A concept devised by Kelvin for solving electrostatic problems associated with a point charge in the vicinity of a conducting surface. It can be shown that under certain conditions the charges induced, by the point charge, on the surface of the conductor have electrical effects identical with those that would be produced by an imaginary point charge located at a particular point relative to and below the surface. This imaginary point charge is known as the electric image of the first charge. The force exerted on the first charge, due to the induced surface charges, may be calculated as if it were due to the image and is called the image force. In the case of an infinite plane surface the image charge is equal and opposite in sign to the inducing charge and at a distance behind the plane equal to the distance of the actual charge in front of it (see diagram).

Electric image