Have you discussed conservation of charge in the course? If you
have then an explanation like "the battery moves electrons from the plate
connected to the positive terminal to the plate connected to the negative
terminal... since the plate connected to the positive terminal initially
was neutral an amount of excess positive charge just equal to the amount
of negative charge moved to the other plate is left on the plate connected
to the positive terminal" might work.
Dr. Mark H. Shapiro
Physics Department
California State University, Fullerton
P.O. Box 6866
Fullerton, California 92834-6866