The electric field in the immediate vicinity of a metal is perpendicular to its surface and is strongest near points of high curvature. Similar reasoning leads to the result that enclosing a region within a metal wall shields the interior region from all external static electric fields. The electroscope then does not respond to external influences. Also see S-10.4 for a version of the Faraday pail experiment. Both attractive and repulsive interactions occur.
Mobility can change dramatically under high stress conditions as in dielectric breakdown.
E.g. a column of air, normally an insulator, becomes an excellent conductor when it ionizes under the strong electric fields that percipitate a lightning strike. Usually current carriers are uncharged. Because the force may be attractive or repulsive, charge may be positive or negative. For two systems in electrical contact charge will flow until the two voltages are equal. For more resources check Electricity and Magnetism Learning Resources .