wo uniformly large parallel thin plates having charge densities `+sigma and - sigma` are kept in the X-Z plane at a distance 'd' apart. Sketch an equipotential surface due to electric field between the plates. If a particle of mass m and charge'- q' remains stationary between the plates, what is the magnitude and direction of this field ?

Explanation

If two uniformly large parallel thin plates A and B having charge densities `+ sigma and-sigma` are kept in the X-Z plane at a distance .d. apart as shown in figure, then electric field is along X-axis.
The field is uniform one having a value `E = sigma/epsi_0`. Hence, an equipotential surface due to the electric field between the plates is a straight surface in Y-Z plane as sketched in figure. If a particle of mass mand charge.-q. remains stationary between the plates, then its weight mg must be just balanced by electric force F=-qE. It is possible only when electric field E is directed in vertically downward direction.
`mg - qE rArr absE = (mg)/q`.