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Field of five charges

  1. Aug 27, 2012 #1
    http://imgur.com/a3vxU


    Really haven't a clue how to go about this, it's the last question on the paper and nothing before it is anything near as complex as this.

    Any help much appreciated
     
  2. jcsd
  3. Aug 27, 2012 #2

    Doc Al

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    Staff: Mentor

    What's the field at the center from a single charge? Then find the vector sum of all the field contributions. Take advantage of symmetry.
     
  4. Aug 27, 2012 #3
    Imagine the top charge is also absent. What would the field be due to the four charges?
     
  5. Aug 27, 2012 #4
    So due to each charge the field is E=kq/r2

    The 4 in the middle cancel, giving a field of either positive or negative of the single field charge above. So would the answer be option 3?
     
  6. Aug 27, 2012 #5
    What is the direction of j? Of the field?
     
  7. Aug 27, 2012 #6
    The picture is all the info given on this
     
  8. Aug 27, 2012 #7

    Doc Al

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    Staff: Mentor

    Good. That's the magnitude of the field, so be sure to count the direction.
    Yes, the 4 in the middle cancel. But I don't know what you mean by "either positive or negative". Which is it? What's the direction of the field from the top charge?
     
  9. Aug 27, 2012 #8
    You have enough information to deduce the direction of the field by the top charge. As for the direction of j, I am sure you have heard of the standard basis vectors i, j and k. What are they?
     
  10. Aug 27, 2012 #9
    Apologies, it's late. The j vector acts in the y direction, so the top charge would act in the upwards of y, +j vector and therefore option 3.

    By positive or negative I was referring to the difference betweent options 3 and 4.
     
  11. Aug 27, 2012 #10
    Why is that?
     
  12. Aug 27, 2012 #11
    Oh damn, they're positive charges, so the field acts downwards away from the top charge.

    This would give a "negative" field in the y direction and so option 4 is correct?
     
  13. Aug 27, 2012 #12
    Correct!
     
  14. Aug 27, 2012 #13
    Thanks very much for your patience, much appreciated
     
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