Because S>& gtd can ignore the edge effect, it is considered that the area of the plate is infinite and the electric field is perpendicular to the plate.

Take a Gaussian surface to surround the flat plate, so 2S*E=q/ε0.

E=q/(2*ε0*S), which is the electric field intensity excited by a polar plate, and the magnetic field is uniform.

The electric field force of the other plate EQ = Q 2/(2 * ε 0 * s), gravity.

(2) Capacitance method

The capacitance of the planar capacitor is c = ε 0 * s/d.

Because q=CU

U=q*d/(ε0*S) Potential difference between two plates.

The electric field between the two plates is U/d=q/(ε0*S).

The electric field is excited by two plates. From the symmetry point of view, the electric field between the two plates should be equal. (The electric fields outside the two plates cancel each other, and the total electric field is 0. )

Therefore, the electric field excited by one plate itself will not exert force on itself, and only the electric field generated by the other plate will provide electric field force.

So f = e/2 * q = q 2/(2 * ε 0 * s)

In fact, the two methods are equivalent, and the key is that the electric field excited by oneself does not exert force on itself.