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Holding together two charges

  1. Mar 15, 2010 #1
    This is an ultra-simple question that I can't wrap my head around.

    Two particles, both with charge +1C are located 1 m apart. If q1 is held in place, what is the magnitude of the force that must be exerted on q2 to keep it from moving?

    The answer is Coulomb's constant k.

    I don't understand why an electric force between two particles of like charge would keep the two charges in place. Or is this exactly what the electric force does? Shouldn't the two particles experience repulsion and move further from each other?

    The way the question is asked it sounds as if the electric force is the force which at the value k prevents charges from moving. Doesn't that go against the fact that the electric force is either attractive or repulsive? What parameters would have to change in this problem for the charge to be repelled from one another?

    I am sorry if this sounds confusing. I am really confused!
  2. jcsd
  3. Mar 15, 2010 #2
    The two charges, being both positive, will repel. That is correct.
    Left like that with no interference, they will both move off in opposite directions due to this mutual repulsion. The mutual force is given by Coulombs Law.
    If you hold one of the charges so it can't move, there is still the same mutual repulsion, but while that one stays where it is, the other, free to move, will move away. To stop it moving away you would need to apply a force on it equal to that original repulsion force; but of course you would need to push the other way to counteract the repulsion. That is, push it towards the other charge.
  4. Mar 15, 2010 #3
    This makes perfect sense. Thank you.
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