Date: Mon, 21 Feb 2011 08:40:08

Author: Cliff Bettis

Subject: Re: Tin Foil Capacitor

Post:

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Sam,

I think that points to the conceptual problem: capacitance is a matter of
geometry, charge distribution and potential differences. In more complete
treatments a set of charged conductors is said to have coefficients of
capacitance that are a measure of how much work it takes to move a test
charge from one to another. An isolated conducting sphere has a capacitance
proportional to its radius; if you morph it into a different shape
preserving its surface area, it will have a smaller capacitance. The foil
example is different in that the area changes because initially the charge
is only on the outer surface of the wrapped up foil but as the foil is
unrolled the area on which the charge is distributed reducing the charge
density and the potential (increasing the capacitance).

It is tricky explaining the use of an electroscope or an electrometer as a
device to measure potential because these instruments themselves have an
input capacitance (as you point out) which, if large compared to the
capacitance of the object whose potential you're measuring, affects the
measurement. So you can think of an electroscope as a gadget to measure
charge, or potential depending on the circumstances.

Cliff

From: tap-l-owner@lists.ncsu.edu [mailto:tap-l-owner@lists.ncsu.edu] On
Behalf Of Sam Sampere
Sent: Monday, February 21, 2011 8:18 AM
To: 'tap-l@lists.ncsu.edu'
Subject: Re: [tap-l] Tin Foil Capacitor

Why do we call this a capacitor? We don't call wires capacitors, thought
they do indeed have stray capacitance? All we're doing is taking charge off
the electroscope and placing it onto another surface.

I tell my students that an electroscope can be used as a charge meter or a
voltmeter. If using as a voltmeter, you're simply hooking a capacitor up in
parallel.

Don't you hook your electroscope to one plate of a parallel plate capacitor
and move it to see a voltage, I mean, charge, I mean voltage, maybe I really
mean both, change???

Thanks,

Sam

From: tap-l-owner@lists.ncsu.edu [mailto:tap-l-owner@lists.ncsu.edu] On
Behalf Of Michael Thomason
Sent: Friday, February 18, 2011 3:22 PM
To: tap-l@lists.ncsu.edu
Subject: Re: [tap-l] Tin Foil Capacitor

Here

?Topic=5

&SubSubtopic=5C10.31&DemoCode=5C10.31

is my tin foil capacitor, manufactured by E B Benjamin, 1800-1914,

Michael Thomason, Director of Physics Learning Laboratories

University of Colorado Boulder Department of Physics

303-492-7117

From: tap-l-owner@lists.ncsu.edu [mailto:tap-l-owner@lists.ncsu.edu] On
Behalf Of Paul Nord
Sent: Thursday, February 17, 2011 8:19 PM
To: tap-l@lists.ncsu.edu
Cc: Paul Nord
Subject: Re: [tap-l] Tin Foil Capacitor

Bill,

It's the last paragraph of the article that has this error.

Here's the citation from December:

Tin foil capacitor

And if it's not bad form to quote the article here:

During the demo, students complete the right col- umn with
their actual observations and explanations. In addition, the sheet has
several concluding questions such as, "Why is it important that the tube in
this demo is made of plastic?"

It might be possible to make another tube that has the foil
sandwiched between sheets of plastic or paper and then roll up the foil. The
plastic should insulate the foil layers when it is rolled up, and therefore
there should be no difference in elec- troscope separation when the foil
sandwich is unrolled and rolled.

Tom - no reference to your article in the December issue. Here's a link to

19th century textbook illustrations
-VII. Surface charge
density

Great illustration!

Paul

On Feb 17, 2011, at 6:46 PM, William Beaty wrote:

On Thu, 17 Feb 2011, Paul Nord wrote:

that he has not tried this. Would a coil of a single foil with a insulating
layer have a higher capacitance than a conducting tube? Wouldn't the charge
simply move to the outside surface?

Yep, all foil parts are at the same potential, so there won't be e-fields
between the layers. Maybe he meant to use *two* foils to create a
capacitor, and ground one of them?

(((((((((((((((((( ( ( ( ( (O) ) ) ) ) )))))))))))))))))))
William J. Beaty http://staff.washington.edu/wbeaty/
beaty, chem washington edu Research Engineer
billb, amasci com UW Chem Dept, Bagley Hall RM74
206-543-6195 Box 351700, Seattle, WA 98195-1700

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Sam, I think that points to the conceptual problem: capacitance is a =
matter of geometry, charge distribution and potential differences. In =
more complete treatments a set of charged conductors is said to have =
coefficients of capacitance that are a measure of how much work it takes =
to move a test charge from one to another. An isolated conducting sphere =
has a capacitance proportional to its radius; if you morph it into a =
different shape preserving its surface area, it will have a smaller =
capacitance. The foil example is different in that the area changes =
because initially the charge is only on the outer surface of the wrapped =
up foil but as the foil is unrolled the area on which the charge is =
distributed reducing the charge density and the potential (increasing =
the capacitance). It is tricky explaining the use of an electroscope or an electrometer =
as a device to measure potential because these instruments themselves =
have an input capacitance (as you point out) which, if large compared to =
the capacitance of the object whose potential you’re measuring, =
affects the measurement. So you can think of an electroscope as a gadget =
to measure charge, or potential depending on the circumstances. =
Cliff From:=
=
tap-l-owner@lists.ncsu.edu [mailto:tap-l-owner@lists.ncsu.edu] On =
Behalf Of Sam SampereSent: Monday, February 21, 2011 8:18 =
AMTo: 'tap-l@lists.ncsu.edu'Subject: Re: [tap-l] =
Tin Foil Capacitor Why do we call this a capacitor? We don’t call wires =
capacitors, thought they do indeed have stray capacitance? All =
we’re doing is taking charge off the electroscope and placing it =
onto another surface. I tell my students that an electroscope can be used as a charge meter =
or a voltmeter. If using as a voltmeter, you’re simply hooking a =
capacitor up in parallel. So, what am I missing?? Why all the excitement about this one? =
Don’t you hook your electroscope to one plate of a parallel =
plate capacitor and move it to see a voltage, I mean, charge, I mean =
voltage, maybe I really mean both, change??? Thanks, Sam From:=
=
tap-l-owner@lists.ncsu.edu [mailto:tap-l-owner@lists.ncsu.edu] On =
Behalf Of Michael ThomasonSent: Friday, February 18, 2011 =
3:22 PMTo: tap-l@lists.ncsu.eduSubject: Re: =
[tap-l] Tin Foil Capacitor He=
re h=
sp?Topic=3D5&SubSubtopic=3D5C10.31&DemoCode=3D5C10.31 is=
my tin foil capacitor, manufactured by E B Benjamin, =
1800-1914, Mi=
chael Thomason, Director of Physics Learning =
LaboratoriesUn=
iversity of Colorado Boulder Department of =
Physics&n=
bsp; =
303-492-7117&n=
bsp; =
bsp; =
=
tap-l-owner@lists.ncsu.edu [mailto:tap-l-owner@lists.ncsu.edu] On =
Behalf Of Paul NordSent: Thursday, February 17, 2011 8:19 =
PMTo: tap-l@lists.ncsu.eduCc: Paul =
NordSubject: Re: [tap-l] Tin Foil =
Capacitor Bill, It's the last paragraph of the article that has this =
error.Here's the citation =
from December:Tin foil =
capacitor And if it's not bad form to quote the article =
here: &=
nbsp; =
During the demo, =
students complete the right col- umn with their actual observations and =
explanations. In addition, the sheet has several concluding questions =
such as, “Why is it important that the tube in this demo is made =
of plastic?” &=
nbsp; =
It might be =
possible to make another tube that has the foil sandwiched between =
sheets of plastic or paper and then roll up the foil. The plastic should =
insulate the foil layers when it is rolled up, and therefore there =
should be no difference in elec- troscope separation when the foil =
sandwich is unrolled and =
rolled. Tom - no reference to your article in the December =
article:19th =
century textbook illustrations—VII. Surface charge =
densityGreat =
illustration! Paul On =
Feb 17, 2011, at 6:46 PM, William Beaty wrote: On Thu, 17 Feb 2011, =
Paul Nord wrote:that he has not =
tried this. Would a coil of a single foil with a insulating layer =
have a higher capacitance than a conducting tube? Wouldn't the charge =
simply move to the outside surface?Yep, all foil parts are at the same potential, so =
there won't be e-fields between the layers. Maybe he meant to use =
*two* foils to create a capacitor, and ground one of =
them?(((((((((((((((((( ( ( ( ( =
(O) ) ) ) ) =
)))))))))))))))))))William J. Beaty =
&=
nbsp; http://staff.washington.edu/=
wbeaty/beaty, chem washington edu =
Research Engineerbillb, amasci com =
&=
nbsp; UW Chem Dept, Bagley Hall RM74206-543-6195 =
&=
nbsp; Box 351700, Seattle, WA =
98195-1700
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From tap-l-owner@lists.ncsu.edu Mon Feb 21 09:45:58 2011

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