Requests for Clarification and Student Suggestions

1. Looking over the past exams... there seems to be alot about transmission lines... the way I see it you were saying that we were not going to be responsible for them. Can you, if it is not too much of a problem, give me a list of fair game for the final (general topics)? [5/26]

   That is hard to answer without seeming coy or difficult. I can say that the final exam is comprehensive; that should come as no surprise since we have successively built new concepts from earlier ones throughout the course. However it would be fair to say that there will be an emphasis on the material not previously covered by our two midterm exams, material that includes ac circuits and transmission lines.

   For the final exam I like to examine situations that bring in several of the concepts that we have studied. Transmission lines are a great context for that since it involves both the capacitance and inductance of a two conductor system, as well as the laboratory experience. Looking over the archived final exam material I see two such problems, so that is probably an appropriate representation of that type of problem. Of course there is no guarantee that I will be put a transmission line on the final... I don't remember saying that transmission line discussion was not fair game; however, I do remember saying that the derivation of the wave equation from Maxwell's equations was not a topic suitable for the final exam.

   Let me know if this helps. I don't mind answering these types of questions, but it is difficult to maintain my flexibility in designing suitable exam questions if I get into too much list-building of what will and will not be on the exam.

2. I see that no homework was assigned for chapter 32, does that mean that it will not be on the exam? [5/26]

   Yes, that is a somewhat correct assessment. I would like you to be familiar with the notion that the displacement current term was a correction of Ampere's law _and_ that this led to the appearance of the wave equation resulting from Maxwell's equations. However there will not be a detailed problem regarding such topics on the exam.

3. This question is in reference to the last point of clarification you posted on the net about electron flow and current flow. When I went to talk to the TA about past exams and we reviewed questions related to that she said to me that current flows in the opposite direction of the electrons. Hadn't we talked about that in class and said how it was weird that we chose current in the opposite direction of electrons? [4/30]

   You are quite correct in your statement that the conventional current flows in the direction opposite to the electrons. What I am saying in my response below is that this concept check is not about the magnetic field created by a current loop, but rather how a charged particle responds to the presence of an external field.

4. As I was looking over the previous exams, I had touble with a few of the concept questions. On the 93 exam, #1 b. if the field is into the page, then the modified right hand rule says the current would be clockwise. But I remember from the beginning of the year that electrons flow in the opposite direction as current, so shouldn't it be counterclockwise?

   And again on 97 #1 c. if the field is pointing out at you, the modified right hand rule says that the current is counterclockwise but if the electron flows the other way, it would be clockwise. Just hoping you could help me out with this. Thanks a lot. [4/30]

   You don't want to think of the circulating charge as a current loop here. (The B field is being created by some other source, not the circulating electron.) The best way to proceed is to figure out which way the force points from q v x B and draw a circle such that force points to the center (the so-called centripetal force). Then see if the velocity that you chose for the charged particle and the circle drawn would make the circulation clockwise or counterclockwise. Let me know if I haven't helped to make sense of this and I will try again...

5. I was just curious as to what you meant when you said that you usually gave C's to students who understood 50 percent of the material we cover. Do you mean that as long as students are doing well in other facets of the course (i.e. Lab) they would need a 50 as an average for the three tests to get a C? Or, do you mean that the 50 percent of material needed for a C encompasses the course as a whole(i.e. the points for Lab, the three tests, and recitiation added together)? [4/28]

   The "50% for a C" was meant to be in reference to the exams, just for general guidance. (I may have misspoken in class.)

   Here is the Fall'97 grading schedule that I used to determine the final grade based on the overall course grade (all points totaled).

6. Are you going to go over RC circuits any further? [3/24]

   Just on the quiz this Friday. ;-)

7. I'm not sure if you said that the stuff we covered today regarding RC circuits will be on the exam. I know you said something about waiting to do these problems in the homework until this weekend, unless I misunderstood you. Can you let me know, please? Thanks a lot! [3/16]

   That's right, RC circuits will not be covered on the first midterm exam. However the second part of the class where I reviewed series and parallel combinations of capacitors as well as the dielectric constant of a material could be.

   My point regarding the RC circuit homework was that recitation session might be best spent this week preparing for the exam and that RC circuits may be reviewed during recitation next week, especially since there are no new homework assignments this week.

8. Why do you keep using the word infinitesimal? Is there another word you could use in it's place? [3/10]

   I guess I just like the way it sounds! Others might use the term differential element but there are already so many other uses sounding the same: differential equation, differentiation, etc. that I prefer to stick with infinitesimal...

9. The binomial expansion theorem keeps popping up on quiz 3 problems. Can you do a quick example in class that would show us how we should be using it in these problems so that if it appears on an exam we're prepared? Thanks! [3/9]

   I demonstrated the use of the binomial theorem in class when I evaluated the result for the electric field on the axis of a charged disk far away from the disk. HRW shows this on page 566. They also show its application to the result for the dipole far away on page 560. I am also making available another sample application of the binomial theorem. Hope this helps...

10. In class today you mentioned that we could solve the three equation systems using a calculator that can perform matrices. Unfortunately, I did not want to shell out the big bucks for one of those calculators- one which I only use 10 percent of its worth. Thus, may I solve for the matrices symbolically or am I required to give a definite answer? [2/24]

    As long as you are comfortable solving algebraically there should be no problem -- that's what I do too. Generally though the bottom-line of the questions raised regarding a particular circuit requires numerical values for current through and voltage difference across each circuit element, so substitution of numerical circuit parameters will be the norm. Often I will substitute in emfs/V and resistances/ohm as dimensionless quantities after the loop/junction rules are applied but before applying the techniques for solving a simultaneous set of equations to get currents.

Fall'97 clarifications

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