# Thought Experiment with Iron wire

1. Jul 25, 2006

### Jdo300

Hello All,

Consider the following. Take a cylindrical magnet (say 2” long by 0.5” diameter and wrap it with iron wire in such a fashion that the windings do not touch (assuming it is uninsinuated). Wrap just enough turns of wire so that the magnet’s field is essentially short circuited through the iron, but use only enough iron to absorb the field you need (should be about saturated).

Now, let’s say that this magnet has a field strength of 5 units and the iron wire is completely saturated with the 5 units of flux from the magnet. If I then apply a DC current in this iron coil that causes it to produce a magnetic field with one unit of strength (while the iron wire is saturated with the permanent magnet’s field) what will happen?

1. Will the flux lines from the permanent magnet that were shorted through the iron wire go through the air since the wire would be saturated from the current?

2. If the flux from the permanent magnet comes out of the iron, will the iron wire release 1 unit of the permanent magnet’s field for every unit of magnetic energy the coil produces, or is this relationship likely to be non-linear?

I would appreciate anyone who could shed some light on this.

Thanks,
Jason O

2. Jul 26, 2006

### Danger

Maybe it's just because of the beer, but that question makes no sense to me. Why, for instance, would you try to wrap uninsulated wire into a solenoid and avoid contact with adjacent coils, when you can just use regular insulated magnet wire?

edit: Ooops! I just noticed that I had the wrong smilie there. Sorry 'bout that. The grumpy one was a typo.

Last edited: Jul 26, 2006
3. Jul 26, 2006

### pallidin

I think you are asking if the "saturated" iron windings no longer provide a magnetic path which would not entail a further "free-space" magnetic expression.
You would be correct. The saturated condition of the windings would act merely as a functional extension of the magnet.

4. Jul 26, 2006

### Jdo300

Hi,

Thank you for your help. So, if I am interpreting what you are saying correctly, once the iron wire is saturated from the input current, the magnetic flux lines of the permanent magnet will no longer go through the iron? What kind of relationship would this be for the iron? I am essentually trying to see if I can completely 'swtch' the flux of the magnet between going through the iron and free air, but I am wondering how much input this requires into the iron to cause this effect. Would I need to put in enough current to create one unit of magnetic flux to release one unit of magnetic flux of the permanent magnet from the iron, or would this relationship likely be non-linear? Is there any way I could calculate this?

Thank you,
Jason O

5. Jul 27, 2006

### pallidin

Ok. Let's examine this:

"...if I am interpreting what you are saying correctly, once the iron wire is saturated from the input current, the magnetic flux lines of the permanent magnet will no longer go through the iron?"

No. Saturation is only an aspect of complete alignment within the potential magnetic domains of the target material. As such, saturation provides for an optimum magnetic pathway and external magnetic flux lines will tend towards a path of optimization.
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"What kind of relationship would this be for the iron?"

The iron acts as an extension of the magnetic expression, even more so under "saturated" conditions.
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"I am essentually trying to see if I can completely 'switch' the flux of the magnet between going through the iron and free air..."

Sure, this can be done, but at a cost of energy expenditure.
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"Would I need to put in enough current to create one unit of magnetic flux to release one unit of magnetic flux of the permanent magnet from the iron, or would this relationship likely be non-linear?"

Ah, the magic question.
Until magnetic monopoles are verified and somehow deemed to be manipulative, there is no "release"

Last edited: Jul 27, 2006
6. Jul 27, 2006

### Jdo300

How would magnetic monopoles be of any use in this case? Also, what do you mean by "release"? I am just interested in understanding the interactions of magnetic field flow through the iron in saturated and unsaturated states. Would you or anyone else here know of any good, relevent resources I can check to understand the physics of this situation?

Thank you,
Jason O

7. Jul 28, 2006

### pallidin

Hi Jason.

I used the word "release" because you used it in the question, so I assumed you were referring somehow to monopoles. Perhaps I was wrong.

Question for you, though: Are you trying to "isolate" a magnetic field from it's source?
Also, when I have more time today, I should be able to locate some good references for you.

8. Jul 28, 2006

### Jdo300

Hi Pallidin,

Sorry for the confusion; when I mentioned the word "release" in I should have said "switch", as in causing the field to want to flow through the open air rather than the iron wire.

As for your second question, no, I don't want to isolate the permanent magnet's field from the magnet itself, I simply want to switch/redirect the flux flow of the magnet from going through the iron to going through the air by saturating the iron wire (not sure if this is how it would work though).

Thanks,
Jason O

9. Jul 28, 2006

### NoTime

It might help to think of it this way.
A piece of iron represents a reduced resistance to the magnetic field.
However, the iron only has a limited number of paths that reduce the resistance.
You seem to have this idea ok.

You seem to need to think about parallel resistors though. The air is still there. Some of the field will always go through the air.

When the magnetic paths in the iron are all filled up
the field will still flow though the area occupied by the iron, it just won't have the advantage or the resistance reducing effect.

For example wrap two copper coils together around a bit of iron.
Use meter to measure L of one coil.
Put a DC current through the second coil.
The measured L will get smaller in the first coil as you increase the DC current up to the saturation point and then stay about the same(see saturation curves).
All the field must still travel through the inside of the coil, which just happens to be filled with iron.