Quote by kb1408
I have used the equation E=(Q/ε_{0})(1/(4*pi*r^{2}))

This looks like the field from an
infinite sheet of charge. You should write it as σ/2ε, where σ is the surface charge density. Not what you want.
I also tried the equation E=(Q/2(ε_{0}))(1(z/(√(z^{2})+(r^{2})))

That's the one you want, but you need to replace Q with σ.