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I'm seeking help with a couple of problems involving electrostatics.

Q1)Calculate the electric field at the centre of a square 0.1m on a side if one corner is occupied by a charge 0.2micro C, and the other three corners are occupied by the charge of -0.9micro C

Q2) A positive test charge Q is placed at the origin of coordinates, and a charge 6Q is fixed to the x-axis at x=2m, find the location of the place along the x-axis where the electric field due to these is zero.

In Q1 I calculate the forces F12, F13, F14which=F12 using Coulombs Law.

Then taking the vector sum of F12 and F14 and adding F13 gives the electric field at the centre of the square. I'm getting confused with the signs in the equations, what am I doing wrong?

In Q2 I add the to E-fields together and set the sum to zero and solve for x,

ie; E1+E2=kQ/x^2+k6Q/(x+2)^2=0

Is this right, or am I doing something really stupid?

Any help with these problems would be much appreciated, as I've spent sometime on them and i've gotten nowhere.

Thanks Heaps

Callisto