Date: Fri, 1 Oct 2004 11:20:13 -0500

Author: Stan Dodds

Subject: Re: what to do with Pasco Parallel Plate capacitor demo


John has, I think, gotten the physics of the set up correct. Stray
capacitance is a significant problem which makes it very difficult to
see the underlying physics. A capacitance meter doubtless leads to
correct and easily interpreted results, but it seems to me that it
obscures the physics. All the student sees is a black box which gives a
number that the teacher says is capacitance. I'm not sure that leads to
much understanding. (The underlying measurement method is either an AC
bridge or one of several charge/discharge tricks, calibrated to
subtract off the instrument capacitance. Good engineering for advanced
students, but not transparent to a freshperson.)

Dale mentions using a home-made coulombmeter to study the parallel
plate capacitor. I don't see that this is much different than using an
electrometer, but perhaps he could explain a bit further. It would be
really nice to have a good, understandable capacitor exercise.


On Oct 1, 2004, at 9:02 AM, Dale Stille wrote:

> John,
> Well, as Richard already mentioned, get rid of the electrometer for
> this
> lab. The meter will fluctuate as people walk around in the room so
> that
> you can get no consistent results. We also found that the placement of
> the connecting wires around the plates and the order of the connections
> themselves was very critical if consistent results were to be obtained
> by every lab station......hence we no longer use electrometers. We
> have
> used the parallel plates for
> years with a capacitance meter as Richard suggests, and more recently,
> we
> have made our own coulombmeter that plugs into a voltmeter for the
> display. The plates then work well for separations up to 100mm with
> repeatability. We also use plates of PVC and polypropylene for
> dielectrics and as an added twist we bought some conductive boxes from
> McMaster Carr that are used to store electronic components and boards
> and
> insert these between the plates also. This will give a good lab on
> capacitors with a conductor between the plates.
> Good Luck,
> Dale Stille, IRS
> U of Iowa
> On Thu, 30 Sep 2004, John Welch wrote:
>> Hi all. My dept has a bunch of the the 20cm dia. Pasco parallel plate
>> capacitors that were bought with the idea of using them for lab
>> experiments.
>> I have been
>> trying to come up with an interesting capacitor lab for first year
>> physics
>> majors using these things, but so far, I'm having trouble finding an
>> activity that works well and is relatively straightforward. Has
>> anyone used
>> these capacitors in a lab activity, and if so, what have you done
>> that works
>> well? I called Pasco and asked them the same question, and they said
>> they'd
>> ask around and call me back, but I haven't heard anything yet.
>> I'm using the Pasco Electrometers to measure the voltage between the
>> plates.
>> (these devices do seem very useful in general). The capacitance of the
>> plates at 2 mm is about 130 pF, and that of the electrometer plus
>> cables is
>> about 120 pF.
>> Here are the things I've tried that don't work too well:
>> 1) Looking at how Capacitance (i.e. V across isolated capacitor)
>> varies with
>> plate separation. You only see any effect with plate separations
>> between 2
>> and 6 mm. (Farther than that you get a constant V) Even then, since
>> the
>> capacitance of the electrometer is on the order of that of the
>> plates, you
>> get a complicated result that isn't just C is proportional to 1/D. The
>> students could take into acount C of the electrometer in parallel and
>> derive
>> the relation, but it's a bit complicated.
>> 2) Effect of dielectric in capacitor: You can't slide the dielectric
>> in
>> between the plates because it gets charged up. Also the behavior
>> again isn't
>> straightforward since C_plates is similar to C_electrometer.
>> 3) Measuring charge density on plates at different separations while
>> plates
>> are kept at constant V: this works OK, but again the results are not
>> straightforward.
>> So, please let me know some of your favorite uses for these
>> capacitors,
>> and/or any of your favorite "Capacitor concepts" lab activities that
>> use
>> other types of equipment.
>> Thanks a lot
>> -John Welch
>> Cabrillo College Physics Dept.
>> Aptos, Ca.
From Fri Oct 1 13:41:05 2004