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Direction of voltage and electric field

  1. Dec 18, 2008 #1
    Hi guys,

    in a physics book I found the equation E = - grad(U) and in another one E=U/d with E:electric field, U:voltage, d:distance. The minus sign confuses me. Can anybody explain that to me, please?

    I am thinking of a room with a positively charged ceiling and a negatively charged floor. So a positive charge will move towards the floor to reduce the potential energy. In that case, is the current I positive or negative?

    Thanks a lot.
    Basti
     
  2. jcsd
  3. Dec 18, 2008 #2

    ZapperZ

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    The former is the correct general form. The latter is probably a simplification where one is only interested in the magnitude of a constant E-field. It is a simplification of the former.

    Zz.
     
  4. Dec 18, 2008 #3

    uart

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    Yes that's correct, the negative sign tells us the the E vector points in the direction of decreasing voltage.
     
  5. Dec 18, 2008 #4

    jtbell

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    It depends on which direction you call "positive" or "negative." If you call the direction from the floor to the ceiling "positive," then the current is negative in this case. If you call the direction from the ceiling to the floor "positive," then the current is positive in this case.

    When there is any chance of confusion in a situation like this, I prefer to specify the direction explicitly, in this case "downward," instead of saying "positive" or "negative."
     
  6. Dec 18, 2008 #5
    Thank a lot. You're great help.
     
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