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Homework Help: Negative sign in coulomb's law/centripetal motion question

  1. Sep 7, 2011 #1
    1. The problem statement, all variables and given/known data
    I had a question in my physics class today that was relatively easy but what I was confused about was a misplaced negative sign. "An electron is rotating around a proton at a distance R. The mass of the electron is me and the charge is -e. Find the period."

    2. Relevant equations

    3. The attempt at a solution
    The solution was calculated equating the two equations given above. The problem arises when I plug in -e and +e for the charges. This leaves one side of the equation negative, which results in a negative period. How does the negative sign go away? Do I not include the sign for charges?
  2. jcsd
  3. Sep 7, 2011 #2


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    No, you do not include the negative sign of charges in Coulomb's Law for the force between two charges. Coulomb's Law gives the magnitude of the force. If you need to know the direction of the force, use the rule that like charges repel and opposites attract.
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