Therefore, the inter-plate voltage u can be obtained according to Q=C*U;

u = Q/C = Q * d/εS；

Then the electric field intensity between plates e = u/d = q/ε s;

The electric field intensity e is the result of the interaction between the positive plate charge +Q and the negative plate charge -Q***.

It can be seen that the positive plate charge +Q and the negative plate charge -Q contribute to E/2 respectively.

The interaction between two plates can be regarded as the charge q of one plate, which is in the electric field E/2 generated by the other plate.

Electrostatic force, f = q * e/2 = q/2 ε s.