To discuss the electric field due to a ring or disk of radius R carrying a uniform charges density, it is not necessary to do the complete calculation striating with Equation given. We have already calculated the force exerted on a point charge qo by a ring of charge or a disk of charge or disk of charge. The force exerted by a ring of charge on a point charge qo on the axis of the ring was given. We can find electric field at a point on the positive z axis due to a ring of charge directly.

Where we have used q=λ(2?R). The electric field is directed along the axis of the ring (the z axis) and away from the ring. Above equation is valid for positive as well as negative z. if the ring is negatively charged, the field points along the axis in the opposite direction (towards the ring).
In similar fashion, e can find the electric field due to a disk of charge.

Here we have expressed the electric field in terms of the surface charge density of the disk using q = oA = o(?R2). This expression gives the field at a point on the positive z axis a distance z from the disk. The field points away from the disk if the disk is positively charged.  Above equation is valid only for z>0. How should it be modified if p is located on the negative z axis.