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Homework Help: Review needed for Electric Charge Problem

  1. May 8, 2007 #1
    1. The problem statement, all variables and given/known data

    Figure a -q1(+)------q2(+)- x (Just the X axis with two positively charged particles at the opposite ends of it, basically)

    The figure shows two positively charged particles fixed in place on an x axis. The charges are q1 = 1.60 x 10^-19C and q2 = 3.20 x 10^-19C, and the particle seperation is R = 0.0200m. What are the magnitude and direction of the electrostatic F12 on particule 1 from particle 2?

    2. Relevant equations

    I have the formula for the net force and know how to apply it to get the magnitude. However, I'm drawing a blank on my earlier Physics stuff and have forgotten how to obtain the direction. (I know that given the one-axis setup it should be obvious, but would like to know the equation/formula for future use.)

    3. The attempt at a solution

    See above. Like I said, I've got the magnitude correct, but have forgotten how to obtain the direction. Could someone help me brush up on this basic part?
    Last edited: May 8, 2007
  2. jcsd
  3. May 8, 2007 #2
    Like charges repel, and unlike charges attract. You only need to know this rule to figure out the direction.
  4. May 8, 2007 #3
    I understand that but I'm still unclear. If like charges repel, and both charges are positive, won't they just both stay where they are until they're too far away to affect each other? Or if they weren't like charges wouldn't they just meet in the center? How do I determine the exact angle using what I know about that rule?
  5. May 8, 2007 #4
    Or to put it another way. If I know that like charges repel, how can I plug that in as an equation? I tried F12 = -F21 but that just gives me the same number on both sides of the equation, just with the right side being negative. Is it just a matter of taking arcsin or arccos or the like or am I making something simple more complicated than it should be?
  6. May 9, 2007 #5
    Yea, I think you are just complicating everything.

    First, forget about the directions, and find the magnitude.
    Once you found the magnitude then try to find the directions.
    Like charges repell, so that means if there are two like charges, the force vector on the right charge would point towards the right direction (away from the left charge)

    Just remember that if they are like charges, the force vectors point away from the other spheres
  7. May 9, 2007 #6
    Actually, I think I've come up with a way of approaching it that seems to work. Let me know if this is correct (or if not, if it's closer to the right idea):

    In this case, it's a 1-dimensional setup, so I know that q1's angle is 180 degrees and q2's angle is 0 degrees. They're both positive charges, so they repel. If they're moving away from each other, wouldn't their combined angle get bigger? So can't I just add the angles in this case?

    If I go counter-clockwise to avoid negative angles I get 0 + 180, or 180 degrees which is the answer. So if I figure each charge's angle individually and then add (If they repel) or subtract (if they attract, IE: the angle gets smaller), is that more or less how it works?

    Though really my problem is that I have forgotten how to find the direction in the first place. Even if my theory is wrong, how do I use the magnitude in order to find the direction? Basically I know how to find the magnitude, but am drawing a blank on the proceedure to obtain the direction/angle.
    Last edited: May 9, 2007
  8. May 9, 2007 #7
    I don't have my textbook yet so I'm having trouble finding practice or clarification for this. Could someone clarify finding the direction further for me? I really am drawing a blank other than my attempt at a theory.
  9. May 9, 2007 #8
    You're really making this a lot more complicated than it needs to be. This is from waaaaaaay back when, when free-body diagrams were the hard part of the problem.

    Choose the two correct force arrows from the attached diagram.

    Attached Files:

    • FBD.GIF
      File size:
      809 bytes
  10. May 9, 2007 #9
    The pair on the left since particle 1 is the one being acted upon according to the description of the problem.
    Last edited: May 9, 2007
  11. May 9, 2007 #10
    I didn't read the last line of the problem before drawing the diagram; I drew what's going on, not necessarily what's being asked for.
    You should pick one force acting on each particle.
  12. May 9, 2007 #11
    Oh well then in that case, if they're both the same charge then each is pushing the other way. The red particle is being pushed left by the blue particle which in turn is being pushed towards the right by the red particle. So the red is being pushed further into the negatives and the blue into the positives.
  13. May 9, 2007 #12
    Very good. So in what direction does the electrostatic F12 on particule 1 from particle 2 point? And what's the direction of F21?
  14. May 9, 2007 #13
    The description states that the electrostactic F12 is the force on particle 1 from particle 2, so it's moving left. In that case since it's just the X axis the direction is 180 degrees. Then F21 would be the other way or 0 degrees.
  15. May 9, 2007 #14
    That's it!
  16. May 9, 2007 #15
    Ohhh I see now. I'm sorry if I tried anyone's patience, just haven't done Physics in a while and am a little rusty. I remember now.
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