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Point Charge Given Field Maps

  1. Feb 6, 2015 #1

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    1. The problem statement, all variables and given/known data
    The electric field map is shown below. (Take it to be a self-consistent scale drawing at a particular moment in time.)

    triple%20charge%20field%20map.jpg
    where [itex]\mathrm{q}_4 = +10 C, \mathrm{q}_6 = +10 C [/itex]

    What is the charge of [itex]\mathrm{q}_5[/itex] ?

    2. Relevant equations


    3. The attempt at a solution

    I've tried the following values for [itex]\mathrm{q}_5[/itex]:

    $$-5,-10,-15,-20$$

    and none of them are correct according to the online homework software. How would I find the charge of [itex]\mathrm{q}_5[/itex]?

    Thank you,
     
  2. jcsd
  3. Feb 6, 2015 #2

    BvU

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    It looks as if halfway q4 and q5 the field is parallel to the axis at small distances x from the axis.
    Write down the three contributions ##\perp## to the axis and make sure they cancel to first order in x.

    And I wonder if three seemingly random shots at the answer counts as an attempt at solution :)
    (If you have unlimited tries, step from -10 to -20 in very small steps ! ;) )
     
  4. Feb 6, 2015 #3

    DEvens

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    What do you suppose it means that the drawing is "a self-consistent scale drawing"? What will that mean with regard to the field lines?

    It's not too often that a physics problem can be solved by a Sesame Street character. "Ten! Ten field lines! Heh! Heh! Heh!"
     
  5. Feb 6, 2015 #4

    BvU

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    I love it! Wish I had thought of that ... :D
     
  6. Feb 6, 2015 #5

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    Thank you for your replies!

    You were right; each field line represents an equal fraction of the original charge. The answer came out to around $$-7.1 \mathrm{C}$$.
     
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