Electric Circuits - starting at the end.

We only have a couple weeks of class left, and there's not enough time to do the traditional Physics B sequence that I've used for electricity with my seniors that asked for a non-AP physics course at the beginning of the year. Normally I do electrostatics for a couple of weeks, talk about electric fields and potential, and then use these concepts to motivate a treatment of electric circuits. I could have stretched that out, but given my freedom in pace and curriculum, I decided to switch everything around.

This year, I started at the end of my sequence to address a pretty big issue I've always seen with my students. As much as they talk about charging (mobile devices, laptops) and basic energy conservation such as turning lights off, they have a pretty fuzzy understanding of electricity and the origins of the energy they use everyday. Some of the last topics in my traditional sequence involve real voltage sources, batteries and internal resistance - the "real" electronics that you need to know if you want to actually build a circuit. You know, the actually interesting part.

There's nothing interesting in looking at a circuit and calculating what current is going through an arbitrary resistor in a given circuit.  It took me a while to come to this realization because I still have some brain cells clinging to the "theory first, application second" philosophy, the same brain cells I've been working to silence this year. These are the sorts of things I want my students to learn to do:

  • Build a charger for an iPod using a solar panel and some circuit components. What is involved in charging a battery in a way that the battery will actually charge up without blowing Nickel and Cadmium all over the classroom?
  • Create a circuit that lights up an LED with the right current so it can outlast an incandescent bulb.
  • Look at an AC adapter that isn't made for a given device, and modify it so that it does work. The fact that it only costs $5 to buy a new one is irrelevant when you compare it to the feeling you get when you realize this is not hard to do. (Thanks Dad!)
  • Generate electricity. Figure out how hard you have to physically work to run your laptop.

This is what we did on day one:

I gave them a solar panel, some small DC motors and LEGO motors, a stripped down version of our FIRST Tech Challenge robot, some lemons, clip leads, and different kinds of wire, and said I wanted them to use these tools to generate the highest voltage they could. There was also a bag of green LEDs on the table there for them to play with. There was a flurry of activity among my five students as they remembered something vaguely from chemistry about sticking different metals into a lemon, and needing to connect one to another in a certain way. They did so and saw that there was a bit of a voltage from the lemons they had connected together, but that there wasn't much there.

I then showed them one of the LEGO motors and had them see what happened on a connected voltmeter when the axle was rotated. They were amazed that this also generated an electrical potential. This turned immediately into a contest of rotating the motor as quickly as possible and seeing the result on the voltmeter. One grabbed an LED and hooked it up and saw that it lit up.

They then turned to the robot and its big beefy motors. They found I had a set of LED lights in my parts box and asked to use it. Positive results:

The solar panel was also a big hit as it resulted in us going outside. They were impressed with how "much" electricity was generated after seeing the voltmeter display over 15 volts - they were surprised then to see that it worked to turn on the LED display, but not any of the motors they tried.

At this point it was the end of the class block, so we put everything away and went on with our day.

Some of the reasons I finished the day with a smile:

  • There was never a moment when I had to tell any of the students to pay attention and get involved in the activity.  The variety of objects on the table and the challenge were enough to get them playing and interacting with each other.
  • While I did show them how to play with one of the tools (i.e. DC motor acting as generator) , they quickly figured out how they might transfer this idea to the other items I made available.
  • They made bits of progress toward the understanding that voltage alone was not what made things work. This is a big one.

The next day's class used the PHet circuit construction kit to explore these ideas further in the context of building and exploring circuits. We had some fantastic conversations about voltage of batteries, conventional vs. electron current, and eventually connected the idea of Ohm's law (which was floating around in their heads from middle school science) to the observations they made.

I was struggling for a while about how to approach electricity because I have always followed the traditional sequence. In the end, I realized that I really didn't want to go through electrostatics - I wasn't excited to teach it this time around.  I also realized that I didn't need to do so, either in order to teach my students what I really wanted them to learn about electricity.

I think this approach will help them realize that electricity is not magic. They can learn to control it. I admit that doing so can be dangerous and expensive if one doesn't know what he or she is doing. That said, a little basic knowledge goes a long way, even in today's world of nanometer sized transistors.

Tomorrow we attempt the LED lighting assignment - feel free to share your comments or suggestions!

3 Comments

Filed under physics, teaching philosophy, teaching stories

3 Responses to Electric Circuits - starting at the end.

  1. pshircliff

    I have not done electrostatics in years. We jump in with lighting various bulbs (next year I might skip the edison style bulbs because they are so expensive ; just use various LED's that are cheap on ebay). We then built various circuits & used PhET, played with generators, built motors. This year I added squishy circuits (conductive "play dough"). I have yet to throw in resistors, but maybe I should. We bridged sound & electricity by building a functional paper plate speaker. I love this exploring part of Physics. Maybe Alternative Energy generation next year.

  2. Evan,
    You've convinced me too. I really like your project ideas, and the idea of starting students with different types of voltage sources (solar cells, batteries, generators) from the very beginning. Did you see the recent post showing a teardown of one of the small apple iPhone chargers? I would really like for my students to be able to take a device (we could start with a toaster before jumping into a charger like this), and figure out the function of all the various parts.

    I think this also brings me a lot closer to the sort of project based work with real world significance I want my students doing.

    • I did see that post - really impressive how much thinking goes into designing even the charger at Apple. I do see it being a bit overwhelming if a student opens something up and doesn't have some sort of guidance on what they are looking at. As recently as several years ago, I felt that way when I wanted to replace a laptop hard drive.

      I think it taps into mindset too. "You're so smart, you know what all this stuff does, so can you tell me what it does?" vs. "Wow - you figured out how it worked? Can you show me how you figured it out?"

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