# Earthing a system of parallel plates

our sir has taught us that when in a system of conducting infinite plates kept parallel to each other, when any one of them is earthed, then the charges present on the free ends(i.e. the open sides of the parallel plates at the 2 ends) will become zero!

Say we have 3 infinite conducting parallel plates $A,B,C$ with charges $q_1,q_2,q_3$. Now say that $A$ is earthed. Then we have been told that the charges on free end are zero, and hence a charge of $-(q_2+q_3)$ is induced on the inner face of $A$.

Can anyone please explain why this occurs?