Date: Thu, 24 Feb 2011 08:40:57

Author: Jerry Hester

Subject: Re: Tin Foil Capacitor

Post:

--=====================_261271238==.ALT
Content-Type: text/plain; charset="us-ascii"; format=flowed

Could it be the "electret" that causes what I know to be discharged
capacitors that I put on the shelf to acquire a charge when I am not looking?

Jerry H.


At 07:06 PM 2/23/2011, you wrote:
>On the Experiment of the Dissectible Condenser
>
>1944
>
>B. Gross
>
>Some di-electric materials have a property coined "electret".
>An electret dielectric has an imagined electric moment, much like a
>dipole moment.
>These "electret" dielectric materials are special because they can
>maintain an electric polarization long after the electric field is
>removed from the capacitor! I imagine it as a di-electric that can
>remember the applied electric field for some period of time even
>after that field is removed. This property is found in some
>dielectric materials, notably some waxes and also in polyethylene.
>
>These electret materials are the best ones to use for the
>dissectable capacitor demo.
>
>I made a homemade dissect-able capacitor demo after I first observed
>it performed at the demo show at the Notre Dame meeting in
>1994. Tom Senior was in that show. A capacitor is constructed with
>a polyethylene trash bag sandwiched between two large sheets of
>aluminum foil and charged with a static charge. The capacitor is
>then dissassembled and the trash bag is removed gingerly and waved
>in the air while the demonstrator asks loudly, "Where is the
>charge?". The capacitor is then reassembled and the charge creates
>a flash of light when it is shorted through a neon tube. It is amazing.
>
>http://en.wikipedia.org/wiki/Electret
>
>- Jerry Z.
>
>
>On Wed, Feb 23, 2011 at 5:56 PM, Jerry DiMarco
> wrote:
> These are good questions to ask, I was thinking about them last
> night. Halliday and Resnick 2nd ed. states that the polarization
> of the dielectric material remains only as long as the electric
> field is there. So that seems to suggest that in order for the
> dissectible cap demo to work, there must be something to maintain
> the electric field after the plates are removed. One possibility,
> as JH stated, is that charge is deposited on the dielectric surface
> from the plates. Are there other possibilities? In an
> electrolytic capacitor there is an oxide coating on the aluminum
> plate that serves as the dielectric. Where does the charge reside
> in that situation?
> The Wiki page on "dielectric" suggests that the dielectric
> constant refers to a material's polarizability. But comparing
> dielectric constants and polarizabilities of materials in the
> Handbook of Chem and Phys shows no clear relationship. So there
> must be additional properties that determine a material's
> dielectric constant. Is the ability to hold surface charge one of
> those factors?
>
>
>Jerry D
>
>
>At 2/23/2011 08:43 AM, you wrote:
>......
>What happens to the electric field inside the dielectric when the
>adjoining plate is removed? Is it pinned there? Is it weaker or
>stronger because there is no conducting surface to "terminate" the field lines?
>
>Paul
>
>
>
>
>
>
>--
>Gerald Zani
>Demonstration Manager
>Physics
>Brown University
>(401) 863-3964

--=====================_261271238==.ALT
Content-Type: text/html; charset="us-ascii"



Could it be the "electret" that causes what I know to be
discharged capacitors that I put on the shelf to acquire a charge when I
am not looking?
Jerry H.

At 07:06 PM 2/23/2011, you wrote:
On the Experiment of the
Dissectible Condenser
1944
B. Gross
Some di-electric materials have a property coined
"electret".
An electret dielectric has an imagined electric moment, much like a
dipole moment.
These "electret" dielectric materials are special because they
can maintain an electric polarization long after the electric field is
removed from the capacitor! I imagine it as a di-electric that can
remember the applied electric field for some period of time even after
that field is removed. This property is found in some dielectric
materials, notably some waxes and also in polyethylene.
These electret materials are the best ones to use for the dissectable
capacitor demo.
I made a homemade dissect-able capacitor demo after I first observed it
performed at the demo show at the Notre Dame meeting in 1994. Tom
Senior was in that show. A capacitor is constructed with a polyethylene
trash bag sandwiched between two large sheets of aluminum foil and
charged with a static charge. The capacitor is then dissassembled
and the trash bag is removed gingerly and waved in the air while the
demonstrator asks loudly, "Where is the charge?". The
capacitor is then reassembled and the charge creates a flash of light
when it is shorted through a neon tube. It is amazing.

http://en.wikipedia.org/wiki/Electret
- Jerry Z.

On Wed, Feb 23, 2011 at 5:56 PM, Jerry DiMarco
<
dimarco@physics.montana.edu> wrote:


These are good questions to ask, I was thinking
about them last night. Halliday and Resnick 2nd ed. states that the
polarization of the dielectric material remains only as long as the
electric field is there. So that seems to suggest that in order for
the dissectible cap demo to work, there must be something to maintain the
electric field after the plates are removed. One possibility, as JH
stated, is that charge is deposited on the dielectric surface from the
plates. Are there other possibilities? In an electrolytic
capacitor there is an oxide coating on the aluminum plate that serves as
the dielectric. Where does the charge reside in that
situation?

The Wiki page on "dielectric" suggests
that the dielectric constant refers to a material's polarizability.
But comparing dielectric constants and polarizabilities of materials in
the Handbook of Chem and Phys shows no clear relationship. So there
must be additional properties that determine a material's dielectric
constant. Is the ability to hold surface charge one of those
factors?



Jerry D


At 2/23/2011 08:43 AM, you wrote:


......

What happens to the electric field inside the dielectric when the
adjoining plate is removed? Is it pinned there? Is it weaker
or stronger because there is no conducting surface to
"terminate" the field lines?

Paul





--
Gerald Zani
Demonstration Manager
Physics
Brown University
(401) 863-3964



--=====================_261271238==.ALT--


From tap-l-owner@lists.ncsu.edu Thu Feb 24 08:51:02 2011

Back