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Determing location where electric field is zero?

  1. Jan 8, 2014 #1
    1. The problem statement, all variables and given/known data


    2. Relevant equations

    See #1

    3. The attempt at a solution

    Based on the information in the question, I did what I could, but in the back of the book they have an answer of -9.9a and I have no idea how?

    For the +3q on the right, I set the denominator to (2a+r)^2, since its distance would be the square of both the 2a between them as well as the additional r distance between the point and -2q. I am thinking they somehow managed to relate a to r to get rid of r, but I can't seem to figure out how I can do that or in some other way get rid of r?
  2. jcsd
  3. Jan 8, 2014 #2


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    Gold Member

    Why (r+2a) and not (r-2a) ?
    Where is the b charge located in your system of coordinates? At x=2a or at x=-2a?
    Why did you expand (r+2a)², does that make your calculations simpler?
  4. Jan 8, 2014 #3
    All is correct except that (r[itex]^{2}[/itex] + 4ar + 4a[itex]^{2}[/itex]) is suddenly changed to
    (r[itex]^{2}[/itex] + 4a[itex]^{2}[/itex]).
    Last edited: Jan 8, 2014
  5. Jan 8, 2014 #4
    Thanks, but I'm not seeing how you are getting that? Wouldn't I want to square their combined sum?
  6. Jan 8, 2014 #5
    I meant that the term '4ar' must be kept and not lost on the way.
  7. Jan 8, 2014 #6
    Oh, yes I did correct that on my personal scribble, just not on that page. Even after I did that however, I still end up with r values in the equation that I can't seem to get rid of as shown here: photo.jpg
  8. Jan 8, 2014 #7
    This thing is due tomorrow and I haven't heard back from the professor either, so if anyone has any suggestions, I'm all ears? I did change up my signs, realizing the direction should be expressed as the point from the source of the charge TO the point I am trying to locate, rather than the reverse which I started with. It's frustrating because our book didn't explain how to approach a problem like this, but our professor threw it at us.
    Last edited: Jan 8, 2014
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