Question

A thin, horizontal square copper plate has a length of 6.35cm and is charged with 2.56 × 108 electrons. If the electrons are uniformly distributed on the surface, calculate the value of the electric field (with the upward direction positive).
why is the strength of the electric field at the plate's centre of mass?

Answer 1

This is either a trick question or a very hard one. In the first case: An electric field cannot occur inside a conductor (or by using the superposition principle you find out that at this point your electric fields cancel each other out)... Or you must use the laplace equation and proper boundary conditions to solve for the electrostatic potential 

Edit: Considering the center of mass of the plate is on the plate