simulation on the left represents a positive charge (+q) near an infinite
grounded conducting sheet. A second animation is shown (see below) that depicts
a similar situation without the conductor present. The rectangle is there for
your reference only and does not carry any charge nor does it affect the
the animation on the right to place charge(s) where they belong to make the
field on the right match the field on the left for x>0. Assuming you could do
this, where are the charge(s)?
A negative charge must be placed the same distance as the original charge is from
the plate, but on the other side, as shown in the figure above. This is a simple
example of the method of images from electrostatics. Ask students to compare the
regions of interest x>0 and the region we do not care about, x<0. What do
they notice? As expected, placing another charge to the left of the rectangle
exactly mimics the field in the region of interest. However, in the region
x<0, this is not the case. The electric field is indeed different there.
Students must remember that since they were specifically interested in the
region x>0, placing a charge outside of this region is completely acceptable
as long as its placement solves the boundary conditions.
Script Author: Mario Belloni