In: Physics
A point charge q is placed a distance L from a grounded plate (z=0). Calculate the charge distribution on the plate.
The simplest example of method of image charges is that of a
point charge, with charge q, located at
above an infinite grounded(i.e.:
) conducting plate in the xy-plane.
To simplify this problem, we may replace the plate of equipotential
with a charge –q, located at
. This arrangement will produce the same
electric field at any point for which
(i.e.: above the conducting plate), and
satisfies the boundary condition that the potential along the plate
must be zero. This situation is equivalent to the original setup,
and so the force on the real charge can now be calculated with
Coulomb's law between two point charges.
The potential at any point in space, due to these two point charges of charge +q at +a and -q at -a on the z-axis, is given in cylindrical coordinates as

The surface charge on the grounded plane is therefore given by

In addition, the total charge induced on the conducting plane will be the integral of the charge density over the entire plane, so:

The total charge induced on the plane turns out to be simply –q.
Because electric fields satisfy the superposition principle, a conducting plane below multiple point charges can be replaced by the mirror images of each of the charges individually, with no other modifications necessary.