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Homework Help: Using Coulombs Law

  1. Sep 16, 2009 #1
    Three charges are arranged as shown in the diagram below. Their magnitudes are:
    q1 = +2.5 x 10^-17 C
    q2 = +3.0 x 10^-17 C
    q3 = +3.5 x 10^-17 C

    (its a right angle triangle, with 50 cm as the hypotenuse, 40 cm as the horizontal, and 30 cm as the vertical) sorry i don't have the actual triangle.

    Calculate the direction and magnitude of the force acting on q1.

    Relevant Equations:
    Fe = kq1q2/r2

    I'm not sure where to start. I believe I need to acquire all of the forces of q2, and q3 and then use Pythagoras theorem to find q1. Not sure if that's correct though
  2. jcsd
  3. Sep 16, 2009 #2
    I don't see a diagram
  4. Sep 16, 2009 #3
    i know, i tried to copy and paste it but i couldn't. so you kind of just have to visualize it, sorry
  5. Sep 16, 2009 #4
    Well, the resultant force on q1 is just the vector sum of the force on q1 due to q2 and the force on q1 due to q3.
  6. Sep 16, 2009 #5
    It's a bit difficult when you don't know the location of the charges, but the problem should be easy to solve using trigonometry(and or Pythagoras) and Coulombs law. I recommend sketching on a paper, in which direction the force of the respective charges work on each other, that way you can get a clear picture of the size and magnitude of the forces working on q1.
  7. Sep 16, 2009 #6
    sorry, what does that mean exactly?
  8. Sep 16, 2009 #7
  9. Sep 16, 2009 #8
    well once i do that, what do i do afterwords?
  10. Sep 16, 2009 #9
    Try to sum together the different forces into one, so that you get one direction and one magnitude. I'm not exactly sure how you should accomplish this though, either use trigonometric functions or pythagoras
  11. Sep 16, 2009 #10
    kk ill try, thanks
  12. Sep 16, 2009 #11
    how i am supposed to know which distance to use though? do i just use the A a, B b, C c function in a triangle, if you know what i mean. cause it doesn't give me specific distances it just shows them labeled in a triangle
  13. Sep 16, 2009 #12
    I'm not quite sure what you mean, but the charges are all positive so you know they repel each other. Use Coulombs law along with the distances given, then you know the force which one charge "feels" from the other two charges. What you need to do is calculate the force between q1 and q2, and between q1 and q3.

    I don't know what you mean by A a, B b and C c, perhaps if you told me in which corner each charge are, then I can help you further if you require it.

    If you don't know which distance that corresponds to what charges I don't see how you could solve this.
  14. Sep 16, 2009 #13
    well for example when you're using cosine law, and in the triangle you'll have A,B and C, and then across from each value is the distance. in this triangle it shows that across from q1 is 40 cm, then across from q3 is 50 cm, and across from q2 is 30 cm. I'm sorry if this isn't making much sense
  15. Sep 16, 2009 #14
    You don't need to use trigonometric functions on the actual triangle with the distances, only on the forces that results. I made a crude sketch, hope it helps. I made some assumptions, but the concept is the same. Hopefully the mods will approve it soon (I uploaded the sketch as a JPEG)

    Attached Files:

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