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Homework Help: Potential and Test Charges

  1. Feb 8, 2008 #1
    I have a few general questions which I need some clarification on.

    1. What is really the relationship between potentials and test charges? I know how they relate to capacitance, but not really to each other. For example would a charge of 5mC be drawn to a potential of -7V or 7V?

    2. I know that two equal charges, like +8 and +8, will cancel each other, but I want to clarify if +6 and -6 cancel as well. I am thinking that they do because they are still of equal force and are in opposite directions, but I just want to make sure.

    3. If you have a line with +3 and -3 and you place a charge of -7 between them do they have the same force on the -7 charge, but are both in the same direction because one attracts and one repels? So, for example, if the force from the +3 on the -7 was 2 N, would the combined forces be 4 N to the direction of the +3?
  2. jcsd
  3. Feb 8, 2008 #2


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    For 3) (which makes the most sense), yes, both forces will act in the same direction. The will only have the same magnitude if the -7 charge is the same distance from both of the 3 charges. For 2) opposite charges cancel if they are sitting on top of each other. Otherwise, they still produce an electric field, just as in 3), the field only cancels if you are directly between them. For 1) (which makes the least sense) charges aren't 'drawn to' potentials. They are pulled by electric fields, which are produced by a gradient in potential.
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