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Direction of electric force

  1. Feb 11, 2012 #1
    Hi all. I don't know if I'm completely over thinking this question or just not getting it. Please tell me if I'm correct.

    A -8.0μC charge is located 0.30m to the left of a +6.0μC charge. What is the magnitude and direction of the electrostatic force on the positive charge?

    F=KQaQb/r^2

    F=(9x10^9)(-8.0x10^-6)(+6.0x10^-6)/(0.30m)^2 = -4.8N...so answer is 4.8, to the right.

    So, I think that because the F is -4.8 means that the direction is going to the right because a negative means that it is an attractive force and the +6 charge will be attracted to the -8 charge. Am I correct?
     
  2. jcsd
  3. Feb 11, 2012 #2

    Doc Al

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    Staff: Mentor

    Ask yourself: Do the charges attract or repel? Given that answer, in what direction must the force act?
     
  4. Feb 11, 2012 #3
    The -8 and +6 will attract because they are opposite charges. So, the positive charge will go to the right.
     
  5. Feb 11, 2012 #4

    Doc Al

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    Right, they will attract.
    Why? If they attract, the force on the positive charge must be towards the negative charge.
     
  6. Feb 11, 2012 #5
    Oh, I see. I think drawing a picture of the charges would have helped. So, because the -8 is to the left of the +6, they are attracted to each other the +6 will move to the left towards the -8.

    I was originally just looking at the "left" in the question and automatically assuming, "ok, then it has to go right."

    Thanks Doc
     
  7. Feb 11, 2012 #6
    Also, does the amount of charge have an affect on the direction of the force? For example...say there was a -4 charge located to the left of a +6 charge...would that still be moving to the left?
     
  8. Feb 11, 2012 #7

    Doc Al

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    No, just the signs.
    Sure. The charges have opposite signs, so they attract.

    (Of course, if there are more than 2 charges involved, you'll need to figure out the net force on any particular charge.)
     
  9. Feb 11, 2012 #8
    Thank you so much! I actually get it now. My teacher was confusing me.
     
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