Maybe I shouldn't have started this blog now, not with everything that's been going on.

Now I’m on to last Thursday’s electricity and magnetism lecture exam. There were only six questions.

The first two I got from their most recent quiz with only minor revisions. For one thing, the dipole potential now had an angle instead of being zero, where only the two non-hypotenuse sides of the right triangle are given. They have to solve for the angle as well as the value of the hypotenuse.

The second one was a cylindrical capacitor whose radii and length were given (from a spherical capacitor last time) whose equivalent square parallel plate capacitor they had to come up with.

Question number three was actually just an exercise (as opposed to a problem) that I got from the textbook. If a capacitor connected to a voltage source were given a dielectric, how would it change the charge, potential etc.? Would they increase, decrease or remain the same? This one was only two points per answer, I asked for the derivation though, which some students confused with differentiation, and I had to clear it up. I thought it was so easy.

Next was just a mix of questions on resistivity, charge, number of electrons, current density and power, given a wire with a current, resistance, diameter and length. For this one I found out that I had to tell them that the area needed was not the cylindrical surface of the wire but the cross-sectional area. These were only four points each, for a total of twenty.

Fifth was the “resistors in series-parallel” problem, which were basically three boxes side by side with one voltage source on the side. This was the first non-simple illustration I was able to get from the internet. I realized early on that there were still some people applying Kirchhoff’s rules to this. I guess my emphasis during a lecture about multi-loops with only one voltage source didn’t quite take.

In the middle of the exam the single engineering dean’s lister of the class asked me if his rewiring of the circuit was correct. I told him it was, and he promptly showed it to the other bright (but distracted) friend of his. Instead I told him to draw it on the board. Another student wanted the resistors labeled. I declined, but allowed Mr. Distracted to label the nodes.

I also had to emphasize to another student that when two branches leave one node and return to another node, they are considered parallel. I forgot to mention that I was asking for only the voltage and current going through one resistor, for ten points each. Of course they had to get the total resistance and current first.

The last problem was also from the internet: Same three side-by-side squares but with two voltage sources now. The students will have to blame the Information Technology Office for taking down the scanner whenever the English teachers are not using it for these ones.

The circuit had four nodes and six branches (and currents). I only asked for the current in one branch though. When I solved the problem, I noticed that two pairs of currents were equal to each other, and there were a couple of expressions using only two currents, which made the substitution easier. Especially since I was only asking for current along one branch.

At least the supposed united “stand” of only answering the given seems to have been thrown out the window, although they did tell one of their underachieving classmates during the test that he could still write down the given.

And as usual I told them that the questions will be their problem set due on Thursday at the start of the class. I have to emphasize that. There were still papers trickling in while I was lecturing last time, and, of course when I turned my back on the desk some of them could have slipped their submissions on the table.

Still on one day per entry. It makes good closure, but I wish I could speed up the backlog. Maybe I’ll try something new tomorrow. Anyway, we’re finished for this class today.