Image force on a line charge above a conducting plane

With no imposed external field, a line of charge density $\lambda$ at height $h$ above a conducting plane has an image $-\lambda$ a distance $2h$ away. The force per unit length is $-{\lambda }^2\hat{\mathbf{z}}/(4\pi {ϵ}_{0}h)$. It is always attractive. The image field gives the force directly; a factor one-half used in some image-energy calculations does not multiply this force.