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Homework Help: Question about voltage sign

  1. Apr 8, 2012 #1
    1. The problem statement, all variables and given/known data

    I know that the work done by an electrostatic force is given by :

    W = Fdcosθ (scalar product)
    W = qE d cosθ

    and I know that the electric potential is V = -W/q ,
    where the work here is the work done by the electrostatic force.

    2. The attempt at a solution

    I have found that the work done by the electrostatic force is positive ,whether it is attraction or repulsion ( of course when the displacement is with the direction of the force)
    (to see that apply the work equation to a positive charge +q and negative charge -q)

    with that result ( when electrostatic force acts alone , the work done is +ve whether attraction or repulsion) ,

    we (or I ) see that V = -W/q is negative when q is positive and positive when q is negative

    why is that ?
    and , how to interpret that (V is -ve for +ve charge , V is +ve for -ve charge) ?
    I am looking for a conceptual answer , I do not need so much mathematical explanation.

  2. jcsd
  3. Apr 9, 2012 #2


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    Homework Helper

    Yes, if a charged particle moves in the direction of the electrostatic force on it, then the work done by the electrostatic force on the charged particle is positive.

    I'm not sure about V=-W/q ... I use V=W/q ... maybe your teacher was explaining specifically about an electron, which has negative charge, and using q to mean the absolute value of the electron's charge? Or maybe he was using W to mean work done by the system on the electromagnetic field?
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