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08 July 2016

So what does an ohmmeter read when it's directly connected to a non-ohmic bulb?

The previous post describes my students' results showing that a flashlight bulb's resistance varies.  Over the available voltage range of 2 V to 8 V, the resistance (determined by the slope of a voltage vs. current graph) varied from about 50 V to 80 V.

The question was, what does an ohmmeter read when placed directly on this bulb?

Consider how an ohmmeter generally works.  It puts an awfully wee voltage across the bulb, and measures the resulting wee current through the bulb.  Then the meter essentially uses ohm's law to calculate resistance.  (That's why you have to disconnect the bulb from the battery in order to use the ohmmeter.)

In the context of our experimental voltage-vs.-current graph above, the ohmmeter is measuring an out-of-range data point, way off down and to the left of the portion shown.  By extrapolating the curve shown, we could guess that we should get a shallower slope and thus a smaller measured resistance.

Sure enough, the meter measured about 8 ohms, a full order of magnitude less than the resistance in the bulb's operable range.  

Again I caution teachers: this is a cool and somewhat unexpected result.  Nevertheless, it's rather irrelevant to the typical practical analysis of a bulb.  The bulb only glows at all with a volt or two across it; the bulb is only rated to about 6 V, meaning it is likely to burn out over that voltage.  In the operable range, the resistance is reasonably steady.  The resistance only drops by an order of magnitude when the voltage is dinky.

The next question: How can we experimentally extend this graph?

My variable DC supply only goes down to 2 V.  I could get a 1.5 V battery to get one more data point, but that's all I can think of.  Does anyone have a suggestion of a way to explore the parameter space below 1.5 V?



  1. Well, keep that 2V source, and if you have a known resistor and a voltmeter and hook the resistor up in series with the bulb and measure the volts across that resistor, you could subtract that voltage from the 2V and effectively get lower voltage data for the bulb, right? Now, they might not be data points at nice even increments (like a tenth of a volt at a time), and there would be some accounting to do for the current because of course there are two resistances to consider, but that's the first thing that comes to my mind.

    1. Ah, of course! The ol' voltage divider. Shoulda thought of that. Thank you, Will! I'll let you know how it works when I try it with my class next February.

  2. Gasstationwithoutpumps tried to comment here, but somehow it didn't get through the spam filter, so I'll make the note for him.

    [ ] is a direct response to your post, and to earlier posts [ ]

    He notes "The resistance was mainly a function of temperature, so the i-vs-v plot for a lightbulb is not well-defined, but I think that my blog post covers that."

  3. And, woah, this is a thesis-worthy experimental presentation of the voltage vs. current for a flashlight bulb. I really, really encourage you to read the linked post if you have any interest in the deeper details of how a bulb behaves at low voltages.

    And, when you need a long-term research project for your research or Physics C:E&M course, well, this is a great one.

    1. Now you have seen how a engineer looks at mundane objects. Now you can think about a true "open question" for the USAYPT, like asking them to measure EVERYTHING they can imagine measuring about a light bulb. Then give them different types, maybe including a neon bulb or even an LED!

      Because the vast majority of my students intend to become engineers, that is the kind of thinking I try to START them on the road to developing as they move from physics into their major. I'm glad Gas Station without Pumps saw your blog and reacted to it, because I see some really valuable ideas in your blog. I'll just mention that I also use the methods you mentioned on 13 June, the "write the equations, not the answer" and "just draw the fbd", in my class. (I go so far as to only ask them to draw the fbd with all components shown clearly. That is the physics.) It helps a lot even though it drives them crazy at first. I might need to get back to blogging, with more of an emphasis on teaching than I have done in the past.