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Homework Help: Electric field

  1. Apr 14, 2008 #1
    Find the electric field at a point midway between two charges of + 3.0 x 10^-9 C and + 60 x 10^-9 C seperated by a distance of 30 cm.

    would we utilize coulums law:

    [9 x 10^9 * 3.0 x 10^-9 * 60 x 10^-9] / [30/2]
  2. jcsd
  3. Apr 14, 2008 #2


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    Yes, but this isn't it:
    What is the expression for Coulomb's Law and what do the symbols mean? You need to find the electric field from each of the two charges at a point 15 cm. away from each of them. Keep in mind that electric fields require vectors: which way does the electric field from each charge point at the place midway between the two charges?
  4. Apr 15, 2008 #3
    F = k*q1*q2 / r^2

    k = contant of 9 x 10^9
    q's = the charges
    r = the radius or distance

    did i forget to square it. is that what i did wrong.
  5. Apr 15, 2008 #4


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    All right -- that's the force equation. The problem is asking for the electric field at a point, so we drop the q2.

    Make a picture of the situation. We have a line segment 0.3 meters long (you'll want to work in meters) with a charge on each end. (Are those supposed to be + 3.0 x 10^-9 C and + 6.0 x 10^-9 C ? 60ยท10^-9 seems rather large compared to the other charge...) Both charges are positive, so which way does the field from each charge point at the position halfway between them? That will tell you whether you want to add or subtract the individual fields.

    Now use the formula for the electric field for each charge at the appropriate distance. (How far is the midpoint from each charge?) You then combine the fields appropriately (have you worked with vectors?) to get the total at that midpoint.
  6. Apr 17, 2008 #5
    Believe it or not, it should read 60. I had to recheck that myself.

    So dropping the q2 should give us:
    [k*q1] / r^2

    Since both charges are negative, I would guess that they would repel one another and face in opposite directions outward.

    I don't know whether to add or subtract the individual fields though.

    And half way inbetween would be 30/2 = 15?

    I haven't really worked that much with vectors. I know the rules but havent had much work with application of vectors.

    Please help so I can apply this to similar problems.

    Last edited: Apr 17, 2008
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