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18 May 2009

Last test in general physics


The school year is winding down now.


(So why haven’t you posted in a week, then?)

Well, because it was busy last week: AP exam and such. I’ll have lots more to say about the AP, especially as I plan to grade problem B2. Until then, though, I’ll talk about finishing the year in my general physics course.

I’ve always done circuitry as the last unit in general physics. It’s not necessarily easy, but it’s different and fun, with lots of hands-on lab work. When I am convinced that students can solve for current and voltage for each resistor in a simple circuit, we move on to building an AM radio. I buy an Elenco AM radio kit for each student. (You can get these through Fischer Scientific… or you can get them even more cheaply online if you search hard enough.) I show them briefly how to solder, then I have them follow the instructions to assemble the radio. It takes about five class days, and about 70% of the radios end up working.

Nowadays, I’m not allowed to give a final exam to my seniors – and all but three of my class are seniors. So I give a last test about a week before the end of school. This isn’t a cumulative final exam, but rather includes topics from the last trimester: optics, circuits, and astronomy.

For optics, I give question 6 from the 2007 AP physics B exam. This is an experimental question in which students graph 1/di and 1/do for a lens. My class did this very experiment a few weeks ago. Thus, I can expect my general class to perform well even on a somewhat difficult AP problem.

For circuits, I give question 3 from the 2007 physics B exam. This question begins with a ranking task about a simple circuit, and then asks for some calculations. Once again… this is EXACTLY what we have been doing in class.

For astronomy, I ask a series of seemingly simple questions about the sun, earth, and moon. Careful… my students consider these kinds of questions quite difficult. This year I spent more time than ever on just basic solar system astronomy, and it’s paid off – but still, very few people got these problems completely correct.

If you ever decide to run a 1-2 week astronomy unit, these questions, among others, can help you set the goals for your topic coverage. Take a look at the problems below.

But first, why did I post the Corona ad in which the moon serves as a lime? Something is astronomically wrong with the advertisement. Post a comment or send an email to suggest what that is.

1. (15 points) The tilt of the earth’s axis is 23o. Consider the view of the sky from the vantage point of the north pole. Justify each answer briefly.

(a) On June 21, what will be the highest altitude of the sun above the horizon?
(b) On the equinoxes, what will be the highest altitude of the sun above the horizon?
(c) On December 21, what will be the highest altitude of the sun above the horizon?
(d) Describe the path that the sun takes through the sky over the course of one day in June. Explain your reasoning, of course.
(e) Now consider a time of day and year that you can see the stars from the north pole. Where in the sky will you see the north star?
(f) Describe any circumpolar stars you would see. Explain your answer.

2. (15 points)
(a) During what phase of the moon can a lunar eclipse occur?
(b) Show with a sketch the relative orientation of the earth, moon, and sun during a lunar eclipse.
(c) Explain why we don’t see a lunar eclipse EVERY month.
(d) At what times during the day and/or night is a full moon visible? Explain.
(e) Explain why we see phases of the moon, i.e. why we sometimes see crescent or half moons.

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