What is the electric field at a point midway between two point charges?

[spark693]

Homework Statement

What is the electric field at a point midway between a -8.50μC and a +6.38μC charge 7.79 cm apart? Take the direction towards the positive charge to be positive. Assume no other charges are nearby.

Homework Equations

E(net) = E(1) + E(2)

E = (k*q)/r^2

The Attempt at a Solution

E(1) = (8.99 x10^-6 * 6.38x10^-6)/ (0.03895)^2
=3.78x10^7

E(2) = (8.99x10^-6 * -8.5x10^-6)/ (0.03895)^2
= -5.037x10^7

E(net) = 3.78x10^7 - 5.037x10^7
=-1.257x10^7 [N/C]

This is not one of the answer choices, and I am not sure what I'm doing wrong. I thought I understood how to do the problem, so it might just be a complete misunderstanding of what the question is asking? I'm not sure...

 

[spark693]

Oh, I added the two final numbers together:

3.78x10^7 + 5.037x10^7 = 8.82x10^7

and got the right answer. But I don't understand - why would you add the negative charge?

 

[haruspex]

A negative charge on the left exerts a field from right to left. A positive charge on the right does likewise, so the field strengths add.