A charge $q=2 \mu \mathrm{C}$ is placed at $a=10 \mathrm{~cm}$ from an infinite grounded conducting plane sheet. Find the force on the charge $q$.
The method of images requires that an image charge $-q$ is placed symmetrically with respect to the plane sheet. This means that the total induced charge on the surface of the conductor is $-q$.The force acting on $+q$ is

$$
F=\frac{1}{4 \pi \varepsilon_{0}} \frac{q^{2}}{(2 a)^{2}}=9 \times 10^{9} \times \frac{\left(2 \times 10^{-6}\right)^{2}}{0.2^{2}}=0.9 \mathrm{~N}
$$

where we have used $\varepsilon_{0}=\frac{1}{4 \pi \times 9 \times 10^{9}} C^{2} /\left(\mathrm{N} \cdot \mathrm{m}^{2}\right)$.