For the problem set this week (HW #4), you will need to do some
reading on your own in the book about electric potential. In
particular, study the section on the potential arising from point
charges, which we did not cover in class today.

Here are some hints on several of the tougher homework problems this
seek:

For problem #4, involving the charged hollow metal pipe, make a
cylindrical Gaussian surface of length L (same as the pipe) at some
arbitrary radial distance r < R_o. Use Gauss's law to determine what
the E-field must be for any r < R_o. Then make the same Gaussian
surface at r > R_o, and use Gauss's Law to compute the E-field for any
r > R_o. Think about why I didn't make any mention of R_i in this
hint.

For problem #11, the uniformly charged ball, figure out the electric
field (as a function of radius r) in the two regions r < R and r >
R. Note that at r = R the E-field magnitudes should match up. In both
cases, use a spherical Gaussian surface and Gauss's Law to figure out
the E-field. Do the r > R case first since this is the easier of the
two.