Two things:

FIRST, an answer to the question regarding the energy in capacitors
which plagued me during class and during this weekend. The question
was how can the three expressions for energy U be consistent with each
other when you change a feature of the plates (such as the area A)?

What was confusing was what to do with charge Q in this case. For
example: if you were to double area A of the plates, then you could
either 1) keep charge Q the same, making charge density sigma half of
its value, or 2) keep charge density sigma the same, thereby doubling
Q.

In case 1: Q stays the same. Electric field between plates E =
sigma/e0, which equals half of the original field since sigma went
down. V = Ed, so voltage V is half the original. Capacitance C =
A*e0/d which is twice the original. Thus the energy U = 1/2*C*V^2 =
1/2*Q^2/C = 1/2*Q*V = one half the original U. So that works OK. 

In case 2: Q gets doubled. Electric field stays the same since sigma
stays the same. V = Ed = the same as original. Capacitance C = A*e0/d
is twice the original. So U is twice the original in each expression
for U.

You can do the same thing for increasing separation distance d and
inserting dielectric and it also works out OK. Come talk to me if you
want more clarification on this.

SECOND, several of the homework problems for this week's problem set
(PS #5) are challenging. In particular, Problems 3, 4, and 5. Here are
some hints to help you get through these.

Problem 3: you must treat this like a parallel plate capacitor where
the curvature is negligible since the distance between plates is so
small relative to the radius in the problem. Once you do this, you
know an expression for C for this setup. Use this expression to get
C/A which is capacitance per area.

Problem 4: Use the C/A you got from the previous problem and recall
that the surface charge density sigma = Q/A.

Problem 5: Capacitance per unit length is C/l where l is length. You
know an expression from before for C/A, and recall that the area of a
cylinder of radius R and length l is A = 2*pi*R^2*l

I hope that helps.  Come see me/call me/email me if you have
questions.