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Homework Help: Zero Point of field between two charges

  1. Mar 19, 2007 #1
    1. The problem statement, all variables and given/known data
    Two point charges lie along the y-axis. A charge of [itex]q_1 = -9.0 \times 10^{-6}\;C[/itex] is at [itex]y = 6.0m[/itex], and a charge of [itex]q_2 = -8.0 \times 10^{-6}\;C[/itex] is at [itex]y = -4.0m[/itex]. Locate the point (other than infinity) at which the total electric field is zero.

    2. Relevant equations
    [tex]E = \frac{k|Q|}{d^2}[/tex]

    3. The attempt at a solution
    All I know is that it's between the two points. Basically, I don't know how to set this problem (or any like it) up. I'm not asking for the answer, just a nudge in the right direction.
    Last edited: Mar 20, 2007
  2. jcsd
  3. Mar 20, 2007 #2
    The E field at any point is a vector.

    In 1D (i.e. along the y-axis in your Q) the E field at a given point is either +ve or -ve, so you have to get the signs right. (Your eqn. won't work as is. Remove the magnitude sign.)

    Then add the E-field components to find E as a function of y. Diff to find point where dE/dy=0
  4. Mar 20, 2007 #3
    Not sure why it was mentioned to take a derivative, but the rest is okay. (the potential isnt mentioned here)
  5. Mar 20, 2007 #4
    Sorry- you're right. You need to equate the fields to find when they're equal and opposite.
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