Three point charges in an equilateral triangle

[Mugwump101]

Homework Statement

I had this exam question for a final exam and I was wondering if I got it right or not. There are three point charges +4q equally spaced apart at the tips of an equilateral triangle with distance .11m apart from each other. What is the magnitude of the electric field at the MIDPOINT between any two charges?

I drew a picture on paint. It's not drawn to scale but it's what was on the exam.

Homework Equations

E=Kq/r^2

The Attempt at a Solution

So basically, I thought the answer had to be zero. E1=-E2 kq/r^2=-kq/r^2 So the summation of the forces is zero because the charges are the same and they are the same distance apart!

However, people have been asking the TA and he says it's not zero, which makes me baffled [PLAIN]http://img638.imageshack.us/img638/3060/physics0012.jpg

 

[Doc Al]

So basically, I thought the answer had to be zero. E1=-E2 kq/r^2=-kq/r^2 So the summation of the forces is zero because the charges are the same and they are the same distance apart!

Well, the field from the two charges on either side of the midpoint will cancel. But what about that third charge at the opposite end of the triangle?

 

[rl.bhat]

At mid point field due to the two charges at the end of the side is zero. But the field due to the third charge on the vertex there will be a field on the mid point. Find the distance between the vertex and the mid point and find the filled.