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I thought circuits had to be closed

  1. Jan 18, 2015 #1
    1. The problem statement, all variables and given/known data
    I am supposed to determine the potential differences between the points. However, I thought circuits had to be closed...
    2w5n9ck.png
     
  2. jcsd
  3. Jan 18, 2015 #2

    Stephen Tashi

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    Such a circuit would have to be closed in order for current to flow, but not for voltages to exist. If there is no current, then no voltage is dropped across each resistor.
     
  4. Jan 18, 2015 #3

    gneill

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    Staff: Mentor

    Circuits have to be closed for current to flow continuously, but potential difference does not depend upon a closed path or on continuous current flow. It is enough that there is some path between the points so that can do a "KVL walk" from one to the other, adding up the potential changes along the way.
     
  5. Jan 18, 2015 #4
    So how then do I calculate potential difference without current?

    I'd usually use the formula V = I*R.
     
  6. Jan 18, 2015 #5

    gneill

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    Yes, and if I=0?
     
  7. Jan 18, 2015 #6
    Well, then there would be no drop in voltage. Does this mean that the potential difference at all three points is the same?

    EDIT: Well I guess there would be current through R2.

    So then would the potential difference between A and B be 9 V, the difference between A and C be 9 V, and the difference C and B be 0 V?
     
    Last edited: Jan 18, 2015
  8. Jan 18, 2015 #7

    gneill

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    Not quite. The battery will maintain a fixed potential difference regardless of current flow or not.
    Yup.
     
  9. Jan 18, 2015 #8

    Stephen Tashi

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    How do you find potential difference across the two terminals of a battery when they aren't connected ?
    What would the potential difference be in the ends of two unconnected wires that had their other ends connected to opposite terminals of a battery?
     
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