# Electric Field Due to a Point Charge

## Homework Statement

Two particles are fixed to an x axis: particle 1 of charge q1= 2.1 * 10^-8 and particle 2 of charge q2= -4.00q1 at x=70 cm. At what coordinate on the axis is the net electric field produced by the particles equal to zero?

## Homework Equations

Enet = E1 + E2
E=F/q
F=k q1q/r2
Let q = point charge in the above formulas

## The Attempt at a Solution

Enet = E1 + E2 = 0
E1 = -E2

k q1/r12 = -k q2/r22
q1/r12 = q2/r22

At this point I thought I should try to isolate for one of the r values and plug it into a second equation to solve for one of the r values. But there is no other equation I can use to set up a system of equations... or is there? Im stuck here.

All help appreciated,
Thanks!

Last edited:

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tiny-tim
Homework Helper
Welcome to PF!

Hi Wables! Welcome to PF! (try using the X2 and X2 icons just above the Reply box )
At this point I thought I should try to isolate for one of the r values and plug it into a second equation to solve for one of the r values. But there is no other equation I can use to set up a system of equations... or is there?
you need an equation relating r1 and r2 supratim1
Gold Member
use r1 + r2 = x = 70 cm

Thanks for the welcome.

r1+x=20
r2+x=70

I can use these!! Ooooh!!

Okay, so:

q1/r12=q2/r22
=> (q1/q2)(1/2)r2=r1

Let (q1/q2)(1/2) = z

r2+x=70
=> r2=70-x

r1+x=20
=> zr2 + x = 20
=> z(70-x) + x = 20
=> x-zx = 20 - 70z

But z is a root of a negative number...?

tiny-tim
Hi Wables! You need a minus in there, don't you? 