Midterm preparation, post 2

(edited Sunday, Oct 24, 2 PM)
In response to the comments on the previous midterm preparation related post, I thought it would be a good idea to start a new post addressing what you should primarily focus on for the midterm. These are general guidelines. My approach to a midterm (and for the final) is to think of it as an opportunity for teaching as well as evaluation. With that in mind, I try to focus on the things that I think are most important for you to learn and remember.

With regard to circuits, you should be able to calculate and understand voltage and current in relatively simple (not complex) circuits involving batteries, resistors and capacitors. You should be able to calculate and understand the current through any resistor, or through any part of a circuit, and to understand how current divides at a junction and recombines at a junction. Our circuits will all be relatively simple. They will not involve complex combinations of resistors and junctions. They will be designed to illustrate basic principles and understanding.
   
The relationship between voltage and charge in a capacitor, as well as the manner in which a capacitor charges as a function of time are both important. Also important is an understanding of the energy stored in a capacitor–-how that is related to charge, voltage and capacitance–- and an understanding of the energy density of the electric field and how that relates to the total energy stored in a capacitor.

With regard to the electric field and electric potential, the two things we studied in the first few weeks of the class, you would want to understand those also. In particular, you should be able to sketch and calculate the electric field at a given point in space from a few discrete point charges. (By discrete we mean not continuous. You don't need to know how to calculate an electric field from continuous charge distribution.) You should also be able to calculate the potential from discrete point charges, though that may be less a point of emphasis. Additionally, it is advisable to understand the relationship between electrical potential and electric field. If you were asked about that it would only be in a fairly simple, quasi one-dimensional, context, like a (parallel plate) capacitor or a situation where the potential depends only on x.

One can see how the capacitor --in which the electric field between the plates played a central role-- connects the first part of the class (the one about fields and potentials) to the middle part (the one about circuits).  The energy carried by electric fields everywhere, not just in capacitors, is an important part of that.  Later in the quarter we will evolve back from talking about circuits to reconsideration of fields in space. We will find that circuits do not make sense unless one considers them in the context of the fields that surround them. So the connection between fields, energy, and circuits is an essential part of this course.

For the midterm,  bring an 4" x 6 " index card with a reasonable number (like 10 or 20) equations written on it. Please do not write too small or cram too many equations on your card. Unless you can remember everything without it, you will really need this card. You will not be given equations at the test.  (PS. put constants on it too, like k and epsilon_0 . also e...)