Past exam of the physics B course of the University of Cambridge / 2015 / Paper 2 / Section A / A2

For the image force on a line charge above a conducting plane, with no imposed external electric field, use the method of images: replace the plane by a parallel image line of charge $-\lambda$ at height $-h$. The image is $2h$ from the real line, and its field at the real line has magnitude $\lambda /(4\pi {ϵ}_{0}h)$. ThereforeHere $\hat{\mathbf{z}}$ points upward from the plane. The force is attractive and normal to the conductor, for either sign of $\lambda$. The image reproduces the induced surface charge field; it is not an extra physical charge, and no factor $1/2$ is inserted in the force on the actual line.