Date: Wed, 6 Oct 2004 15:26:39 -0700

Author: "John Welch"

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

Post:

Thanks for all the capacitance lab ideas everyone.
I tend to agree with Stan that a capacitance meter is a bit more of a "black
box" than I would like.
I'm interested in Dale's method of using a homemade coloumbmeter - is it a
substantially different thing than a faraday ice pail connected to the pasco
electrometer? Could you describe it?
I tried out Steve Wonnell's experiment which has students use a scope to get
Q from a plot of I vs t for a discharging capacitor. I thought it was a
great idea, and I got good numbers, but in our situation, students won't
have studied current or resistance when they're doing this lab, and I'm
reluctant to introduce those concepts (as well as RC time constants) in a
lab writeup.

We did the lab last week, and what we ended up doing was having them charge
up a .1 uF capacitor (a 'real' capacitor, not Pasco's ) to 10 V, then
connect an uncharged capacitor in parallel with it and predict the resulting
voltage across the two. They measured this voltage with the electrometer,
which gave good results. We had them do it for various configurations of
capacitors in series and parallel. Then they did the same thing with a 120pF
capacitor and considered the electrometer as an unknown capacitor in
parallel with that, in order to find it's capacitance. Then knowing that
capacitance, they could try to measure the capacitance of Pasco's capacitor.
It went OK, but it was a lot of series and parallel stuff, and not as much
stuff about general capacitance concepts. And it didn't require Pasco
Capacitors at all.

Someone from Pasco wrote me to say that he is working on developing a
straightforward experiment with their capacitors. We'll see what he comes up
with.

thanks again
-John

----- Original Message -----
From: "Stan Dodds"
To:
Sent: Friday, October 01, 2004 9:20 AM
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.
>
> Thanks
> Stan
>
>
>
> 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 dickheckathorn@sbcglobal.net Thu Oct 7 06:14:09 2004

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