Unit 4: Magnetix

Finally, the part I was most looking forward to. Playing with magnets. How could I have been so simple minded? We weren't going to just play...we were going to dig deep down into the pit of confusion and chaos, of right hand rules and mysteriously moving wires. Of magnetic field lines coming out or going in- who knew which?

Better fuel up....

The morning selection...

Ok, let's get to work. We started with compasses and flowing charge in a wire.

field around a wire

We mapped the direction of the field and decided that the field swirls CCW when the current is up and CW when the current is down.

looks like we are learning to tell time!

Vectors

Then we were able to take some measurements of the strength of the field. We changed the current and measured how the compass was deflected, then we changed the distance. We "logicked" through the kind of curves they were- distance looked linear but couldn't be, because it would cross the axis and change direction! So it turned out to be inverse. Current looked linear.

Next we got magnets to play with - a bar magnet and face magnets. We mapped the field around these magnets and then had an interesting and protracted (always!) discussion about which way the earth's magnetic field goes. Now I thought I had some background info on this because I teach astronomy and I show a video called "Magnetic Storm" about how the earth's magnetic field is changing, but it turns out I really wasn't clear on it even after watching that video eleventy umpteen times. So the field goes OUT of the north pole and INTO the south pole. By convention. So the end of a compass is "north seeking." But..."it's been known for many years"...that the magnetic poles of the earth do switch and it is actually long overdue for such a switch.

We delved into some worksheets to help figure out directions of currents field and forces, and through much trial and error, we did come up with a right hand rule. Colleen actually thought through it as her table mates seeded her way and came up with the hand rule herself! Bravo!

so...we could actually use our right hands to help remember...

Give her a hand!

B, F and I are all at right angles to each other, which is what made visualizing it a three dimensional abstract jungle gym nightmare of mental contortions.

So there's a rule for current in a wire: thumb in direction of current and fingers curl in the direction of the magnetic field. Easy peasy. But for force- the fingers go in the direction of the B field, the thumb in the direction of current, and your palm OF YOUR RIGHT HAND (I used my left once and then had to face palm myself with it...) is the direction of the force.

3-D Craz- E

bending our brains...

We "learned" the notation for in and out of the page, so we could better keep track of which way fields were tending. After a lot of fiddling with magnets and drawing fields, we had a recap from Laura about all the things we'd discovered. I think this is important- because I had been getting lost in activity after activity. It's good to pause and take inventory of what you've learned and what is still out there to be explored.
So:
1. We mapped the mag. field and concluded we could use the RHR to show direction of field.
2. Mag. field came from charge flow.
3. Mag field comes out of north (convention)
4. Mag field changes in different contexts
5. inverse relationship between B and d
6. B is prop. to current and inversely prop. to distance

Next came the coils. The Copper coils. And bar magnets. And meters to measure any nonsense going on.

copper coil

Here's what we think:

field around coil

Things get pretty tricky pretty quickly now. We were finishing up in the final days of the workshop. We'd been at it over two weeks- we'd had no break from teaching, laundry was piling up, families needed attention, etc, and things at the workshop got tougher. But it would have to. How else could you understand how a moving charge could create a magnetic field, or that moving a magnet through a coil could cause a current. It's tricky stuff. But very cool. And very useful.

put the magnet in the hole...pull it out...

The next lab was a demo lab because the equipment was expensive, but we could actually "weigh" the force with  a charge balance. We changed the magnet strength, the thickness of the wire, and the length of the wire. After we graphed our data we saw linear relationships for all- concluding that
F = IBl.
We had more debriefing  from Don and he made a connection with springs and how they add. This is where we codified our information about resistors, capacitors, and how we treat them in series versus parallel. We talked more about the reading we had done and he made some connections with the terminology- "surface charge" and "steady state" and "quasi-steady state." It made much more sense now that we had so much experience and context in which to put it.

One of the final activities we did was a lab I called The Mortal Coil. We shoved the magnet in and out of the coil, and figured out which way the current was going by using all our appendages, right hands, left brains, and abstract thought...and we ended up with the old magnet through a copper tube trick which I've seen many a time, but Don went through the explanation of how it could be that dropping a magnet through a copper tube could produce a field opposite in direction to the magnetic field of the magnet- thus slowing down its progress and acting against gravity.

This explains it all! 

Unfortunately, it wasn't just 3:59pm on a friday when the learnin' gets tough, it was 3:59 on a friday of the last day of a three week intensive study of E and M in the Modeling framework.
That's tough times five. 

But I grasped just enough of it to make me want to think more and read more and think more. I know there is more because I took the E and M semester of physics at HFCC when I was getting my teaching degree 8+ years ago. And I remember words like flux and faradays and webers and rotating loops of wire.

Now, I'm almost 100% sure I won't get to this material in my high school classes. But how much richer is life when you really get a feel for how things work? And how much richer is learning when you get the chance to construct meaning and experiences that lead you to a deeper understanding.

So now the workshop is over, but I have so much more work to do. Planning and plotting and getting things ready and thinking about how I will proceed. Luckily i won't be alone- I met and worked with a cohort of very cool and very smart people who I will use as resources.

Physics discussions continue at a more informal setting...