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Electric Forces and net electric force?

  1. Jan 27, 2017 #1
    1. The problem statement, all variables and given/known data
    Problem # 4:

    A thin semicircular rod has a total charge +Q uniformly distributed along it. A negative point charge -Q is placed as shown. A test charge +q is placed at point C. (Point C is equidistant from -Q and from all points on the rod.)

    http://imgur.com/AeVfddb
    upload_2017-1-27_18-50-50.png


    Let FP and FR represent the force on the test Charge due to the point charge and the rod respectively.

    Part C:
    A second negative point charge -Q is placed as shown.

    http://imgur.com/eYEOkPc
    upload_2017-1-27_18-51-45.png

    Is the magnitude of the net electric force on +q greater than, less than, or equal to the magnitude of the net electric force on +q in part b? Explain.

    Whole Problem
    http://imgur.com/VbAtlg5

    2. Relevant equations
    F = kq1q2/r2
    Also basic knowledge of electric forces based on vector math and x/y - components.

    3. The attempt at a solution
    So I solved part b (hopefully correct), but I want to say that the magnitude of the net electric force on +q will be less than the one found in part b since the -Q charges would cancel out as they are the same distance thus same magnitude, but opposite directions. However, I am confused on how the semicircular rod affects this as it is right there between it and the -Q charge.
     
    Last edited by a moderator: Jan 27, 2017
  2. jcsd
  3. Jan 27, 2017 #2

    gneill

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    Staff: Mentor

    Your problem statement is all but undecipherable. Please provide a clear, visible description of the problem that helpers can immediately see.
     
  4. Jan 27, 2017 #3
    Sorry, I placed the image link at the bottom rather than the top by accident. I fixed it and the link is now at the top.
     
  5. Jan 27, 2017 #4

    gneill

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    Staff: Mentor

    Even with the image, your text description should be more complete. Helpers shouldn't have to wade through a whole scanned image to discover what the problem is about. Take the time to describe the problem in the problem statement, use attached images to provide supporting diagrams and figures.
     
  6. Jan 27, 2017 #5
    Sorry to inconvenience you tonight. I don't understand the hostility, but that is besides the point. I re-did the write-up with clear directions, diagrams to support, and finally the an image of ONLY the problem to eliminate any confusion. I tried my best to set-up the problem, but clearly that was not enough. Hopefully everything I did will clarify what I need help on.
     
  7. Jan 27, 2017 #6

    gneill

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    Staff: Mentor

    The write up is much better now. Thanks. The idea is to make it easier for our volunteer helpers to help you.

    I placed captured/cropped versions of your images in-line in the problem statement so that helpers don't have to follow off-site links to see them.
     
  8. Jan 27, 2017 #7
    Thanks, How did you do the cropped images? I tried to get it imbedded into the description, but the site didn't accept my links from imgur. Just in case for the future.
     
  9. Jan 27, 2017 #8

    gneill

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    Staff: Mentor

    I just used a screen image capturing tool (Windows' snipping tool) to grab the images and pasted them. I'm aware that not every software's capture to the clipboard can be pasted directly, but the snipping tool works well. You can also upload the images to the PF site via the UPLOAD icon at the lower right of the edit window. Then you can insert the images where you will in the text of the post.
     
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