On Purcell's Chpt. 4

There is a lot of important information in Chpt. 4. Please go over it this week, carefully, and try to understand it all. I will review it very quickly next week and you won't be able to follow unless you go over it in advance. Have your questions ready. I list below the items you should grasp.

Definition of electric current, I. Units.
Definition of current density, J. Relation between J and I. Note that J is a vector, I is not.
Condition on divJ from charge conservation.
Ohm's law relating I to V, or E to J.
Relations between resistance (R), resistivity (rho), and conductivity (sigma). Why does one define both resistance and resistivity?
Understand why there is an electric field inside a conductor that is conducting electricity, even though we learned in Chpt. 3 that there are no electric fields inside conductors.
Sect. 4.4 is pretty long-winded. The bottom line is the relation between conductivity and collision time tau, Eq. (20), or simpler and more important, the first equation in section 4.5.
You can ignore most of the details of Sect. 4.6.
Circuits, resistors. 3 conditions at the bottom of page 151 are important.
Resistors in parallel and in series, you should derive Eqs. (22) and (23).
Concept of electromotive force. Networks with several voltage sources.
Energy dissipation in current flow.
Circuit with a capacitor and a resistor, time dependence of current and voltage.
How can there be a current in such a circuit if electrons can't jump from one plate of the capacitor to the other?

Also, do the assigned hw problems. There is no problem session this week unfortunately, but the solutions are posted.