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Coulomb's law with more point charges than one

  1. Jun 23, 2011 #1
    Hi everyone,

    I got a little question. Let's say we have 3 point charges on a line, for example on the x axis.


    something like this. If I want to calculate the Force on point charge one, why am I allowed to just add the forces 12 and 13. I saw it in a book and was confused. Why don't I have to consider the interaction between the particel 2 and 3?

    Thanks for your help.
  2. jcsd
  3. Jun 23, 2011 #2

    Doc Al

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    Staff: Mentor

    Why would the force that particles 2 and 3 exert on each other matter to particle 1? All you care about are the forces on particle 1.
  4. Jun 23, 2011 #3

    1: -
    2: -
    3: +

    I thought that 2 and 3 attract each other and the distance between 1 and 2 increases, because 1 and 2 repel aswell. Further distance -> force decreased. Or do I have to handle the charges like they are on a constant place? That's what I was wondering about.
  5. Jun 23, 2011 #4

    Doc Al

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    Staff: Mentor

    You'll usually be given the locations of the charges. Assume they are fixed in place unless told otherwise. In any case, you are probably asked to find the force on particle 1 when the charges are in the locations given. If later they move, who cares? That's a different problem.
  6. Jun 23, 2011 #5
    It's called superposition and its a wonderful thing as it makes calculation so much easier that can be applied in many aspects of physics.

    Basically superposition says that each effect taken separately can simply be added togerther.

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