Point Charge Given Field Maps

  • #1
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Homework Statement


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] ?

Homework Equations




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,
 

Answers and Replies

  • #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 ! ;) )
 
  • #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!"
 
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  • #4
BvU
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I love it! Wish I had thought of that ... :D
 
  • #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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