I ask students to read their textbooks very, very rarely. Textbooks can be excellent references, they can provide sources of problems, examples of problem solving processes in action... but it is rare that a first-time physics student can simply read and understand a section in a textbook.
Well into the school year, after I have hammered the class about the difference between physics and math, when the students themselves are demanding demonstrations and physical (rather than mathematical) explanations of new topics... then and only then will I consider assigning some reading. And even then, that reading must have a clear and useful purpose.
One reading assignment I've used repeatedly is to introduce the magnetic force on a charge and the first right hand rule. Serway's presentation is pretty good, though equation-heavy and technically written.
If I were to present this topic in class, I'd be stopped with questions at every turn: "Is a magnetic field the same thing as an electric field? What does the θ mean in F=qvBsinθ ? Does a negative charge get forced the opposite way as a positive charge? Why doesn't a charge experience a force if it moves along the magnetic field lines? Why do we use B for magnetic field, not M?" I'm quite proud of my class for their inquisitiveness, I'm pleased that they expect these sorts of questions to be answered. There have been numerous times in class when I've encouraged, nay, DEMANDED such questioning. It's difficult for me to communicate "Shut up right now and listen for ten minutes while I just show you the fundamentals. You'll have time to play with these new ideas on homework and in class tomorrow, but for now, I just have to feed you this information. So be patient and quit buggin' me."
I assign the brief section of text in which the equation F=qvBsinθ and the right hand rule is introduced. Sure, this is pretty confusing, even to my experienced AP students. But now they know what questions to ask. And the text answers many of these questions, so I can go straight to a presentation on the right hand rule, followed by several demonstrations in which I use a manget to deflect a beam of electrons in the direction predicted by the right hand rule.
Here's how I set up the reading assignment. Instead of two homework problems on, say, Thursday night, one of the problems is replaced by the reading assignment. Here's what I say:
Problem 2: READ Serway chapter 19-3, about magnetic fields. Magnetic fields are different from electric fields. I expect you to know equation 19.1, including what each term means; and, you should look at how to figure out the direction of a magnetic force. Friday's quiz will be basic questions about this reading. You will be allowed to use your notes on this quiz if the notes were personally handwritten by you.
Note that I've referenced the only relevant equation in the section, though Serway provides others that are not particularly relevant. Also note that by promising a quiz I ensure that a student who has difficulty doesn't just throw up his hands and say "I don't get it." He's welcome to say that, but he'll fail the quiz.
(I'm particularly fond of the "you may use personally handwritten notes" proviso. I credit Haverford history professor Roger Lane for this idea. Half of the course grade in US History was based on periodic pop quizzes based on the assigned reading. We were always allowed to use our notes, but only if these notes were handwritten by us, not xeroxed or highlighted. So we all took pretty good reading notes. And, the quizzes weren't so hard, 'cause we had paid such good attention to the reading, 'cause we had taken such careful notes. Insidious, that Roger Lane.)
Anyway, here's the quiz. Note that questions 3-5 are as much about knowing when a magnetic field DOESN'T produce a force as about the right hand rule.
1. What are the units of a magnetic field?
2. State the equation for the magnetic force on a charged particle. Define each variable.
3. A positive charge moves to the right in a magnetic field that points toward the top of the page. State the direction of the force on this charge.
4. A negative charge moves to the left in a magnetic field that points to the right. State the direction of the force on this charge.
5. A positive charge is at rest in a magnetic field that points toward the bottom of the page. State the direction of the force on this charge.
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