Superposition of conductors

Question

If a put some charged conductors in an isolated system i could use the superposition principle to calculate the electric field. The difficult thing is to know in which way the charges will redistribute in the conductors' surfaces.

Nevertheless, when we study capacitors ( = two plane conductors placed close to each other) this redistribution seems not happen; so we can apply directly the superposition , knowing the field generated by each conductor. Why in this case the redistribution doesn't happen ?

Answer 1

Usually when you say parallel plate capacitors, you are considering them large enough to neglect any edge effects. Now, suppose you have two parallel plate capacitors placed closed to each other. Initially, you have charge uniformly distributed on them. Now, when they are close to each other, think about a particular charge. For the charge, there is no preferred direction to move in because of the symmetry of the problem. If it moves someplace, that would mean the symmetry is violated. So, by this simple argument, you can see that the charge redistribution won't take place as long as you are neglecting edge effects. However, a more complete analysis will show that the charges at the edge do have a preferred direction,i.e ,towards the center where the electric field is more stronger. Hence, in reality, the redistribution does take place.