Grading / Quiz 1

I will have graded things back to you on Friday, sorry for the delay.

Also: quiz 1 and a solution.

Update: having graded the quiz, I'm just going to throw out the first question. Apparently I was a bastard for asking it ... we'll go over the quiz tomorrow in recitation.

Pig Flu / Aporkalypse Now

If, for some reason, you end up missing an exam due to the pig flu, I will weight the final proportionally more.

If you miss something else due to the apparent pig flue epidemic (e.g., homework, quiz, lab), you can either make it up (if done within a short time) or receive a 'bye' on that particular assignment.

I do not become ill while teaching, as it turns out. I did teach with nearly a dozen hour-old stitches in my right hand and a head full of painkillers, but illness is right out. So I'll be there.

Just sayin'. Apparently, this is a Big Deal. Wash your hands, go to the med center, procure notes, etc.

(I am not belittling the public health threat here. I am, in fact, mocking the general response to and hysteria surrounding the public health threat.)

Homework 2 #10

Just for fun, here's a picture of the electric field surrounding the two rods for HW2.10. The lines correspond to contours of constant electric field. I guess you can figure out where I placed the rods ...


Yes, this is what I do when I have some free time.

Question 5 / HW2

I misspoke in class today ...

We calculated the potential due to a semicircle of charge, that was OK. I forgot about the line segments ... they do not cancel each other out, since they are both positive, and potential is a scalar (so there is no 'opposing direction').

What you need to do is superimpose on the semicircle result the potential due to two line charges a distance r away (along the line axis). Since both line segments are the same, find the result for one line and double it ...

See, e.g., PH106, F08, HW3, Q5.

Since I misspoke in giving you a hint, I won't count off if you miss the line bits.

It could be worse.

A truly pathalogical function. Continuous everywhere and differentiable nowhere.

A nice quote:

While it's not very common that badly-behaved functions arise in physics, there are functions which at least don't always remember to say please and thank you. They have to be gently corrected, but they're good at heart. The mathematicians are the ones who have to deal with the truly shady functions, the ones who form prison gangs and don't play by the rules and obey the laws. Or theorems.

I have an image of rogue mathematical symbols ganging up on me now. Great.

He's no Wolfram, but then again, who is?

Look, a derivative calculator!

Also, Dr. Wolfram & Co. have more tricks up their collective sleeves. No identity too obscure, no function too pathalogical.

Problem 9 / HW 2

Problem 9 on homework 2 is the same as Griffiths problem 2.41, by the way. However, I think it is conceptually easier to tackle the problem by first finding the field from a short line charge, and then building a plate out of line charges. If you do this, you will need an obscure identity to recover the same form as Griffiths.

\tan^{-1}{\left(\frac{2z}{z^2-1}\right)}=2\tan^{-1}{\left(\frac{1}{z}\right)} \pm n\pi

Here n is an integer. Just saying ... if you solve the problem the way I demonstrate in class (which is, I think, conceptually easier and leads to the appropriate limits more easily), there is some work involved to check that is the same as Griffiths' result.

I'm sure you realized that you can't use Gauss' law by this point. The fields of a finite square plate have an icky symmetry to them, as does anything square-ish when you're dealing with radial fields.

Also, problem 10 is the nearly same as a PH106 problem I assigned last year. Excepting that the integrations involved are more painful.