display | more...

This year I'm a physics teaching assistant for a freshman electromagnetism course (got to pay the bill somehow), and what I always find remarkable when I'm TAing, is that students in an honors-level course at a top institution have so much trouble with the concept of physical units. Some students just don't understand that if the problem asks for the power some circuit draws from the power supply that just giving a number without units is completely meaningless, and doesn't correspond to a quantity of power. Maybe it's my fault. Maybe I'm not getting the concepts through to them, but I don't think so. For example, the other day I had a student come to my office hours to complain about my marking him down on a problem set, and I spent nearly ten minutes trying to explain to him why his answer didn't make sense and about the importance of specifying units. Here's roughly how the conversation went:

TA: Like here, for problem 1. You just wrote "1.4" as the answer. "Watt" is missing.

Student: I don't know. Is anything missing?

TA: Watt

S: You say there's something missing.

TA: Yes.

S: You marked me down for this problem.

TA: Yes.

S: Well, then what is missing?

TA: Yes.

S: I mean, what is left out?

TA: Yes.

S: What is the thing I got wrong?

TA: Yes.

S: Why did you mark me down?

TA: "Watt" is missing.

S: If I knew, I would have written it down. I should have written what?

TA: Yes.

S: You think something is missing?

TA: "Watt"

S: I don't know, you said there's something missing! Look maybe you were grading this when you were up late at night and you got confused? You graded this problem what hour?

TA: No. Not this problem. The second problem you should've written "Watt-hour".

S: I'm not asking you when I should've done which problem. We're talking about the first problem.

TA: "Watt" is missing in the first problem.

S: I don't know. What is missing?

TA: Then why are you saying you don't know.

S: Because I'm not sure what is missing.

TA: You're saying I might be wrong?

S: Well, I don't know. You wrote the solutions to problem 1 what hour?

TA: No, that's problem 2.

S: OK. Let's talk about problem 2. what is missing on my answer to problem 2?

TA: No, "Watt" is missing in problem 1.

S: Hold up, one problem at a time!

TA: Don't change the units around. Units are very important. Without units the numbers are meaningless. For example, the answer to problem 3 is a flux density. So, the units are...

S: I don't know.

TA: Gauss.

S: I don't know.

TA: The units are... Gauss.

S: I don't know, the units are what?

TA: No, Watt's the unit of power.

S: What's the units of power?

TA: Yes.

S: I don't know. I think about it, and it hurts.

TA: That's frequency!

S: What's frequency?

TA: No, Watt's power.

S: I don't know the units of power. Maybe if I knew the units of flux density, it would help me understand the units of power. The units of flux density are ...

TA: Gauss.

S: They are ...

TA: Gauss.

S: They are ...

TA: Gauss.

S: Hold on, I'm trying, cool on.

TA: That's electric charge.

S: What's electric charge?

TA: Watt's power.

S: Stop asking me that, I'm getting a headache. Arrgh, it hurts!

TA: No, that's frequecy!

S: What's frequency?

TA: Watt's power.

S: Just tell me the units of power.

TA: Watt's power.

S: You're the TA. Just tell me the answer.