Saturday, February 7, 2015

Thoughts on Beginning Magnetism

It has been a while since I have been able to get magnetism into the Honors Physics course, but the lack of fourteen million snow days this year has certainly helped. I'm putting it at the end of the second term, which is the term in which we studied gravity and circular motion, so it fits in pretty well - a non-contact, field-based phenomenon which, like gravity, which causes circular motion.

Since it has been several years, I thought that I'd completely revamp my treatment of magnetism. Here are some notes on the first day, and a brief outline of the plan of the rest. It's mostly bullet points, and at least as much for me remembering the thought process as for anything.

My favorite bit is that it's a whole 95-minute day that fleshes out the idea of field relatively well, giving students some concrete experience puzzling them out and creating representations based on their own investigations. Hopefully, this will give them a better mental picture of fields in space.

General ideas: 
- Magnetism is similar to gravity, in that it's a non-contact force - it's "invisible.'
- It's all about "fields," and we're going to need to figure out what the heck a field is at some point. That's pretty much the goal for the day.

Operational def’ns: We're going to use an operational def'n of the field (at least the B field direction) today. What’s our op. def. of temperature? It's what a thermometer measures! For B field directions, it’s going to be “where a compass points."

Let’s explore that a little:
- Map of the field in the room. Need better compasses, or maybe use phones? (yes, phones worked much better on the second day) They draw their vectors on board, "complete" them to form field lines.
- Let’s look at another field together: field of wire apparatus (through table), with compasses. Combining little arrows into loops, change i direction to see the opposite direction loop
- They investigate with bar magnet, horseshoe magnet, current loop (?), solenoid - your goal is to draw a good diagram of your object's B field on the WB.

Sharing whiteboards (unfortunately, didn't get any pics here of the boards):
- Bar first - what dir. are the field lines? (N to S) What does that tell us about Earth? (The north geographic pole is near a south magnetic pole!). 
- Horseshoe: what’s the orientation of the (unlabeled) poles?
What’s in common so far? 
- loops (are they closed? Yes - we couldn't see the part inside the magnet for these, but they're there)
- from N to S
- distance-dependent strength (how can we tell this from the diagram? Density of field lines!)
- opposites/likes
Now the solenoid: puzzling out the shape (if they didn't figure it out - one class did)

RHR - wire current

Current loop, using RHR1 - generates RHR2… let’s check it for the solenoid (go to solenoid)

Looping back around (see what I did there?): what is a field?
- has a value (magnitude, direction) at every point in space
- affects objects that are in it

Which field have we dealt with? Gravitational field, though we haven't called it that, really.
- Planet g field shape; where’s it strongest? same deal; not closed loops, though - that’s a difference.
- What objects create g fields? (masses) what objects does it affect? (masses)

For B fields, what creates them? perm. mag., currents -> moving charges (spin, domains, etc.), and it’ll affect moving charges, too! We'll look at the effects next time!

- Using this applet to examine the effects of B fields on charges - helps to figure out that the direction of the force on the charge is always perp to v, dependence on q, etc.
- Lorentz force 
- Applying that to a current; forces on wires
- Quantifying the fields of wires, solenoids

That's pretty much what we'll have time for before exams!


  1. I always get frustrated with physical compasses because they really only measure the direction of the field in the horizontal plane. Way up here in MN, for example, the earth's fields has a huge vertical component. Do phones help with that?

  2. Interesting! They would, I think, though we didn't check. The Hall effect sensor shouldn't be vulnerable to orientation like an accelerometer would be.

    I did find (accidentally) an Android compass app that gives the strength of the magnetic field, in microTesla. It seemed to be at least plausibly accurate - certainly qualitatively better than my B field sensor was when I tested it.