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05 July 2016

More on the light bulb that doesn't obey Ohm's law

Data collected by my students showing a non-ohmic bulb
Before I get into a discovery about the non-ohmic nature of a flashlight bulb, an important caveat:

Until the very end of your circuit unit, treat bulbs as regular old resistors.

Like everything in introductory physics*, it's important to start simple and build complexities in gradually.  Teach your students to deal with ohmic bulbs.  The only difference between a bulb and a resistor should be that a bulb produces light; the brightness of the light depends on the power dissipated by the bulb.


* And in high-level physics research, as well

Then, ask them in the laboratory for experimental evidence that the bulbs actually do or do not obey ohm's law.  My students' evidence is shown above -- click to enlarge.  Over the available range of voltages of about 2 V to 8 V, the bulb's resistance (determined by the slope of the V-I graph) varies from about 50 ohms to 80 ohms.  

Importantly, that doesn't mean that the first approximation of a constant-resistance light bulb is a bad one, any more than the first approximation of no air resistance invalidates the study of kinematics.  In most laboratory situations in introductory physics, the ~30% difference in resistance -- less difference if the voltage range being used is narrow -- will still produce quantitative and qualitative predictions that can be verified experimentally.  For example, the typical "rank these bulbs by their brightness" will give correct results pretty much irrespective of the non-ohmic nature of the bulbs.

Asking a new question -- what will a resistance meter measure?

In my AP Summer Institute in Georgia last week, a couple of participants set up this experiment (it's based on the 2015 AP Physics 1 exam problem 2), getting results pretty much exactly as reported above.  Then the question came up, what would a resistance meter measure?

Here's where, in class, I'd give everyone a minute or two to write their thoughts down on a piece of paper.  You can do that too.  I'll wait.

In fact, I'm not giving the answer yet.  I've posted a twitter poll here where you can give your thoughts.  Answer coming in a few days.

(Yes, Jordan and Hannah who did this experiment... you may vote.  Just wait to comment here until the votes are tallied.  :-)  )

GCJ

2 comments:

  1. I am also fond of recording voltage and current starting from 0v up to some reasonable level, and then keep recording as you step the voltage down.

    The graph can be interesting.

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    Replies
    1. You could make a simple voltage divider with 10 equal resistors ... they should be available in tight tolerances from like a Radio Shack ... and then you have your 2V in 0.2V increments. Or you can use however many you want, like 20 will give you 0.1V increments. you just stack them in series between the + terminal and gnd. And then take it off wherever you want. That'd be a pretty simple way to do it. And if you used, say, 10% tolerance resistors (as opposed to 5%), you'd also have the opportunity to have a good discussion about experimental error. It's just another avenue to that "ways to improve the reliability" discussion we're always trying to encourage.

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