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Net Force of Point Charges

  1. Mar 16, 2009 #1
    1. The problem statement, all variables and given/known data
    Two point charges, q1 and q2, are placed 0.30m apart on the x-axis, as shown in the figure(http://hiviz.org/apb89/ap/problems/2006-3.jpg). Charge q1 has a value of -3.0x10^-9 C. The net electric field at point P is zero.

    A) What is charge of q2 (Positive/Negative)
    B) Calculate magnitude of charge q2
    C) Calculate magnitude of electric force on q2 and give direction.
    D) Determine x-coordinate of point on line between 2 charges at which electric potential is 0.
    E) How much work must be done by an external force to bring an electron from infinity to the point at which the electric potential is zero?


    2. Relevant equations
    F = k*|q1|*|q2|/r
    V = kq/r

    3. The attempt at a solution
    A) I believe the charge is positive because if the net force is zero, then the positive charge would have to counteract the negative charge of q1.
    B) I think I am supposed to use F = k*|q1|*|q2|/r. Do I just set that equal to zero, since the net force is zero at point P?
    C) Don't know where to start here
    D) V= kq/r? There is only one q in that equation though, so I'm not sure how to do it.
    E) Don't know how to start this one either.

    Any suggestions/hints would be great. Thanks!
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Mar 16, 2009 #2
    A) Yes you are correct on this one. Good reasoning.

    B) You might try a different equation. The one you are using will work for part C, but here you need to find the magnitude of the electric field at a point. Try thinking about E = (k* (q1)/r^2) + (k*(q2)/r^2)

    C) After doing B I think you will find this much easier

    Unfortunately must go to class. Sorry wasn't able to give comments on last 2. Good luck.
     
  4. Mar 16, 2009 #3

    LowlyPion

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    For d) you have to solve for the condition that the two |E| are equal at some point Q, where Q is distance r from q1 and distance (r + .3) from q2.

    That yields something like q1/(r2) = q2/(r +.3)2
     
  5. Mar 16, 2009 #4

    LowlyPion

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    For e) what is the equation for Work that relates voltages and charges?
     
  6. Mar 16, 2009 #5
    V=w/q?
     
  7. Mar 16, 2009 #6

    LowlyPion

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    Isn't it more like W = q*ΔV ?

    And what is the V at ∞ ?
     
  8. Mar 16, 2009 #7
    0 maybe? Sorry I don't really know
     
  9. Mar 16, 2009 #8

    LowlyPion

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    It's not maybe. That's the way it's defined. Your R is ∞ after all, so it must be 0 right. (V = kq/r ) Now moving a charge to your point where it's also 0 is what they are asking.

    So how much work is involved in moving a charge from 0 potential to a 0 potential point? The product of the charge and the voltage difference perhaps? And that is what again?
     
  10. Mar 16, 2009 #9
    Would it be 0 again?
     
  11. Mar 16, 2009 #10

    LowlyPion

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    Sounds good to me.
     
  12. Mar 16, 2009 #11
    Alright thanks for the help, I appreciate it
     
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