  We re-examine a familiar problem given in introductory physics courses, about
determining the induced charge distribution on an uncharged
``infinitely-large'' conducting plate when placing parallel to it a uniform
charged dielectric plate of the same size. We show that, no matter how large
the plates are, the edge effect will always be strong enough to influence the
charge distribution deep in the central region, which totally destroyed the
infinity assumption (that the surface charge densities on the two sides are
uniform and of opposite magnitudes). For a more detailed analysis, we solve
Poisson's equation for a similar setting in two-dimensional space and obtain
the exact charge distribution, helping us to understand what happens how charge
distributes at the central, the asymptotic, and the edge regions.
