On the Plus Side…

By cjohnson | February 22, 2006 9:44 pm

…. I gave the midterm for my Electromagnetism class today. Stayed up until 1:30am putting the finishing touches on it, and then up again four and a half hours later to go in early and get it and its accompanying formula sheet (yeah, yeah, I’m getting soft, I know) photocopied, and ready to give to them at 8:45am in class. They had one hour of furious computing -and a bit of thinking- to do. These ones are hard to get right since you’ve got to gauge their ability to do enough in the short time allowed. Get it wrong (like I did with the equivalent mid term last year) and they just can require way longer to do it than they have time for. It is really important to recall that they are still young and don’t take all the same computational shortcuts that one does later in life. And recall that for upper-level courses I like to have a bit of fun new stuff in there too, so I had to think carefully about how to do that in one hour too. (The plan was to do this all with the aid of the atmostphere at the Cat and Fiddle, like last week’s midterm prep session, but in the end I got sucked in by several incredibly campy Queen videos -how come we never saw that aspect so clearly as kids?- on You Tube…., which made for a background which is more relevant than you think… more on that later.)

So it worked! And they (most of them I think) really liked it (at least a number said so immediately after, and they were all chatting happily about bits and pieces on the exam after they’d done….. which is good)… there did not seem to be that air of them having been treated unfairly that happens when an exam misses the mark. This is easier with 12 students, than say, the 250+ freshmen I (and my colleague) set a midterm for last week. That one went ok, but was actually too easy, and now we have to alert the students to the fact that they should not feel overly confident about how they are doing….

For my E+M midterm today, it was just right, and as a bonus I think they learned something about superconductors and superconducting rotating spheres and the Meissner effect… oh, and why the electron is not a tiny rotating classical spherical shell of charge, which everyone should know, of course. So what’s not to like? (as they say).


CATEGORIZED UNDER: Academia, Personal, Science
  • Chris

    I don’t suppose you publish old midterms on your website?

  • http://blogs.discovermagazine.com/cosmicvariance/clifford/ Clifford

    Only for internal consumption. Same with homeworks, etc… This is because I also post solutions, etc, with such material (and I sometimes borrow heavily from the literature -there are only so many good problems out there-), and then I end up getting email from some professor in Mexico (I think it was, last time) saying that her students are getting all the answers to her homeworks from my website.



  • http://deferentialgeometry.org/ Garrett

    Instead of a formula sheet, why not give open-book tests?

  • http://blogs.discovermagazine.com/cosmicvariance/clifford/ Clifford

    Open-Book is a different dynamic, and a different kind of test. And you know what? In a short test, an open book exam does the student more harm…. rather than think about what they are supposed to do they instead waste time looking for stuff in the book or in their notes. This does not help them. Better – in a fixed time- to have a fixed task in mind, and a short sheet of formulae if neccessary. Then if they don’t immediately know something, tehy know immediately that it is because they have to figure it out, rather than spend time looking for things….. etc. etc…. open book is appropriate for a differnt kind of exam, testing a different kind of thing than I am testign here. I want them to learn how to calculate, first and foremost. No amount of opening books is going to tell them how best to set up a coordinate system, or keep track of the flow of energy, etc., etc.



  • http://feynman137.tripod.com/ Science

    ‘… and why the electron us not a tiny, rotating classical shell of charge…’

    May I ask what kind of reasoning you used, please?

    1. The equator would have to go at a speed of 137c:

    ‘It had been an audacious idea that particles as small as electrons could have spin and, indeed, quite a lot of it. … the ‘surface of the electron’ would have to move 137 times as fast as the speed of light. Nowadays such objections are simply ignored.’ — Professor Gerard t’Hooft, In Search of the Ultimate Building Blocks, Cambridge University Press, 1997, p27.

    2. The centripetal accelerative motion of charge would radiate energy (like QFT says it does if gauge bosons carry energy and are continually exchanged between charges to create electromagnetic force fields)
    3. The electron is really a string in 10/11 dimensions
    4. The half-integer spin of the electron means it would have to rotate 720 degrees to get back where it started (like the Moebius strip-loop with half a turn, so you trace out a single line twice the circumference, i.e. two rotations to return to the starting point)
    5. The electron is really a Penrose twister of some sort
    6. Heisenberg’s uncertainty principle says the electron will move when measured, so we can forget classical ideas
    7. The Standard Model says the electron’s mass is due to the Higgs field, so the electron is confined electromagnetic energy

    Just curious!

  • http://countiblis.blogspot.com Count Iblis

    Clifford, where you awake enough during the exam to be sure that no one cheated? :)

  • http://blogs.discovermagazine.com/cosmicvariance/clifford/ Clifford

    Science: Yes, #1 is the first thing you notice with the classical computation.


  • http://feynman137.tripod.com/ Science

    If they’re clever enough to cheat, they deserve to pass 😉

  • http://blogs.discovermagazine.com/cosmicvariance/clifford/ Clifford

    They are a good bunch, and would not cheat. Anyway, as an invigilator/proctor, I was as sharp as a … very sharp thing.


  • citrine

    Exams are a necessary evil of college life – until someone comes up with a better alternative or at least better ways of testing someone’s competence in a particular subject area.

    When people complain about exams I tell them that according to Socrates, “the unexamined life is not worth living”. :(

  • http://valatan.blogspot.com bittergradstudent

    re: cheating. A friend of mine loves to tell the story of the time that he decided to force students to sit in randomly determined seats for exams. This football-player looking student of his, who he had been thinking was cheating throughout the course of the semester, looked down, stressed out at his paper, and ended up getting up after 15 minutes and turned in a blank sheet of paper.

    And congrats, clifford. I love exams that push the taker to do a new calculation and learn something new within the course of the hour. I don’t think anyone got into physics in order to be able to write down the Green’s function for a given differential equation, in and of itself.

  • http://countiblis.blogspot.com Count Iblis

    From my own experience, it is almost impossible to have a good one hour exam. Most exams here are 3 hours and usually we have to extend it by half an hour. The best way to test students is to give them some projects to work on. It’s also more fun for the students.

    Science about point 2, see here.

    Also it is Gerard ‘t Hooft not Gerard t’Hooft :)

    The problem of neutral conducting rotating spheres is actually very interesting too, see here.

  • http://blogs.discovermagazine.com/cosmicvariance/clifford/ Clifford

    As long as what is asked matches the duration appropriately, an exam of any length can have value, depending upon what you are testing…. from five minutes to five days….. but it takes care to make sure to get the balance right…..


  • Stephan

    I agree about open book exams being harder than closed book ones. My astrophysics course last semester had an open book exam that was an hour and a half rush, mainly because I spent so much time searching for things. The final was closed book (with a formula sheet), much more pleasant and much easier, although it asked questions at about the same level of difficulty.

  • http://eskesthai.blogspot.com/2006/02/history-of-universe-and-standard-model.html Plato

    From a layman’s view.

    Within context of the fundamental forces, understanding certain attributes, in relation to distance, makes this very interesting, when applied to the “unifying of forces” in a global perspective?

    For instance, electromagetic and gravitational forces grow stronger at shorter distances, while the strong force, grows weaker. View seen in QQ distances, in regards to strong force.

    So idealization about experimentation of the gravitational force, let’s say at shorter distances, becomes interesting perspective and question?

    Froma geometrical standpoint Inverse square law becomes applicable.

  • http://feynman137.tripod.com/ Science

    ‘… while the strong force, grows weaker…’

    This is why quarks have asymptotic freedom: because the strong force and electromagnetic force cancel where the strong force is weak, at around the distance of separation of quarks in hadrons. That’s because of interactions with the virtual particles (fermions, quarks) and the field of gluons around quarks. If the strong nuclear force fell by the inverse square law and by an exponential quenching, then the hadrons would have no volume because the quarks would be on top of one another (the attractive nuclear force is much greater than the electromagnetic force).

  • Scott O

    There is nothing harder than writing good exam and HW questions. Especially when I teach particle physics, I find that there are really only a small handful of different kinds of questions that an undergraduate is capable of calculating in a reasonable time. You can only do so much particle physics with a group of students that doesn’t know any field theory.

    On the other hand, I once had a comment on my student evaluations that I asked too many different kinds of questions, instead of repeating types of problems in order to give the students more practice. You can’t win, I tell you.

  • http://eskesthai.blogspot.com/2006/02/history-of-universe-and-standard-model.html Plato


    I’m trapped in a way of seeing.

    The situation in regards to strong force, run contradictory(in relation to strength) to distant measures in gravitational and electromagnetic forces. While the strong force gets weaker as the QQ distance gets shorter, the opposite was true, of the other two forces.

    While I had been focused on the cosmo at large, the very beginning had to be included in the standard model. The standard model being complete picture minus gravitational force carrier(?) had to have ways in which to look at the “strength’s and weaknesses of that analogy of the membrane.”

    It was just “ways” of looking at a membrane that had me puzzled, while in those abstract spaces, such concepts had to amount to something? “Gaussian coordinates” were simple, as a understanding of that field?

    Layman scratching head.

  • http://feynman137.tripod.com/ Science


    It is well known you can’t isolate a quark from a hadron because the energy needed is more than that which would produce a new pair of quarks. So as you pull a pair of quarks apart, the force needed increases because the energy you are using is going into creating more matter.

    This is why the quark-quark force doesn’t obey the inverse square law. There is a pictorial discussion of this in a few books (I believe it is in “The Left Hand of Creation”, which says the heuristic explanation of why the strong nuclear force gets weaker when quark-quark distance decreases is to do with the interference between the cloud of virtual quarks and gluons surrounding each quark).

    Between nucleons, neutrons and protons, the strong force is mediated by pions and simply decreases with increasing distance by the inverse-square law and an exponential term something like exp(-x/d) where x is distance and d = hc/(2.Pi.E) from the uncertainty principle.


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