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Homework Help: Physics 2 Electric Fields

  1. Sep 2, 2010 #1
    Hey I'm a new time member but I hope to use this forum for my college Physics 2 class, anyways:

    1. Question: Two charges are D = 2.5 m apart and lie on the x-axis. What must be the magnitude of the charge on the right one, Q2, (in C) so that the electric field is zero at x = 1.75 meters. Let Q1 be at the origin and have a charge 17 C.

    2. Equations I'm using.

    3. My attempt:
    E1=(8.99E9)(17C) / (1.75^2) = 4.990E10
    E2= k*Q / (1.75^2)
    E(total) = E1 + E2
    0 = E1+E2
    Q= (-4.990E10)(1.75^2) / (8.99E9)
    Q= -16.9987

    I've just learned electrical fields / coulombs law, for a problem like this I just need some explanation on what I'm doing wrong. I thing what the biggest problem is and confusing this is it initially is 2.5m apart but they want you to find it at 1.75m, I'm going to keep working but I feel like the 2.5 is relevant and I'm not using it. Thanks in advance.
  2. jcsd
  3. Sep 3, 2010 #2
    it's quite simple if you understand the concepts.....see electric field is a vector, so you should take the direction under consideration before setting the equation,According to data,Q1=+17C, so the field due to it on point(1.75,0) is towards +ve x-axis.So to nulify its effect and make field zero at (1.75,0),Q2 would be -ve and direction of field due to it would be towards -ve x-axis.Now if we only talk about the magnitude,then,.....
    so, Q1/r1^2=Q2/r2^2
    so, 17/(1.75)^2=Q2/(1.25)^2
    so, Q2=8.673C.
    This was the magnitude that we found.Now, as we discussed earlier, Q2 has to be -ve.So,
    Hope this was helpful....
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