Square array (electric field question)

[mujadeo]

Homework Statement

Four charges q1 = q3 = -q and q2 = q4 = +q, where q = 6 µC, are fixed at the corners of a square with sides a = 1.3 m. (see attached .gif)

Calculate the x- and y-components of the net electric field at the midpoint M of the bottom side of the square.

Homework Equations

Why don't the charges cancel in the field??
The distances and angles, are all equal
The charges are opposite.

SO how can there be a net electric field at that point (M)?

The Attempt at a Solution

Homework Statement

havent calc'd anything cause i don't get why field shouldn't be zero

Homework Equations

im totally stuck
thanks for any help sum1.

The Attempt at a Solution

 

[Doc Al]

If you were asked to find the field in the middle of the square, then you'd be correct: The field would be zero.

But not at point M. Note that on one side of M there's a +q and on the other side there's a -q. They don't cancel! (If M were in between two similar charges, then you could say that their field cancels.)

 

[robphy]

It's always a good idea to go back to the basics, especially when confused, rather than seeking the shortcut that might not be there.

For each point charge, draw a vector representing its electric field at point M (...paying attention to direction and some sense of relative scale). Draw the vector sum of those electric field vectors. Does it look like the [total] electric field vector at M is zero?