Homework Help: Net Electric Field of 2 charged particles fixed to X-axis

1. Sep 22, 2010

Oaksmack

2 Particles are fixed to an x-axis:
q1=2.1*10^-8 C @ x=.2m
q2=-4.00q1 @ x=.7m

We are trying to find the coordinate on the x-axis where the net electric field produced by the particles = 0.
The equation I have is

E=((8.99*10^9)q)/r^2

I got what i thought was q2 first, by multiplying 2.1*10^-8 by -4.00 to get -8.4*10^-8. From there I equated E1 & E2:
((8.99*10^9)(2.1*10^-8))/r^2 = ((8.99*10^9)(-8.4*10^-8))/r^2
from there, I had no idea how to solve for r.
I simplified the constant 8.99*10^9 with the respective q's, but just couldn't figure out how to solve for r without it canceling into 1 and having two numbers equaling each other.
Please help, I have a number of problems like this to do for this chapter.

2. Sep 22, 2010

Awatarn

Almost correct except $$r$$ in your equation
$$r$$ is a distance from the charge you are considering. So each $$r$$ on the left side and right side of your equation should not be equal. One should be include the distance between two point charges.

3. Sep 23, 2010

Oaksmack

Wait, so one of the r's should be .5m, the distance between the two particles?

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