1. The problem statement, all variables and given/known data Fig. 1 and 2 show a dielectric slab being inserted between the plates of one of two identical capacitors, capacitor 2. Select the correct answer to each of the statements below (enter I for `increases', D for `decreases', or S for `stays the same'). http://img99.imageshack.us/img99/763/prob45.gif [Broken] A. In Fig. 1, the potential difference between the plates of capacitor 2 _______ when the dielectric is inserted. B. In Fig. 1, the charge on capacitor 2 _______ when the dielectric is inserted. C. In Fig. 2, the capacitance of capacitor 1 _______ when the dielectric is inserted. D. In Fig. 2, the potential energy stored in capacitor 1 _______ when the dielectric is inserted. 2. Relevant equations For Capacitors in parallel: Ceq=C1+C2+C3... In series 1/Ceq=1/C1+1/C2+1/C3... The introduction of a dielectric increases capcitence to C=kC0 Energy stored on a Capacitor is uc=(1/2)C(deltaVc)2 C=Q/(deltaV) 3. The attempt at a solution For A, since the capcitors are in series, the charge on each must be constant, as no charge flow could reach the region between the two capacitors. Thus when the dielectric is added to #2, the potential difference across it must go down from deltaV=Q/C. Therefore, D, for decreases. For B, due to the above reasoning, the charges can't have changed, so the answer would be S, stays the same. For C, they are in parallel, so the potential difference across each is a constant. When the dielectric is added to #2, it's capacitence would go up, increasing the charge on that one, however, this shouldn't influence the capacitence on #1, so again, S, for stays the same. For D, the energy is uc=(1/2)C(deltaVc)2, so since the potential difference hasn't changed, and C hasn't changed, this would also be S, stays the same. I feel reasonably confident that C and D should have the same answer, so I suspect if one of those two is wrong, the other is wrong in the same way. (I.E. if C is really I, then so is D.) I know theres at least one mistake in my reasoning somewhere, I'm just not sure where.