Electric Field Lines and how Martin Kirby introduces them

Readers, forgive me, for I have sinned. I have not taught my AP physics B class how to sketch electric field lines.

Now, before you call the official College Board Audit Police, I ask that you hear my reasons. After all, I certainly teach about electric fields in general, what electric fields do to a charge placed in them, and even how parallel plates or point charges can themselves produce electric fields. We even do an extensive variety of exercises in which we determine the magnitude and direction of the electric field produced by a multitude of charges at various positions in space. (See this post, for an example of fundamentals quiz questions on electrostatics.)

I’ve never introduced the field line representation formally. If I mention field lines at all, it’s because my class has done an old Physics Bowl test question asking about them. I’ll then note that field lines just tell us the direction of the electric field at any point, with the closeness of the field lines indicating the strength of the field. I say this in sort of “by the way” manner, making it clear that electric field lines are not something that the class needs to know about. And until this year, AP exams have not been particularly concerned about field lines and their meaning.[1]

Why don’t I pay more attention to field lines, which any physics professor would consider part of the cannon of introductory physics knowledge? Because first-year physics students have enough trouble merely understanding what an electric field is without trying to represent the electric field in an abstract way. On the rare occasions I have introduced field lines, students have tend to memorize pictures without grasping the reason for them. Second year students seem to get the idea, but newbies? Not a chance.

I may change my approach next year, if I can find a little bit of extra time. Martin Kirby, a fellow long time AP physics reader, described to me his approach to electric field lines. After his class has mastered[2] the idea of what an electric field is and what it means, he guides his students through a discussion of the electric field near a point charge. “Describe [in words] the electric field near this positive charge,” he says. His students indicate that the field points away from the charge, and is stronger near the charge than far away. Then comes the magic question:

“Okay, if you were asked to DRAW the electric field, how would you do it?” Then he lets the students chew on that for a while. They come up with all sorts of clever ideas, often involving the degree of shading, or colors, or annotations with numbers. After about 20 minutes of discussion and student attempts, he shows them an idea. “What if we drew lines,” he asks. “We point the lines in the direction of the field. The more lines we draw, and the closer those lines are to each other, the stronger the electric field.” Finally, Mr. Kirby shows the class how his idea works for a single point charge, as well as for two point charges.

Because the students have spent so much time trying to make their own representation of an electric field, they tend to latch on to Mr. Kirby’s simpler option.





[1] Take a look at 2009 AP physics B problem 2(a), which asks students to sketch electric field lines near two positive point charges.
[2] For the highest available value of “mastered,” of course

Just so you know I haven't disappeared

Yeah, I haven't posted in nine days. I've been reading a thousand AP exams every day, and that takes a lot out of the ol' brain, thank you very much. I'm grading problem B2, the one about two hanging point charges. I'm seeing electric field lines -- both correct and incorrect -- in my dreams at night.

I think of the AP reading as a physics teachers version of a fundamentalist revival. The people here are awesome teachers. Yeah, we all complain now and again (and again) about some of the stupidiy we have to deal with in educational institutions, about our colleagues who have not the first clue what does or should go on in a physics classroom. It's nice to be around people who understand and sympathize.

Obviously, kvetching isn't the only thing we do here. Mainly, we talk shop. I've written down idea after idea for future blog posts, and for my class next year. Problem is, I don't have the mental energy after a nine hour grading day to write up each post.

So today you get a bit of a preview of upcoming posts. After the reading ends this weekend, I'm going to Georgia for a week-long summer institute. I may or may not be able to post from there (though I'll get many more wonderful ideas from the critical mass of physics teachers who will congregate). Soon, though, I should be able to flesh out posts about:

* AP physics at the 9th grade honors level
* How to introduce the concept of electric field lines
* Using folders to speed and organize grading
* The importance of language and notation
* Argumements and ideas about how to teach Newton's second law
* Eddie the Electric Elephant

Don't abandon this blog. I'll be back. If there's something you want me to talk about, post a comment!

GCJ

Off to the AP reading

I leave Monday morning for Fort Collins, Colorado, where I'll be a table leader for the 2009 AP physics reading.

No, there's no use offering me a bribe to give a few extra points to your class (though every year some overly optomistic student encloses a buck in his or her test). I won't even know who your students are; and with ten or so readers on each problem, there's only a small chance that I, personally, will even be the one grading any of your school's tests.

I'll spend the first couple of days developing the rubric to problem 2 on the physics B exam. That's the one with two charges hanging from strings. I have a preliminary idea in my head of how to distribute the 10 available points. Tuesday I'll have a day-long conversation with my partner -- Denver-area teacher Briant McKellips, whom I've graded with for nigh on six years, now -- to mesh our ideas together. Then we'll go through a few hundred tests to try out our rubric, and adjust it where necessary. On Wednesday we have to defend our rubric in front of 20-odd table leaders, who are collegial, friendly, and merciless in physics arguments. Wednesday's discussions are the primary manner in which we ensure that the grading standards for each problem are of the highest quality.

I'll write several times from the reading, but not to dish College Board dirt. You won't hear how awful the Chief Reader is -- primarily because Chief Reader Bill Ingham, of James Madison University, is one of the most awesome people in the universe. You won't hear specific stupidicisms that I read on the exams, either -- I've only once encountered a truly clever and original non-physics item on an AP test. You can generally read better teachery humor at random internet sites.

What you might hear are summaries of the great shop talk that takes place at the reading. I spend ten evenings or so gabbing about physics teaching, absorbing ideas from the best physics teachers in the country. The Reading is the best professional development I've ever been to. Even though this is my tenth reading, I have never failed to learn something new.

For now, though, wish me luck in the monstrosity that is the country's air transportation network. For conversation at the first night of the reading tradionally consists of complaints and bitter war stories of everyone's adventures getting to Fort Collins.

GCJ

Sporcle!

Regular followers of Jacobs Physics know that I am not ever in favor of “wasting” time in class. Every class session begins with a short quiz, all the better to avoid five minutes of getting settled. I teach all the way to the end of the period. If I have five minutes left at the end, I don’t give time to start the homework, I don’t shoot the breeze with the class, and I don’t let the class go early; usually I’ll pull out a “check your neighbor” question, or I’ll preview tomorrow’s quiz, or I’ll give a Socratic hint for the night’s homework assignment. Maybe I’ll even begin the next day’s lecture.

I’m very strict about these time usage guidelines at the beginning of the year. But in the spring, especially as the end of the year approaches, I become ever more relaxed. The students – and I – have earned a break sometimes. In fact, in general physics, the final assignment is for students to solder together an AM radio from a kit, and to do so at their own pace. I’ve found this project to be an excellent way to keep the attention of the seniors in their last week of school.

Sometimes in the last couple of class days there’s just no way to avoid wasting time. I was in this situation this year when I had three juniors left to teach because the rest of my class consisted of seniors on their culminating experience. We finished the lab cleanup and organization. We had 20 minutes left in class. What did we do?

I took my cue from colleague Jacob Sargent’s bag of tricks: I called up sporcle.com on the projector. Not familiar with Sporcle? It consists of a large number of “quizzes” which are sort of an online version of that old game Scattergories. For example, sporcle will show a map of the United States and ask you to list the capitals. As you type in “Lincoln,” the location of Lincoln, Nebraska will light up on the map. But hurry… the timer is counting down. When the timer expires, or when you give up, you see the answers you missed. Then you can click for a histogram of past performances. Generally, between 10,000 and 100,000 people have taken each quiz. You can see which state capital was most frequently identified, and which was most frequently missed; you can see where your score ranks.

Of course, sporcle has much more than just state capitals. I did pretty well on the “countries of Africa,” but not so well with “flags of the world.” Beyond geography, they have Literature (list the Shakespeare plays, name the 50 different words in “Green Eggs and Ham”), Sports (list the Stanley Cup winners since 1950, who was on the roster of the 2004 Red Sox), and all sorts of random or arcane topics (name things beginning and ending with “d,” name the Western Roman Emperors from 300-470 AD).

Now, Sporcle could easily be used for physics purposes. They have a wonderful “Can you name the S.I. units?” quiz, which I recommend for you and your class. Users can author quizzes; next year, a class assignment will be to write a quiz with “relevant equations for the AP physics B exam.”

But my class wasn’t too interested in the science category (where the “anatomy of the heart” quiz might have helped their performance on the biology final). They preferred the VH1 top 100 songs of the 1980s: we saw a list of 100 songs, and we had to name the artist. Oh, boy, did everyone get into that. It was a fun and relaxing way to end an intense year.

GCJ

In opposition to the summer assignment

This time of year, many physics teachers, especially AP physics teachers, are preparing summer assignments for their upcoming classes to submit in the fall. While I understand the intended benefits of such assignments, I suggest that, in every circumstance I can think of, a summer assignment is a BAD IDEA.

First of all, imagine that your summer assignment will produce every possible benefit you imagine to the start of next year’s course. At what cost have you achieved these benefits?

Your students do not enjoy doing physics. Okay, maybe by the END of the course, some students will begin to see the intellectual attractiveness of physics. After all, one of the reasons we teach physics is in order to help students see the beauty in our subject. But in August, even the brightest 16-17 year olds in your class would rather be pursuing other wholesome and not-wholesome summer activities than doing your assignment. By requiring summer work, you begin the year with a hostile class: they come the first day with their assignment thinking, “Why’d you make us do this? It had better be worth it.”

Furthermore, even if your assignment is perfectly designed to produce maximum learning benefits, how many of your students will complete the assignment with appropriate care and attention to detail? It is far more likely that your students race to just get the dang thing done, minimizing any benefit they might receive through their effort.

For that matter, how many of your students will complete the assignment at all? A student who didn’t do the summer assignment has now forced you into an early test of wills. Either you make him do the assignment and take off substantial credit, setting yourself up from day one as a hostile entity; or, you give this student a mere slap on the wrist, showing your colors as a pushover whose assignments aren’t REALLY important. Why not save that fight for an assignment without a built-in excuse?

[I’ll make an aside here about enrollment rules. Depending on your school, AP enrollment might be set in stone in May, or might be still in flux all the way through August. If students are allowed to make decisions during the summer, then a summer assignment will reduce enrollment – AP physics is a much tougher sell if it includes a packet of stuff to do. And what about students who might be new to the school? A summer assignment deters them, too.

Thus, I can see one time when a summer assignment might be useful. If your course is oversubscribed with students of moderate talent but considerable feelings of entitlement – the “pushy parent” types – a straightforward summer assignment could be used as a filter. Students who are being pushed by mom and dad to take AP physics but who really don’t want to do the work (or who are not capable of doing the work) can use poor performance or lack of effort on the summer assignment as a reason to drop the course. I would not recommend this course of action except in extreme circumstances where you are in desperate need of a “weeding” mechanism.]

And finally, let’s look at that assumption of maximum learning benefits. If it is done carefully with attention to detail, what should a student expect to get out of a summer assignment? Most summer assignments I’ve seen have been heavy on the mathematics. Well, it’s been shown[1] that math review in isolation does little good, and likely does harm. Math assignments give a false impression of what physics truly is about. Good math students become overconfident, then frustrated when they don’t pay attention to how to set up problems. Mediocre math students become frustrated right off the bat, even if they might become excellent physics students due to their conceptual understanding.

If not a math review, what would that summer assignment be? Are you going to ask students to learn kinematics or Newton’s Laws on their own? I wouldn't recommend that course of action, as the most likely result of such an attempt is that common misconceptions become further ingrained before you have a chance to address them in class.

My fundamental conclusion is that whatever positive impact a summer assignment might possibly have on next year’s course is not outweighed by the substantial cost incurred by requiring summer work.

Think of it this way. It is trendy for AP classes across disciplines to make summer assignments. History and English especially tend to load students with summer reading in order to get a head start on a difficult and broad course. And, your AP students are likely taking at least one or two of these other AP classes. Consider how much political capital and general goodwill you can generate by showing up on the first day of class saying, “Let’s learn some physics from the ground up” rather than, “All right, how many of you didn’t do your summer work?”






[1] in the classic Arons “How to Teach Physics” text, repeatedly in the physics teaching literature, through anecdotal evidence from good teachers, through my own experience…