[QUE/EM-02013]

The electric field due to a line segment of length $2a$, and carrying
a uniform line charge $\lambda$, at a distance $d$ above the mid point is given by
$$ E = \frac{1}{4\pi\epsilon_0} \, \frac{2\lambda a}{d \sqrt{d^2 +a^2}}$$
Use this result to find the electric field of a {\bf square lamina}
(side $2s$), carrying uniform surface charge density $\sigma$, at a distance
$z$ above the center of the disk.