Student "edition" found at {csi dot journalspace dot com}.

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

In the last Electromagnetic Theory class since my most recent post, I started with the translation of axes for all three… axes, and not just two rotating around one which I had last time, that had only one adjustment angle.

To specify the seriousness of this configuration I gave the two points at the end of the line charge, and the coordinates of the point charge relatively nearby as well.

The important thing to get here was the point along the extension of the line that is closest to the point – in other words at perpendicular distance from it, because this will serve as the origin of the translated system. Then the distance to the point and the nearest end of the line of charge could be obtained.

We started by getting the plane in which all three points exist, which is of course the new y-z plane. We also got the vector normal to this plane, which was now our new negative x axis. From the two end of the line we also could get new z axis.

Using the cross product again, we could get the new y axis from new x and new z.

Next we could get the equation of the new x-z plane, where the line of charge lies, where the constant could be obtained by substituting one of the two points.

We could also get the equation of the line perpendicular to the plane and passing through the point charge by using the equation of the line in three dimensions given a point and a vector. Thus afterwards we could get the intersection of the plane and the line, which is our new origin. Then we solve for the distances.

These we used to solve for the forces, after which we had to translate to the original axes using the three new angles of movement we solved, based on spherical coordinates (I also gave them cylindrical, and the translation of each).

I told them this would be part of their exam on the next meeting, so when could they pass the assignment? The day before the exam, of course, so I could tell if they really did know it or not. Because otherwise I could just assume they knew it and placed a question about it in the exam. I wasn’t surprised the chose the option to pass the assignment the day before the exam.

I also gave plus points in recitation to those who answered my theoretical and recap questions correctly.

Session 1501 just copied and didn’t listen, as evidenced by low exercise grades. Class dismissed.