[Phys-L] Re: The bulb-with-one-wire task: too tricky?

[Carl Mungan <mungan@USNA.EDU>]

> "I know I enjoy the sport [testing hapless subjects on the
> one-wire-plus-battery-plus-light-bulb task] as well as the next guy .
> . . . but I was just struck by the idea that the bulb task is, in
> fact, so tricky it may not tell us what we think it does."
>
> I agree that it's probably a poor test for K-8 or even K-12 students
> who have not actually taken a bulb apart to see how it works.
>
> However, I think MIT graduates - the future elite of U.S.
> engineering/technology - should be able to figure out how to make the
> connections from their knowledge or observations that:
>
> 1. most metals DO conduct electricity,
>
> 2. most glasses DO NOT conduct electricity,
>
> 3. there are two metallic contacts on the light bulb which are not in
> EXTERNAL electrical contact,
>
> 4. current must flow into the filament and then out of the filament
> that can easily be seen though the glass,
>
> 5. ergo, why not try connecting the battery so its positive pole is
> connected to one of the metallic contacts, and its negative pole is
> connected to the other metallic contact?

I have often heard people say that this test is too "tricky" even for
undergrads. So this semester in the labs on simple circuits (for
General Physics II) I posed it to a number of groups of college
sophomores. Every group eventually got it, most without too much
difficulty. I will say however, that I asked it directly after they
had answered a question of the form of 3 above: Where are the two
contacts on the bulb? This was a much more difficult question for
them, even after being shown a cross-sectional cutaway diagram of a
bulb. But once they got that, then lighting the bulb with one wire
and a battery wasn't generally difficult. To put it another way, if
you hand them a bulb IN A SOCKET with two female banana jack plugs,
they can do it right away. So my conclusion is that it is not at all
obvious that one connection to the bulb is to the screws on the side
of the case, and that as with many things, simply "telling" them this
isn't visceral enough for them to understand. Can we blame them? Carl
--
Carl E. Mungan, Asst. Prof. of Physics  410-293-6680 (O) -3729 (F)
       U.S. Naval Academy, Stop 9C, Annapolis, MD 21402-5040
mailto:mungan@usna.edu       http://usna.edu/Users/physics/mungan/