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Voltage - Sign convention problems

  1. Dec 11, 2007 #1
    Hey guys, I need some help clearing up voltage signs, its always confusing me and I have my final this Friday.

    First off I'm not talking about voltage in a circuit, but rather if there is an electric field caused by a charge distribution.

    First off, I understand potential energy for gravity. ANYTHING with a higher height is considered to have a higher potential energy. But with voltage you can have + and - charges. So my confusion is if you have an electric field caused by two oppositely charged plates


    ---------------->
    (+) ------------> (-)
    ---------------->

    1) Which side has higher potential energy? The + or the - side.

    2) So say the potential is 500V, and the book says, now assume the V is -500, how is it possible to have a different potential without switching the charges around?


    I don't know how to really put this in words but if you can figure out what I am confused about then I would appreciate some explanation.
     
  2. jcsd
  3. Dec 11, 2007 #2

    chroot

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    Voltage is a strictly relative concept. You cannot simply say "this point has a voltage of 500V." You must also provide some point of reference which you are using to judge the voltage. For example, you could say "this point has a voltage 500V higher than the other point."

    The electric field is conventionally described by its effects on a positive test charge. Therefore, electric field lines point away from positive charges and towards negative charges. Since voltage is the line integral of those field lines, voltage decreases as you walk along a field line.

    In the picture you've drawn, you are free to label either side of the diagram as having a potential of 0V. If you declare that the left hand side has a potential of 0V, then the right hand side has a potential of -500V. If you declare that the right hand side has a potential of 0V, then the left hand side has a potential of +500V.

    - Warren
     
  4. Dec 11, 2007 #3

    rbj

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    depends on if you're a positively charged object or particle or a negatively charged one. if you're positively charged, you have more potential energy if you are close to the + side. if you're a negatively charged particle, you have more potential energy when you're close to the - side.

    i think that's what happens when you change the voltage of something (relative to the same whatever point of reference): charge moves around.
     
  5. Dec 11, 2007 #4

    chroot

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    To avoid confusing the issue, rbj, remember that it's conventional to consider positive test charges. If you use a positive test charge, it will experience force in the same direction as the electric field line it's on. That makes it all pretty easy to remember.

    - Warren
     
  6. Dec 11, 2007 #5

    rbj

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    oh, all right.

    i still think your avatar is just tits, Warren. that has to be the coolest looking, clearest and most symmetrical nebula around.
     
  7. Dec 12, 2007 #6
    its sort of convention i think... electrons are electricity... and electrons are stored at the negative polarity node... hence in one sense you could say that the negative side is higher potential. however in the words of my high school teacher, we are used to things flowing from positive to negative.. high to low. hence the direction of conventional current is from positive to negative.... but the direction of flow of electrons is from negative to positive.
     
  8. Dec 12, 2007 #7

    chroot

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    tenseiga,

    It is absolutely just a convention; however, it's important to follow the convention if you wish for anyone else to understand you.

    By the way, electrons are not "stored at" the negative terminal of a battery; the battery pushes them in that terminal's direction when current is able to flow, but they're not piled up there by the millions, waiting to be used.

    Don't confuse voltages with concentration of electrons -- they are two entirely different concepts. You can still have a voltage across a wire, even if the electron concentration is the same everywhere in that wire (this is the usual case).

    - Warren
     
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