Date: Tue, 25 Oct 2011 14:01:12

Author: --- "George Herold"

Subject: Re: Image Current Demo

Post:

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Hi Steve, Thanks for the links and discussion. You seem to have it in
hand. I just wanted to say that the eddy current depends on a changing
B-field and once your current is started (and constant) the eddy current
effects should die away. =20
=20
George=20

________________________________

From: tap-l-owner@lists.ncsu.edu
[mailto:tap-l-owner@lists.ncsu.edu] On Behalf Of Steve Wonnell
Sent: Tuesday, October 25, 2011 10:17 AM
To: tap-l@lists.ncsu.edu
Subject: Re: [tap-l] Image Current Demo
=09
=09
Hi Jerry,

The "method of images" is a mathematical method for solving
problems.

You know that if you hold a charged electrical object next to a
thick piece of metal, it will be attracted because the charged object
induces an opposite charge on the metal.

It turns out that the electric fields in this situation, created
by the original charge and the induced charges together, behaving
exactly as if there were only two charged objects -- the original
charged object and an "image charge" buried inside the plate.

What I've just learned is that you can apply the same "method of
images" technique to an electrical current above the surface of a
magnetic material.
I think what's really going on is that the magnetic field
produced by the electrical current aligns the magnetic dipoles of the
magnetic material, and these aligned magnetic dipoles create a magnetic
field that then repels the magnetic field created by the current.

Apparently, according to those who've done the math, the
magnetic field configurations of this complicated situation appear to be
identical to those created by two charged currents: the electrical
current above the surface of the magnetic material, and an image current
buried inside the magnetic material.

There must also be an induced eddy current in the magnetic
material since the magnetic material is usually also a conducting
material, but evidently this eddy current repulsion is weaker than the
image current's field.

In Wolfgang's results, he should see that the attractive force
between the current and the ferromagnetic plate is stronger that the
repulsive force between the current and the copper bar.

That's my take so far.

Steve

On Oct 25, 2011, at 9:24 AM, Zani, Gerald wrote:

Hmmmm.
=09
Is the "image" charge an imaginary charge, not a real
charge? =20
=09
No?
=09
And the image charge method makes no distinction between
the types of conductor used? =20
=09
No?
=09
So a copper bar would behave the same as an iron bar,
they would both be attracted?
=09
Just trying to understand what is happening. - Jerry
=09
=09
On Tue, Oct 25, 2011 at 8:56 AM, George Herold
wrote:
=09

=09
Well an image current is moving image charges.
You must know image charges from the electro-static shpere and Farady
pail/ electrometer experiments... that sometimes work. (At least in my
limited experience)=20
=20
George H.
=09

________________________________

From: tap-l-owner@lists.ncsu.edu
[mailto:tap-l-owner@lists.ncsu.edu] On Behalf Of Rueckner, Wolfgang
Sent: Monday, October 24, 2011 5:04 PM
To: tap-l@lists.ncsu.edu
Subject: Re: [tap-l] Image Current Demo
=09
=09
So what exactly is this "image current"?
I just realized it's not eddy currents because they would create a
repulsion of the current carrying wire, not attraction. If I had a
similar copper bar (rather than iron) next to the wire, I suppose I
would get a repulsion?=20

On Oct 24, 2011, at 4:54 PM, Rueckner,
Wolfgang wrote:

I'll bring my camera in tomorrow morning
and take a picture for you. I'll measure the resistance of the whole
shebang and calculate the current. My guess is 200 to 500 amps.=20

On Oct 24, 2011, at 4:51 PM, Adam
Beehler wrote:

Well, there you have it. Thanks, Wolf!
Would you mind sending me a picture offline? How much current do you
estimate flows through your copper braided wire?
=09
=09
On 10/24/2011 2:25 PM, Rueckner,
Wolfgang wrote:=20

I placed and iron bar (2"x4"x24") on the
smooth cement floor in our demo room and laid down our braided copper
conductor (that we use for repelling (or attracting) current carrying
wires) next to it. About 1" separation between iron bar and copper
wire. Closed the moose knife switch to short out our truck battery
(just for half a second, mind you) and the copper wire was attracted
(and touched) the iron bar. -- Wolf=20

On Oct 24, 2011, at 4:15 PM, Zani,
Gerald wrote:

Wolf,
=09
What did you use for a ferromagnetic
plate and how did you arrange it?
=09
How much did it move?
=09
- Jerry
=09
=09
On Mon, Oct 24, 2011 at 4:09 PM,
Rueckner, Wolfgang wrote:
=09

Just tried it. It works remarkably
well!
=09
=09
On Oct 24, 2011, at 3:54 PM, Adam
Beehler wrote:
=09
> I have an instructor that tends to
believe we ought to be able to put
> together a demo showing "image
currents." This is what he proposed to
> me. Run a current-carrying wire along
the surface of a ferromagnetic
> plate/sheet. If the current is high
enough, then there will be a force
> between the current-carrying wire and
its "image current" running
> parallel to it in the ferromagnetic
surface just next to it. This would
> need to be a whole lot of amps, like
500-1000!
>
> So...has anybody tried anything like
this? Is there wire that can
> handle such currents for brief
moments, yet not be so bulky that the
> attractive forced created is
outweighed by the wire's bulk? I need
>
=09
=09
=09

--=20
Gerald Zani
Demonstration Manager
Physics
Brown University
(401) 863-3964
=09
=09

--=20
Gerald Zani
Demonstration Manager
Physics
Brown University
(401) 863-3964
=09
=09

**********************************************
Steven K. Wonnell, Ph.D.
Manager, Physics Instructional Resources
Department of Physics and Astronomy
The Johns Hopkins University
3400 N. Charles Street
Room 534 Bloomberg Center
Baltimore, MD 21218
Phone: 410-516-5468
E-Mail: wonnell@pha.jhu.edu
**********************************************

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Hi Steve, Thanks for the links and discussion. =
You seem=20
to have it in hand. I just wanted to say that the eddy current =
depends on=20
a changing B-field and once your current is started (and =
constant) the eddy=20
current effects should die away.

George=20

From: tap-l-owner@lists.ncsu.edu=20
[mailto:tap-l-owner@lists.ncsu.edu] On Behalf Of Steve=20
WonnellSent: Tuesday, October 25, 2011 10:17 =
AMTo:=20
tap-l@lists.ncsu.eduSubject: Re: [tap-l] Image Current=20
Demo

Hi Jerry,

The "method of images" is a mathematical method for solving=20
problems.

You know that if you hold a charged electrical object next to a =
thick=20
piece of metal, it will be attracted because the charged object =
induces an=20
opposite charge on the metal.

It turns out that the electric fields in this situation, created =
by the=20
original charge and the induced charges together, behaving exactly as =
if there=20
were only two charged objects -- the original charged object and an =
"image=20
charge" buried inside the plate.

What I've just learned is that you can apply the same "method of =
images"=20
technique to an electrical current above the surface of a magnetic=20
material.
I think what's really going on is that the magnetic field =
produced by the=20
electrical current aligns the magnetic dipoles of the magnetic =
material,=20
and these aligned magnetic dipoles create a magnetic field that then =
repels=20
the magnetic field created by the current.

Apparently, according to those who've done the math, the magnetic =
field=20
configurations of this complicated situation appear to be identical to =
those=20
created by two charged currents: the electrical current above =
the=20
surface of the magnetic material, and an image current buried inside =
the=20
magnetic material.

There must also be an induced eddy current in the magnetic =
material since=20
the magnetic material is usually also a conducting material, but =
evidently=20
this eddy current repulsion is weaker than the image current's =
field.

In Wolfgang's results, he should see that the attractive force =
between=20
the current and the ferromagnetic plate is stronger that the repulsive =
force=20
between the current and the copper bar.

That's my take so far.

Steve

On Oct 25, 2011, at 9:24 AM, Zani, Gerald wrote:
Hmmmm.Is the "image" charge an =
imaginary=20
charge, not a real charge? No?And the image =
charge=20
method makes no distinction between the types of conductor =
used? =20
No?So a copper bar would behave the same as an iron =
bar,=20
they would both be attracted?Just trying to understand what =
is=20
happening. - Jerry
On Tue, Oct 25, 2011 at 8:56 AM, George =
Herold =20
wrote:

Well an image =
current is moving=20
image charges. You must know image charges from the =
electro-static=20
shpere and Farady pail/ electrometer experiments...=20
that sometimes work. (At least in my limited=20
experience)

George =
H.

From: tap-l-owner@lists.ncsu.edu [mailto:tap-l-owner@lists.ncsu.edu] On Behalf Of=20
Rueckner, WolfgangSent: Monday, October 24, 2011 =
5:04=20
PMTo: tap-l@lists.ncsu.eduSubject: Re: =
[tap-l]=20
Image Current Demo
So what exactly is this "image current"? I just =

realized it's not eddy currents because they would create a =
repulsion of=20
the current carrying wire, not attraction. If I had a =
similar=20
copper bar (rather than iron) next to the wire, I suppose I =
would get a=20
repulsion?=20

On Oct 24, 2011, at 4:54 PM, Rueckner, Wolfgang =
wrote:

I'll bring my camera in =
tomorrow=20
morning and take a picture for you. I'll measure the =
resistance=20
of the whole shebang and calculate the current. My guess =
is 200=20
to 500 amps.=20

On Oct 24, 2011, at 4:51 PM, Adam Beehler =
wrote:

Well, there you =
have it. =20
Thanks, Wolf! Would you mind sending me a picture=20
offline? How much current do you estimate flows =
through your=20
10/24/2011 2:25=20
PM, Rueckner, Wolfgang wrote:=20
I placed and iron bar (2"x4"x24") =
on the=20
smooth cement floor in our demo room and laid down our =
braided=20
copper conductor (that we use for repelling (or =
attracting)=20
current carrying wires) next to it. About 1" =
separation=20
between iron bar and copper wire. Closed the moose =
knife=20
switch to short out our truck battery (just for half a =
second,=20
mind you) and the copper wire was attracted (and touched) =
the iron=20
bar. -- Wolf=20

On Oct 24, 2011, at 4:15 PM, Zani, Gerald =
wrote:
Wolf,What did you use =
for a=20
ferromagnetic plate and how did you arrange =
it?How much=20
did it move?- Jerry
On Mon, Oct 24, 2011 at 4:09 =
PM,=20
Rueckner, Wolfgang =
wrote:
Just tried it. It works =
remarkably=20
well!On Oct 24, 2011, at 3:54 PM, Adam =
Beehler=20
wrote:> I have an instructor that tends to =
believe=20
we ought to be able to put> together a demo =
showing=20
"image currents." This is what he proposed =
to>=20
me. Run a current-carrying wire along the =
surface of a=20
ferromagnetic> plate/sheet. If the =
current is=20
high enough, then there will be a force> =
between the=20
current-carrying wire and its "image current" =
running>=20
parallel to it in the ferromagnetic surface just next =
to it.=20
This would> need to be a whole lot of =
amps, like=20
500-1000!>> So...has anybody tried =
anything like=20
this? Is there wire that can> handle such =

currents for brief moments, yet not be so bulky that=20
the> attractive forced created is outweighed by =
the=20
wire's bulk? I need> help,=20
Beehler--=20
Gerald ZaniDemonstration =
ManagerPhysicsBrown=20
University(401)=20
=
863-3964-- Gerald ZaniDemonstration=20
ManagerPhysicsBrown University(401)=20
863-3964

**********************************************
Steven K. Wonnell, Ph.D.
Manager, Physics Instructional Resources
Department of Physics and Astronomy
The Johns Hopkins University
3400 N. Charles Street
Room 534 Bloomberg Center
Baltimore, MD 21218
Phone: 410-516-5468
E-Mail: wonnell@pha.jhu.edu
**********************************************

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