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A property of conductors is that all the free electrons come to its surface. We can explain this in the following way. If there was any charge inside the conductor, it would create an internal electric field. To compensate for that, an equal electric field would be created in the opposite direction to cancel out its effect. In this way, the inside of the conductor is absent of any free charges.

These are the free electrons that are responsible for all electrical phenomenon, so, if a conductor is a negative charge the free electron come to the surface of the conductor. If the conductor is given a positive charge, electron move away from the surface and leave a positive charge on the surface of the conductor.