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Homework Help: (Simple Electromagnetics) A charge placed between 2 charges

  1. Jan 12, 2016 #1
    1. The problem statement, all variables and given/known data
    A charge (q) is placed in-between two other equal charges named Q.
    a) If both Q's are positive, is charge q in equilibrium? If yes, is it a stable or unstable equilibrium?
    b) What about if one Q is negative while the other is positive?

    3. The attempt at a solution
    a) I think the charge q will be in equilibrium, considering the attraction or repulsion forces will cancel out. However, I have no clue how to know if it is unstable or stable.

    b) q will not be in equilibrium, the forces will be in the same direction and thus not cancel out, giving q an acceleration.
  2. jcsd
  3. Jan 12, 2016 #2


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    I'm not sure about the answer to a. It depends whether tjis is to be treated as purely a one dimensional problem or whether other dimensions should be taken into account.
    What is the criterion for an equilibrium to be stable?
  4. Jan 12, 2016 #3
    It is one-dimensional, and nothing to overthink I believe. It's a first-year intro course so nothing too complicated.

    The issue is the criterion wasn't provided, so I'm trying to determine this on my own.
  5. Jan 12, 2016 #4


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    A stable equilibrium is one in which the system will tend to return towards the equilibrium position if slightly perturbed from it.
    If the middle charge is pushed a little to one side, will it move back, keep going, or stay where it is put?
    Those three possibilities correspond to stable, unstable, semistable.
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