I don't think it is correct that when the power supply remains hooked
up that the slab must be pushed in. I think the electric field still
does work on the slab as the field pulls the slab in. Therefore we do
not need to look for the weird interaction Joseph Scherrer is inquiring
about.
In the first case (power supply disconnected) the capacitance goes up
(because of a higher dielectric constant being inserted), the charge
stays the same, the voltage goes down, the energy goes down. In the
second case (power supply connected) the capacitance goes up, the
voltage stays the same (assuming voltage-regulated power supply), and
the charge goes up. Hence, the power supply provides more charge. The
energy difference between the two situations has been provided by the
power supply.
Michael D. Edmiston, Ph.D. Phone/voice-mail: 419-358-3270
Professor of Chemistry & Physics FAX: 419-358-3323
Chairman, Science Department E-Mail edmiston@bluffton.edu
Bluffton College
280 West College Avenue
Bluffton, OH 45817