Zero Point of field between two charges

[duelle]

Homework Statement

Two point charges lie along the y-axis. A charge of $q_1 = -9.0 \times 10^{-6}\;C$ is at $y = 6.0m$, and a charge of $q_2 = -8.0 \times 10^{-6}\;C$ is at $y = -4.0m$. Locate the point (other than infinity) at which the total electric field is zero.

Homework Equations

$$E = \frac{k|Q|}{d^2}$$

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.

 

[christianjb]

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

 

[robb_]

Not sure why it was mentioned to take a derivative, but the rest is okay. (the potential isn't mentioned here)

 

[christianjb]

Not sure why it was mentioned to take a derivative, but the rest is okay. (the potential isn't mentioned here)

Sorry- you're right. You need to equate the fields to find when they're equal and opposite.