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Parallel - Plate Capacitor
 
A parallel-plate capacitor consists of two parallel conducting plates, separated by the distance d.
This distance is usually small in comparison with the dimensions of conducting plates. Much of the field in such a capacitor is localized in the region that lies between the plates. In this region, the field is approximately uniform and it equals to
E = Q/(ε0S) = σ/ε0.
In this equation, Q is the charge of each plate, and S is the area of the plate. The ratio σ = Q/S is the magnitude of surface charge density of the plates.
A force of magnitude F = qE = qσ/ε0 will act on a point charge q if this point charge is situated in the region between the plates.
If q>0, the direction of the force coincides with the direction of the field lines; and if q<0, the direction of the force is opposite to the direction of the field lines.
A "fringing" effect takes place on the edges of the region between the plates, where the field is not uniform and the field lines are bent. There is also an electric field present outside of the capacitor, but it is much weaker there. In a number of problems (but not always!), the electric field that lies outside of the capacitor can be neglected.
Remember: the electrostatic field of a capacitor is a potential one. The electric field does no work on a charge during a two-way displacement.
 
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