Built on Facts

Class Day & and Induction Quiz

Posted by Matt Springer on July 7, 2009
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Today was my first day teaching recitation for the summer session Physics 202, the algebra-based second half of intro physics. Physics 202 focuses mostly on electricity and magnetism, where the first course (Physics 201) was mostly classical mechanics.

The summer semester is split into halves for university-wide administrative reasons that are somewhat obscure. The gist of it for me is that I’m only the TA for the second half, replacing the previous guy. As it happens my first topic was electromagnetic inductance, and after going through inductance, mutual inductance, energy in an inductor, and inductors in circuits I gave them a quiz with two questions.

1) How much energy is converted to light and heat by a 100 W bulb in 1 hour?

2) If the amount of energy found in 1) is stored in an inductor carrying a current of 10 A, what is the inductance?

I didn’t make this quiz myself, it’s verbatim from the book homework with the numbers changed. First day, don’t want to be too obnoxious.

A surprising number of people missed part 1. This is either after-lunch lethargy or I have a lot of repair work to do on their prior physics education. For those of you who haven’t had physics, you find the answer like this: a watt is a unit of power, which is energy per time. In the SI system energy is measured in joules while time is measured in seconds. A watt is one joule per second. If power is energy per time, energy has to be power multiplied by time. So we have (100 J/s)(60 s / minute)(60 minute/hour) = 360,000 J.

Part 2 is just plug-and-chug since we went over the equations in class and in fact I printed it on their quiz:

i-d4bb9fe09d6b78eb832985821d8a307f-1.png

Solve for L:

i-88516ca505d84f969b7ab15a965c3fb3-2.png

Plug in and you get 7,200 H, where H stands for henrys, the unit of inductance. This is a whopping huge amount of inductance; the quiz is designed to drive home the point of scales to develop intuition for these sorts of problems. It looks like we have a ways to go, but that’s why I rake in the big bucks.

(I don’t actually rake in the big bucks)

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Comments

  1. #1 CCphysicist
    July 7, 2009

    That teaching system is nuts, even if it is needed as a way for the faculty to do research half of the summer while still covering some classes. If you had mentioned it earlier (maybe you did and I missed it), I would have advised you to sit in on a class or two before you took over, just to get some sense of the style of the previous instructor.

    If they miss the first one, they simply aren’t doing their homework. That would be the most likely explanation.

    It is a fair question, and a perfect example of the kind of skill they will be expected to have going into the MCAT. You might do the basal metabolic rate problem — given a diet of 2000 Calories = 2000 kcal, what is the average power output of a human body — at the start of the next class. (The answer is about 100 W, for a nice tie-in.) It only takes a few minutes. Or give them a quiz to find out whether their knowledge gap concerns the number of seconds in an hour or the definition of Watt.

    PS – What a crazy place to break the course! That makes it almost like a tag-team match.

  2. #2 MartinB
    July 8, 2009

    May it be that students were confused by question 1 because they thought (as I did, reading the question) that they have to calculate the two energies separately for light and heat?

  3. #3 CCPhysicist
    July 8, 2009

    Interesting comment from MartinB.

    When I have asked that sort of question, I usually say something like “light (both visible and infrared)”. But that group of students would probably be confused more because they don’t yet know that infrared “light” is what you experience as heat.