Dot Physics

This summer I taught the part II of algebra-based physics. It is odd, but I rarely teach this course. I usually end up teaching the calculus-based version (with Matter and Interactions). There is something strange. If you look at the algebra-based physics I and calculus-based physics I, they are just about the same course. Both essentially cover forces, momentum, energy, and angular momentum. Oh, I know – some also cover waves and pressure and sound and stuff.

I think what tipped me off to the big difference in the algebra-based physics II is the theme in Matter and Interactions. Yes, there is a theme for Matter and Interactions. The whole part II is essentially just about Maxwell’s equations (the relationship between charge, electric fields and magnetic fields). Since Maxwell’s equations are sort of mathematically complex, the algebra-based course doesn’t really get there. This is what most of these courses cover:

  • Electric forces, electric fields
  • Electric potential – sometimes capacitors are in here too
  • DC circuits* (and maybe RC circuits)
  • Currents and magnetic fields
  • Faraday’s law and stuff (changing magnetic fields and induced EMF)
  • AC Circuits
  • Light and stuff
  • Optics stuff

You get the idea. Almost all of the algebra-based texts are similar to this. There are a couple of problems. First, very few texts make a solid connection between DC circuits and electric fields and electric potential. For most, it is this little side street that is explored. But why? If it is not connected, why go down that road? A common faculty reply will be: but they have to cover DC circuits. You may have me there. This is a service course, it is not for physics majors. Who takes this course? It is mostly:

  • Biology majors
  • Kinesiology
  • Nursing
  • Industrial Technology, Engineering Technology
  • Computer Science – just a few. Most CS students take the calc-based physics

So the question should be: what do these major programs need? Do they need circuits? I really don’t know the answer. I guess they do, aren’t circuits on the MCAT?

This problem of disjointedness continues in the text. The farther you get, the more hand-waving there is. It just leaves a bad taste in your mouth.

Comments

  1. #1 Patrick LeClair
    August 13, 2010

    I teach the service course at U. Ala. (PH102) fairly regularly, and I do teach both ac & dc circuits. My argument is that it would be sad if I skipped something that they are surrounded by every day just to spend more time on relativity or point charges …

    I mainly try to keep it practical. DC circuits we do quantitatively and qualitatively, AC circuits mostly qualitatively since the math gets hard for them (e.g., is this a low-pass or a high-pass filter? what is the cutoff frequency?). Some random examples I’ve used

    *voltage dividers as volume controls
    *which bulb should be brighter?
    *series vs parallel, Xmas lights
    *multi-loop circuit: jump starting a car
    *audio filters & crossovers (L+C+R low/hi pass)
    *simple photoresistor & LED circuits (e.g., proximity sensors)
    *inductance & large MRI magnets
    *inductive & capacitive pickup in measurements (shielding)
    *why do we use ac power, and how to turn it into dc?

    I do cover the microscopic nature of current & resistance, but I am in the minority here I think. I don’t know if circuits are on the MCAT, but I try to convince them that a basic understanding of circuits is a very handy thing in everyday life and in understanding medical technology.

    They really seem to like audio circuits – a common question I ask is to show them the circuit diagram for a crossover with several speakers, and have them identify which ones are woofers and which ones are tweeters. They might care less about most of the courses, but a good number of them perk up when you start to explain how their stereo works …

    Most everything I’ve ever used for the course is here, btw:

    http://faculty.mint.ua.edu/~pleclair/ph102/

    All creative commons if there is something you like.

  2. #2 Patrick LeClair
    August 13, 2010

    Supplemental comment: the hand-waving is awful in most texts, agreed. I try to at least derive special cases, and then assert the general case.

    For example, from electric forces & length contraction you can get B from E for a special case (which is why I start with relativity in the algebra-based course, rather than ending with it). Then the magnetic field is not another mysterious field, just the same one they already know. The derivation is hard for them, but they get the point.

    From E & B forces you can derive the induced voltage across a conducting bar moving in a magnetic field. From that, you can derive the current in a loop moving into a region of nonzero magnetic field. From that, Faraday’s law is at least plausible.

    You can’t derive the most general laws, but you *can* get the special cases that are enough to establish plausibility. The price is very careful thought-experiments and well-reasoned arguments. For me, making the logic ironclad without math is much harder than deriving a string of equations.

    If you can do that, even partially, you can also tie the whole thing together, since you’ve at least motivated everything on fairly general grounds.

  3. #3 Rhett Allain
    August 13, 2010

    @Patrick,

    I think you could come up with an argument for every topic in the text as to why it should be taught – and I agree that circuits are very important. The problem is that if you try to go through the whole book, it is like trying to make students drink from a fire hydrant. Sure, some of the water will get in them, but their clothes get wet too.

    But, like I said before – since this is a service course I guess it is really up to those departments we are serving to say what topics are covered.

  4. #4 CCPhysicist
    August 22, 2010

    All intro courses look like a fire hydrant. The key is to figure out what to skip in exchange for depth. For example, I guess you could leave dynamical systems like the oscillator out of calc-based physics 1 if you figured it would be done in conjunction with AC circuits or just let the math department do it in a DE class, but mechanical engineers would question your motives. Ditto for never teaching any thermodynamics.

    Students in an algebra-based physics 2 class need to know circuits, particularly DC circuits, and optics, particularly corrective lenses, for their major. (They also need to know something about fluid flow and gasses from physics 1.) It is on the MCAT because they assume students know this when they get in their medical courses. Nerves. Vision. Blood. Dissolved gas. Muscle-skeletal leverage. Electrostatics as the core for basic atomic physics used in chemistry. They need those skills at the 100% level. They don’t need to know relativity or that magnetism arises mathematically from a relativistic transform except as a topic of discussion with other MDs after a day on the golf course.