Date: Tue, 25 Oct 2011 10:50:08

Author: --- "Rueckner, Wolfgang"

Subject: Re: Image Current Demo

Post:

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That's right Steve, the attractive force between the current and the ferrom=
agnetic bar is stronger than the repulsive force from a copper bar. Since =
the resistance of iron is significantly greater than copper, one would also=
expect that the magnitude of the eddy currents in the iron bar are less th=
an in the copper. So the magnitude of ferromagnetic attraction versus eddy=
current repulsion is confounded by the differences in resistance. -- Wol=
fgang

On Oct 25, 2011, at 10:16 AM, Steve Wonnell wrote:

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 oppos=
ite charge on the metal.

It turns out that the electric fields in this situation, created by the ori=
ginal 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" te=
chnique 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 e=
lectrical current aligns the magnetic dipoles of the magnetic material, and=
these aligned magnetic dipoles create a magnetic field that then repels th=
e magnetic field created by the current.

Apparently, according to those who've done the math, the magnetic field con=
figurations of this complicated situation appear to be identical to those c=
reated by two charged currents: the electrical current above the surface o=
f the magnetic material, and an image current buried inside the magnetic ma=
terial.

There must also be an induced eddy current in the magnetic material since t=
he magnetic material is usually also a conducting material, but evidently t=
his 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 be=
tween 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 charge, not a real charge?

No?

And the image charge method makes no distinction between the types of condu=
ctor used?

No?

So a copper bar would behave the same as an iron bar, they would both be at=
tracted?

Just trying to understand what is happening. - Jerry

On Tue, Oct 25, 2011 at 8:56 AM, George Herold > wrote:
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)

George H.
________________________________
From: tap-l-owner@lists.ncsu.edu [mailto=
:tap-l-owner@lists.ncsu.edu] On Behalf O=
f Rueckner, Wolfgang
Sent: Monday, October 24, 2011 5:04 PM
To: tap-l@lists.ncsu.edu
Subject: Re: [tap-l] Image Current Demo

So what exactly is this "image current"? I just realized it's not eddy cur=
rents because they would create a repulsion of the current carrying wire, n=
ot attraction. If I had a similar copper bar (rather than iron) next to th=
e wire, I suppose I would get a repulsion?

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.

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

Well, there you have it. Thanks, Wolf! Would you mind sending me a pictur=
e offline? How much current do you estimate flows through your copper brai=
ded wire?

On 10/24/2011 2:25 PM, Rueckner, Wolfgang wrote:
I placed and iron bar (2"x4"x24") on the smooth cement floor in our demo ro=
om and laid down our braided copper conductor (that we use for repelling (o=
r attracting) current carrying wires) next to it. About 1" separation betw=
een iron bar and copper wire. Closed the moose knife switch to short out o=
ur truck battery (just for half a second, mind you) and the copper wire was=
attracted (and touched) the iron bar. -- Wolf

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

Wolf,

What did you use for a ferromagnetic plate and how did you arrange it?

How much did it move?

- Jerry

On Mon, Oct 24, 2011 at 4:09 PM, Rueckner, Wolfgang > wrote:
Just tried it. It works remarkably well!

On Oct 24, 2011, at 3:54 PM, Adam Beehler wrote:

> 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
>

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

--
Gerald Zani
Demonstration Manager
Physics
Brown University
(401) 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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That's right Steve, the at=
tractive force between the current and the ferromagnetic bar is stronger th=
an the repulsive force from a copper bar. Since the resistance of iro=
n is significantly greater than copper, one would also expect that the magn=
itude of the eddy currents in the iron bar are less than in the copper. &nb=
sp;So the magnitude of ferromagnetic attraction versus eddy current repulsi=
on is confounded by the differences in resistance. -- Wolfgang<=
div>On Oct 25, 2011, at 10:16 A=
M, Steve Wonnell wrote:Hi Jerry,<=
br>The "method of images" is a mathematical method for solving p=
roblems.You know that if you hold a charged elect=
rical object next to a thick piece of metal, it will be attracted because t=
he charged object induces an opposite charge on the metal.It turns out that the electric fields in this situation, created b=
y 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 th=
e electrical current aligns the magnetic dipoles of the magnetic mater=
ial, and these aligned magnetic dipoles create a magnetic field that then r=
epels the magnetic field created by the current.A=
pparently, according to those who've done the math, the magnetic field conf=
igurations of this complicated situation appear to be identical to those cr=
eated by two charged currents: the electrical current above the surfa=
ce of the magnetic material, and an image current buried inside the magneti=
c material.There must also be an induced eddy cur=
rent in the magnetic material since the magnetic material is usually also a=
conducting material, but evidently this eddy current repulsion is weaker t=
han the image current's field.In Wolfgang's resul=
ts, he should see that the attractive force between the current and the fer=
romagnetic plate is stronger that the repulsive force between the current a=
nd the copper bar.That's my take so far.SteveOn Oct =
25, 2011, at 9:24 AM, Zani, Gerald wrote:Hmmmm.Is the "image" charge a=
n imaginary charge, not a real charge? No?And the ima=
ge charge method makes no distinction between the types of conductor used?&=
nbsp; No?So a copper bar would behave the same as an iron b=
ar, they would both be attracted?

Just trying to understand what is happening. - JerryOn Tue, Oct 25, 2011 at 8:56 AM, George Herold > wrote:

Well an=20
image current is moving image charges. You must know image charges fr=
om=20
the electro-static 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=20
[mailto:t=
ap-l-owner@lists.ncsu.edu] On Behalf Of Rueckner,=20
WolfgangSent: Monday, October 24, 2011 5:04 PMTo:=20
tap-l@lists.ncs=
u.eduSubject: Re: [tap-l] Image Current=20
Demo
So what exactly is this "image current"? I just realized=
it's=20
not eddy currents because they would create a repulsion of the current=20
carrying wire, not attraction. If I had a similar copper bar (rathe=
r=20
than iron) next to the wire, I suppose I would get a repulsion?

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

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

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

Well, there you have it.&nb=
sp;=20
Thanks, Wolf! Would you mind sending me a picture offline? =
; How=20
much current do you estimate flows through your copper braided=20
wire?Adam BeehlerOn 10/24/2011 2:25 PM, Rueckner, Wol=
fgang=20
wrote:=20
I placed and iron bar (2"x4"x24") on the sm=
ooth cement floor=20
in our demo room and laid down our braided copper conductor (that w=
e use=20
for repelling (or attracting) current carrying wires) next to it.=20
About 1" separation between iron bar and copper wire. C=
losed=20
the moose knife switch to short out our truck battery (just for hal=
f a=20
second, mind you) and the copper wire was attracted (and touched) t=
he=20
iron 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 d=
id it=20
move?- Jerry
On Mon, Oct 24, 2011 at 4:09 PM, Rueck=
ner,=20
Wolfgang =20
wrote:
Just tried it. It wor=
ks remarkably=20
well!On Oct 24, 2011, at 3:54 PM, Adam Beehler=20
wrote:> I have an instructor that tends to believe w=
e=20
ought to be able to put> together a demo showing "image=
=20
currents." This is what he proposed to> me. =
Run a=20
current-carrying wire along the surface of a ferromagnetic&=
gt;=20
plate/sheet. If the current is high enough, then there wi=
ll be=20
a force> between the current-carrying wire and its "imag=
e=20
current" running> parallel to it in the ferromagnetic su=
rface=20
just next to it. This would> need to be a whole lo=
t of=20
amps, like 500-1000!>> So...has anybody tried any=
thing=20
like this? Is there wire that can> handle such cur=
rents=20
for brief moments, yet not be so bulky that the> attract=
ive=20
forced created is outweighed by the wire's bulk? I=20
Beehler-=
-=20
Gerald ZaniDemonstration ManagerPhysicsBrown=20
University(401)=20
863-3964<=
/blockquote>
-- Gerald ZaniDemonstr=
ation ManagerPhysicsBrown University(401) 863-3964

**********************************************Steven K=
. Wonnell, Ph.D.Manager, Physics Instructional ResourcesDepartment of Physics and AstronomyThe Johns Hopkins Universi=
ty3400 N. Charles StreetRoom 534 Bloomberg CenterBaltimore, MD 21218Phone: 410-516-5468E=
-Mail: wonnell@pha.jhu.edu=
**********************************************<=
br>

=

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