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

I had the second back-to-back session in Trigonometry yesterday, but I still have to catch up on the events of the day before that, so again the previous day will have to wait until next time.

Now, onto the electricity and magnetism exam last Thursday: the first two questions were the two most “difficult” from the earlier fifty-point quiz.

The first was the electric charge after two spheres are momentarily joined, which uses the quadratic formula. The second was the electric field at the third point of an equilateral triangle with an electron and a proton at the other points.

I didn’t even change the numeric values. Yet there were still questions on how to find the charge of a proton and an electron, even after some of the guys had already stated that the problems looked familiar.

These items also had a larger weight: fifteen points each, which roughly took care of one third of the hundred point total as I promised.

The third problem asked for the flux on the individual faces and the total for a gaussian cube aligned along the positive axes, with the electric field given as two vectors along the x and the z-axes. The flux for two of the faces is at right angles with the perpendicular vector of the area, and for the other four are similar values with opposite signs. Thus the total flux is zero. We did a similar solution for one face in class. Since this one had six parts: one computation for each face, one for the resultant electric field and one for the total flux, I assigned forty points for this one.

The fourth question, still on Gauss’ Law, was exactly the same as a given seatwork: getting the charge, flux on the surface and electric field (at five points each) of a conducting sphere given the radius and the surface charge density. As far as I know this is the only question that can be solved by direct substitution on the given equations, and I already heard that some were able to answer this easily.

The last question (on electric potential) has three particles on three corners of a square, and asked for the electric potential energy of the system and the potential at the fourth corner, for fifteen points each, which is simpler than some of the examples I’ve given (with the charges arranged on a three dimensional pyramid).

That’s a total of one hundred fifteen points over a hundred. I still heard reactions that some students found it difficult though.

By the way, one of the students made one paper with all the equations he had in his notes, which I photocopied and distributed along with the questionnaire. There were some equations missing, such as the area of a sphere and the relationship of surface charge density with charge and area. I also had to correct some instances where “q” (charge) was mistaken for “a”. I instructed them that for the next exam, they should include the units and the scientific notation equivalents of the prefixes such as “micro” and “nano” so I wouldn’t have to keep answering questions about those anymore.

And just like in the first quiz, I told them that the last three questions (not the first two, obviously for me) would be another problem set for them that is due on Thursday next week.