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## 25 January 2010

### Introduction to circuits: laboratory exercise

I've already posted about how frustrating electrostatics can be, both to teach and to learn.  Why?  Because I've never found a way to do quantitative, or even qualitative, demonstrations.  Sure, you can hang a balloon from a wall or use an electroscope, but these aren't nearly as satisfying as, say, measuring the pressure at the bottom of a flask to be exactly the 103 kPa you predicted.

I teach circuits immediately after electrostatics.  It's pretty straightforward to do quantitative demonstrations with circuits -- all you need is a voltmeter.  (Or, if you want to be fancy, Vernier voltage and current probes.)  Thing is, I don't do more than a couple quick quantitative demos.  Instead, I run a laboratory exercise over the course of several days.

Especially in the wake of electrostatics, I want the class to get their hands dirty experimentally.  I want them to see, once again, that physics is not a math class, even though the vector addition from electric fields may have given that impression.

So:  on day one of circuits, I give a crash course of definitions and memorization material.  I write on the board how to deal with series and parallel resistors, including the equivalent resistance formulas and definitions (current through series resistors is the same for each; voltage across parallel resistors is the same for each).  I give one brief example of how to use Ohm's law to deal with a set of two series resistors.

The next day I stop with the theory.  Instead, I show briefly how to use a breadboard to connect resistors to a power supply.  I bring out a large set of labeled resistors, all between 5k and 200k, and a set of voltmeters.  Each student gets a nine-page packet which they are to fill out.  I must sign off on each page before they proceed to the next.  Completing the packet will take about four class periods -- and it's worth every minute.

Here's the first page:

This page takes a while for most folks.  In fact, it might take the entire first class period.  The conversion to microamps should be trivial, but often is not; getting the correct circuit connected on the breadboard is not simple.

The good news is, the class tends to help each other.  Once one person makes a connection, then that information spreads throughout the class quickly.  I want them to learn from each other as well as from me.

Once I'm satisfied that a student has both predicted and measured voltage across each resistor, I initial the page and tell the student to move on to the next page.  What's on the next page, you ask?  It's the exact same set of questions... but with a different circuit diagram.  At the end of the post are pictures of the circuits on pages 2-9.

Students work at their own pace.  I ask everyone to get through the first seven circuits over four class periods.  The last two are essentially "extra credit" for the quick workers.  Those who finish early are given time to work on problem set problems, or to help the others in the class.

Now, I'm pretty picky about the answers to the questions.  If a student says, "The voltage is bigger across the bigger resistor because V=IR," I don't accept that.  "Because V=IR"  is not a reasonable explanation for anything.  For the series resistors, students must indicate that current is the same for each, meaning that bigger R leads to bigger V in Ohm's law.  For the combination circuit, they must show me that they are simplifiying the circuit to a set of series resistors.

Does this approach work?  Is it worth the four class periods?  I certainly think so.  (If I didn't, I wouldn't do it, right?  :-)  )  I find that my class has performed better on circuit problems on tests and quizzes since I started this exercise about five years ago.  More importantly, when someone does miss a question, we all have a common experience that I can refer to.  "Remember on the circuit lab?  Did the biggest resistor ALWAYS have the biggest voltage?  Remember when the resistors were in parallel?  Oh, yeah..."

GCJ

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