Date: Wed, 26 Oct 2011 13:53:26

Author: --- Adam Beehler

Subject: Re: Image Current Demo

Post:

Oh boy, what a day! I finally had time to mess with this "demo." This
is what I have learned.

Wolfgang, the connecting red wire is definitely influencing the swinging
rod, despite it being "away." I confirmed this by moving it closer and
farther, as well as switching sides. I can now make the swinging rod
swing either direction or not at all depending on wire orientation.

Cliff, the iron rod I was using is now very, very, very weakly
magnetized. I am not sure if it was when I started. I started testing
all kinds of iron and steel objects around my area. I walked around
with a tiny little compass needle and watched to see its deflection
right up against the objects. I was surprised to learn that most of the
iron/steel objects I have just lying around were ever so slightly
magnetized. Anyway, I finally found some that were not and used them in
the "demo." They worked as well, i.e., the swinging rod was attracted
to them.

I even set up the swinging rod so that it would repel one way from the
side wire, while at the same time placing an unmagnetized iron rod so it
would swing the opposite way. In other words, the repulsion and
attraction forces should be competing. It still worked - the swinging
rod was attracted to the iron rod.

Adam Beehler


On 10/26/2011 6:14 AM, Cliff Bettis wrote:
> Adam,
>
> A piece of ferromagnetic material placed in a magnetic field will have an
> induced magnetic dipole moment. It doesn't matter if the source of the field
> is another magnet or a current carrying wire. Whether or not the
> ferromagnetic material experiences a force or not depends on geometry (e.g.
> there is no net force if the applied field is uniform, only a torque (again
> depending on geometry)).
>
> I am skeptical about this demonstration because the theoretical derivation
> requires an infinite wire. In the real case, the iron rod is finite and
> there is a current loop in at least one side of the magnetic circuit. Should
> the iron rod be made a closed circuit so current can flow? If so aren't you
> talking about two current loops? This reminds me of the problems that arise
> when you try to deal with the question of what frame of reference a current
> carrying wire is neutral in and explaining the Biot-Savart law
> relativistically (from the electrostatic point of view). There, too,
> calculations involve an infinite wire and it turns out that while the field
> quantities are easily transformed from one frame of reference to another,
> the source terms are not in realistic cases.
>
> Cliff
>
> -----Original Message-----
> From: tap-l-owner@lists.ncsu.edu [mailto:tap-l-owner@lists.ncsu.edu] On
> Behalf Of beehler@physics.utah.edu
> Sent: Tuesday, October 25, 2011 6:44 PM
> To: tap-l@lists.ncsu.edu
> Cc: beehler@physics.utah.edu
> Subject: Re: [tap-l] Image Current Demo
>
> When I first held copper up to the swinging rod, I was all excited to see a
> slight repulsion. But no luck, instead I saw a slight attraction. I was
> not sure what to think. I just simply tried other materials and shapes and
> thicknesses. Same thing. Soft ferromagnetic materials attracted the
> swinging rod conspicuously, and everything else either caused no movement or
> initial attraction. Even when I had nothing next to the swinging rod, it
> still seemed to move inward. Part of it is due to me physically shaking the
> unit when I depress the "on" button, but I am not convinced that is the
> whole story. I tried to be pretty careful at times.
>
> I thought another reason could be the mini electrical arcs that can happen
> in the small contact points mounting the swinging rod. I do not always see
> them but I can quite often hear them. They could be jiggling the swinging
> rod and allowing it to adjust due to gravity. (The swinging rod unit has
> leveling feet for adjusting the unit so that the swinging rods tend to swing
> back after attracting or repelling. I tried to adjust those feet so that
> the one swinging rod would swing back, and away, from the fixed rod after
> any interaction. Sometimes the swinging rod would stick or come to rest in
> slightly different positions though.)
>
> I then wondered what you mentioned. Could it be repelled from the other
> wires or posts involved with the set-up? Short answer - I'm not sure but
> I'm leaning towards probably not. The swinging rod should be "attracted"
> to the post, so that isn't it. The current in the same-side, red wire would
> be going the opposite direction and thus provide a repulsive force, but
> since it is relatively far away from the swinging rod, I am not yet
> convinced it is the culprit. I am at home right now and will test this
> theory tomorrow by moving that wire even farther away. I can also change
> sides with the swinging rod and test that. This theory would sure explain
> it, though.
>
> As for the iron rod having already been magnetized, I cannot confirm nor
> deny at this point. In other words, I did not check. I will tomorrow. I
> did however try several different iron objects (not all rods), and they all
> showed attraction. True, they might all have been pre-magnetized, but that
> would be unlikely.
>
> I am also curious about the possible connection between this demo and a
> solenoid pulling in an iron core. I always considered the iron core as
> becoming magnetized and then attracting to the coil's magnetic field.
> Maybe one can think of it as the iron core's magnetic field exerting a
> Lorentz force on the moving charges in the coil. I don't know. I guess if
> one can calculate the same resultant forces using an image current, then
> maybe so.
>
> Adam Beehler
>
>
>
>> Well that certainly works and makes the point! Is the slight movement
>> (with the aluminum and rubber rods) due to repulsion from the other
>> wire off to the right? -- Wolfgang
>>
>>
>>
>> On Oct 25, 2011, at 3:23 PM, Adam Beehler wrote:
>>
>>> I was able to do the demonstration myself. Hooray! But I did it
>>> differently. I have posted a little video I took of it here:
>>> http://www.physics.utah.edu/~beehler/tap_l/ImageCurrentDemo.mp4
>>> I do not yet have the supplies to do it Wolfgang style, so I tried it
>>> with my "Scientific Supplies, Inc." "Force Between Two Parallel
>>> Currents Demonstrator."
>>> http://www.unitedsci.com/detail.php?id=1519
>>> I removed one of the swinging/pivoting rods and mounted an iron rod
>>> next to the remaining swinging/pivoting rod. It worked. The
>>> swinging rod was attracted to the iron rod. I then replaced the iron
>>> rod with aluminum, copper, and rubber. I was not able to see any
>>> repulsion, but I did not see attraction either.
>>> Adam Beehler
>>>
>>



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