, if that matters. If all you need to find is the current, not its direction, you won't need to know.
     
  4. Mar 13, 2010 #3
    lets say my circuit is a direct circuit and I have a battery with plus and minus terminals connected to a load resistance. there are also wires between the load resistance and the battery terminals. And lets say

    1) I go along the wire starting from the plus terminal all the way to the negative terminal and along the way I pass through points A and B , passing through A first. I know that there is a change in potential as I go from A to B. According to my text book this change in potential is Current X Resistance.

    2) I go along the wire but this time I start at the negative terminal of the battery and end at the positive terminal of the battery. Along the way I pass through points B and A. This time I pass through B first. IT SEEMS the change in potential is also = Current X Resistance which is the same as the change in potential in 1) (going from A first, then B), but how can this be because isnt it generally true that the change in potential from point1 to point2 is different from the change in potential from point2 to point1.
     
  5. Mar 13, 2010 #4

    Doc Al

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    (1) The change in potential from A to B is just the negative of the change in potential from B to A.
    (2) The + terminal of the battery is the higher potential. Thus A is at a higher potential than B.
    (3) The conventional current travels through the circuit from higher to lower potential. So going from A to B the potential drops by an amount equal to current X resistance.

    When they say ΔV = current X resistance, they are often just talking about the magnitudes of those quantities. You put in the signs as explained above.
     
  6. Mar 13, 2010 #5
    cool that was very helpful but how can one talk about the magnitude of ChangeInPotential when current in the formula can be negative?
     
  7. Mar 13, 2010 #6

    Doc Al

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    You can talk about the magnitude of any quantity, if that's all you're interested in.

    But try this. Imagine moving from A to B across the resistor. To find the voltage difference between A and B, including the sign, use: ΔV = -current X resistance. The sign of the current will be + if the current goes from A to B, otherwise it will be negative. In this case, the current is positive, so ΔV is negative.
     
  8. Mar 13, 2010 #7
    So when my book says the "voltage drop as we go from higher potential A to lower potential B across the resistor is equal to Current X Resistance" , it is actually talking about the magnitude of the change in potential? Not the actual change in potential as the words "voltage drop" seems to imply?

    thanks, this helps
     
  9. Mar 13, 2010 #8

    Doc Al

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    Since you are moving in the same direction as the current, the change in potential is negative. As your book says, it's a voltage drop across the resistor.
     
  10. Mar 13, 2010 #9
    thanks you were very helpful. Im going to now read everything you said and try to put it all together.
     
  11. Mar 13, 2010 #10
    As noted in the formula you'll need some information on orientation unless just the magnitude is asked.

    In an actual circuit, if there is a ground symbol shown, the convention is to use that as the negative reference...but in some circuits it Is possible for positive to be the grounded...but that is very uncommon....

    As a further confusion, conventional (positive) current flow gives one reference, actual current flow (negative electrons) gives another....

    After a while you'll get used to the conventions and they become second nature....
     
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