Permeability of iron

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Summary:
I want to know what happens as a magnetic field passes through iron.
I know that iron is typically used in electromagnets to create a stronger magnetic field. But what is the exact nature in which that happens? When it goes in is it slowed down at all while traveling through the metal or does it still keep going at the speed of causality? Also does it have the reverse polarity as it leaves the other side or is it the same? Finally, at lower frequencies iron reflects a return signal that cancels out the incoming magnetic wave. I learned that when I found out why they used iron shavings in the paint of stealth jets. But what are those frequencies and what is the delay? I would like to know more about this.
 

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  • #2
Baluncore
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I know that iron is typically used in electromagnets to create a stronger magnetic field. But what is the exact nature in which that happens?
https://en.wikipedia.org/wiki/Ferromagnetism#Magnetic_domains

When it goes in is it slowed down at all while traveling through the metal or does it still keep going at the speed of causality?
It is slowed down.

Also does it have the reverse polarity as it leaves the other side or is it the same?
It enters from both sides at the same time, one side forwards, the other side backwards, meeting in the middle.

Finally, at lower frequencies iron reflects a return signal that cancels out the incoming magnetic wave. I learned that when I found out why they used iron shavings in the paint of stealth jets. But what are those frequencies and what is the delay? I would like to know more about this.
Grains of iron tend to be big and so work at low frequencies, other forms of iron such as in ferrites are small so can work at microwave frequencies. That is why different frequency radar is reflected differently.
https://en.wikipedia.org/wiki/Skin_effect
 
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  • #3
tech99
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I am not aware that magnetic fields have a velocity. Where we see a flow of energy we have an EM wave. The magnetic field is an energy store.
 
  • #4
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It is slowed down.
By how much?
It enters from both sides at the same time, one side forwards, the other side backwards, meeting in the middle.
You might want to rephrase that because I am having trouble understanding it.
I am not aware that magnetic fields have a velocity.
I am referring to magnetic field propagation.
 
  • #5
DaveE
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By how much?
The speed of light in materials is primarily a function of three parameters: Permeability (magnetic, sort of); Permittivity (electric, sort of); resistivity (energy losses, sort of). Magnetic materials have high permeability.

The exact formulas are here: https://en.wikipedia.org/wiki/Speed_of_electricity
 
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  • #6
Baluncore
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You might want to rephrase that because I am having trouble understanding it.
Imagine a wire being used to cut cheese. You hold the wire, with some tension, between your two hands. You can move it into place through air very rapidly, but a cheese obstructs and slows down the cutting progress. The wire will quickly align on the surface of the cheese, before it begins to cut slowly into the cheese. Different types of hard or soft cheese can be cut at different rates.

Imagine a virtual “line” of magnetic field, being used to cut magnetic and conductive materials. The field line is held, under some tension, between the two poles of a magnet. The field line can cut sideways through space at the speed of light, but magnetic and conductive material slows it down. Different materials slow it down by different amounts. The high conductivity of copper slows the magnetic lines down to your jogging pace. Superconductors can effectively stop them. Magnetic materials attract the field lines, and magnetic objects are pulled together by the tension in the lines, all lines beginning and ending at the poles of a magnet.

Now refine your mental model of a magnetic field to be a huge number of filaments that cut space, all repelling each other, so keeping apart, while being kept under tension between the poles of the magnet. Those fields of magnetic lines diffuse sideways at predictable speeds, at the speed of light through space, or at reduced speeds through different materials.

That model is for beginners only. At some point the more general mathematical model must take over. The cheese model is over-simplistic, and does not for example differentiate between B and H.
 
  • #7
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The speed of light in materials is primarily a function of three parameters: Permeability (magnetic, sort of); Permittivity (electric, sort of); resistivity (energy losses, sort of). Magnetic materials have high permeability.

The exact formulas are here: https://en.wikipedia.org/wiki/Speed_of_electricity
Are these formulas explaining how fast the em wave travels through a material when receiving an oscillating wave coming FROM a circuit, or as PART of a circuit? Are they talking 90 degrees away from the direction of the current or in the same direction of the current?
 
  • #8
DaveE
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Are these formulas explaining how fast the em wave travels through a material when receiving an oscillating wave coming FROM a circuit, or as PART of a circuit? Are they talking 90 degrees away from the direction of the current or in the same direction of the current?
An EM wave traveling through the material. Where it came from is a more complex question that depends on the details of the antennas, etc. These are fundamental properties that describe how all EM fields behave, much more general than a specific circuit or current flow. This would normally be introduced with plane waves, which are simple mathematically, and often a good approximation in practice, but actually impossible in nature.
 
  • #9
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That model is for beginners only. At some point the more general mathematical model must take over. The cheese model is over-simplistic, and does not for example differentiate between B and H.
Sure it can't be as simple as your cheese analogy, right? I mean what about reflections? You said they come out of both sides at the same time. EXACTLY or APPROXIMATELY? Because if they come out both sides at EXACTLY the same time wouldn't the material have literally no thickness and therefore not even be there? Please explain more.
Where it came from is a more complex question that depends on the details of the antennas, etc.
Let me put it to you this way. Let's say we have a single, straight wire, carrying a very high frequency AC, next to an iron plate. How would the EM wave travel through the plate? Would there be any reflections or anything? Or is it as simple as traveling through the iron slower? Also, would iron be a good dielectric or conductor? I'm leaning towards a good conductor.
 
  • #10
Baluncore
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You said they come out of both sides at the same time. EXACTLY or APPROXIMATELY? Because if they come out both sides at EXACTLY the same time wouldn't the material have literally no thickness and therefore not even be there? Please explain more.
It enters from both sides at the same time, one side forwards, the other side backwards, meeting in the middle.
When you cut cheese with a wire the wire enters and exits the cheese on both sides at the same time. The wire is not like a twist drill that makes a hole from one side to the other. Likewise a continuous magnetic line is never broken and must be pulled into the material sideways, so it virtually enters and exits at the same time.

Let's say we have a single, straight wire, carrying a very high frequency AC, next to an iron plate. How would the EM wave travel through the plate? Would there be any reflections or anything? Or is it as simple as traveling through the iron slower?
The vast majority of the HF would be reflected from the surface by the conductivity of the iron. The small amount that was not reflected would continue to diffuse through the solid iron at walking pace, until it merged with it's other half phase.

Also, would iron be a good dielectric or conductor? I'm leaning towards a good conductor.
Iron is a good conductor, not a dielectric. Dielectrics tend to be good insulators, metals are good conductors.
 
  • #11
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When you cut cheese with a wire the wire enters and exits the cheese on both sides at the same time. The wire is not like a twist drill that makes a hole from one side to the other. Likewise a continuous magnetic line is never broken and must be pulled into the material sideways, so it virtually enters and exits at the same time.
I thought electromagnetic fields don't propagate faster than the speed of light. Are we talking about the same direction of the wave traveling through the metal?
 
  • #12
Baluncore
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I thought electromagnetic fields don't propagate faster than the speed of light. Are we talking about the same direction of the wave traveling through the metal?
Are we talking about the same orthogonal component of the wave passing through the metal ?
The wave and energy travel in the direction of the Poynting vector, which is mutually perpendicular to the electric field and the magnetic field lines.
https://en.wikipedia.org/wiki/Electromagnetic_field#/media/File:Onde_electromagnetique.svg
https://en.wikipedia.org/wiki/Poynting_vector

The incident magnetic field lines are pulled tight against the conductive surface as the wave begins to enter everywhere along the line at the same time, travelling very slowly into the material, perpendicular to the surface.

The group velocity of the wave is between zero and the speed of light, depending on the material. The phase velocity of the wave is between the speed of light and infinity depending on the angle of arrival. Since the magnetic line moves into the metal everywhere at almost the same instant, it has a phase velocity very close to infinity.
At the beach, surfers prefer a lower phase velocity, an infinite phase velocity is a dumper.
 
  • #13
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Okay, and when the magnetic field reflects from the iron, does it become stronger (when using high frequency ac)? They use iron in electromagnets to make them stronger which is why I am asking.
 
  • #15
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You didn't answer any of my questions.

Also going back to the cheese analogy, are you assuming I have wires on BOTH sides of the iron? Is that what you meant when you said "enters both sides at the same time"? Because I'm not trying to do that at all, the wire for my example is just on one side of an iron plate. I apologize for not clearing that up.

So here is what I assume would happen, and correct me where I am wrong. The wire is at a very small distance from the iron plate while conducting very high frequency AC. The magnetic field, propagates towards the iron plate. When it reaches the plate it reflects off of it even stronger with the vector field lines still curling in the same direction. Also the reflection is elongated because of the thickness of the iron and some of the field reflects sooner than other of it. So am I correct?
 
  • #16
Baluncore
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So am I correct?
No.

When it reaches the plate it reflects off of it even stronger with the vector field lines still curling in the same direction.
What do you mean by "even stronger", when you look in a mirror, is your image brighter ?
What do you mean by "curling" ?
 
  • #17
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No.


What do you mean by "even stronger", when you look in a mirror, is your image brighter ?
What do you mean by "curling" ?
So then how DOES iron make an electromagnet stronger? Does it do so by distorting the field lines or what?
 
  • #18
Baluncore
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Does it do so by distorting the field lines or what?
It takes time for the magnetic field, B, to get deeper into the iron, where it can be stronger. You should think of the magnetisation, H, increasing in proportion to the permeability of the material that B passes through.

Probably the best model for a beginner without mathematics is to think of the magnetic field lines as being under tension, then getting thicker and so pulling stronger where they pass through a magnetic material like iron.
 
  • #19
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Another thing I'm having trouble understanding is if magnetic field propagation is slower through iron then why don't electrical transformers have phase shifts? (Except for intentional ones.)

And by the way, I have quite a bit of a mathematical background.
 
  • #20
Baluncore
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Another thing I'm having trouble understanding is if magnetic field propagation is slower through iron then why don't electrical transformers have phase shifts?
Because they have laminations that are thin. The magnetic field travels at about 60% of the speed of light through the insulation between the laminations, then moves very slowly into the surface area of the laminations.

There is a thread here that discusses lamination thickness and skin effect.
https://www.physicsforums.com/threa...transformers-how-to-apply-them.1002399/page-2
 
  • #21
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Let me put it to you like this. Say we have a wire warped around an iron cylinder, i.e. a solenoid. So basically you are saying the magnetic field propagates out through the polar ends first during the first wave of the AC at the speed of light, but only at the edges first? And part of the field that propagates out of the polar ends approaches the center at a reduced rate? Is THAT what you are trying to tell me? If so I think I understand now.

Anyway, I want to share with you why I am asking you this. I am trying to find out how to enhance induction motors for very high frequency applications, and how the Lorentz force comes into this. Now I know even the first induction motors had an efficiency between 94 and 95 percent, which is awesome. But I am wondering if this can be enhanced further. That is why I am trying to understand the EXACT nature of how iron shapes the propagation of an alternating magnetic field, especially at extremely high frequency, so much so that relativity is a factor. And don't worry too much about centrifugal forces tearing things apart, it's more of a linear application.
 
  • #22
Baluncore
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So basically you are saying the magnetic field propagates out through the polar ends first during the first wave of the AC at the speed of light, but only at the edges first?
The field appears almost instantly about the iron cylinder. It then diffuses into the solid iron very slowly. A solenoid wound on a solid iron core would take time for the magnetisation to build up or to release. But the complete magnetic circuit, the external path between the poles, will be important.

I am trying to find out how to enhance induction motors for very high frequency applications,...
The stator field coils in a 3PH induction motor induce currents in the metal rotor. Normally, the field windings will have a laminated core with slots for the phase windings. The core must function at the full power frequency. It is most unlikely you will get shaft power efficiently from an induction motor for power frequencies above 1 kHz.

For high frequencies only a very thin outer layer of the rotor can be involved so the rotor surface current will be limited by the resistance of the surface layer. The frequency of the currents flowing in the rotor would be zero when it is running at synchronous speed. To get full power from the motor you will need to have about 5% slip below synchronous speed, so the rotor will have currents at 5% of the field winding frequency. That is why a solid metal rotor can be used in a typical induction motor, since 5% of 50 Hz is 2.5 Hz. But 5% of 1 kHz is 20 Hz, so less rotor conductor will be involved, resistance will be higher and losses greater.

So what do you exactly mean by “High Frequency”.
 
  • #23
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The field appears almost instantly about the iron cylinder. It then diffuses into the solid iron very slowly. A solenoid wound on a solid iron core would take time for the magnetization to build up or to release. But the complete magnetic circuit, the external path between the poles, will be important.
But you are saying that the filed moving out of the polar ends of the solenoid does so first closer to the windings, right?

So what do you exactly mean by “High Frequency”.
If you must know, gigahertz. I know what you are going to say. "It will never work for a 3PH induction motor". Well I'm not trying to create a 3PH induction motor. It works through induction but the architecture is different. Beyond that I''m not going into much detail.

But I have another example. let's say we have an iron bar with two windings at each end, one circuit being the sender, and the other being the receiver. Now, as the sender has AC flowing through it, how much of a delay would there be before the receiving circuit matches it through induction? Would it be at the speed of light or would it be slower as the magnetic field propagates through the metal?
 
  • #24
Baluncore
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Would it be at the speed of light or would it be slower as the magnetic field propagates through the metal?
There are multiple paths. One through the air outside the iron core, the others at different depths through the iron. The result will depend on the sum of the signals through those two parallel paths, and that will depend on the frequency of the AC.

High frequencies will travel at the speed of light, while sine waves at any frequency will be phase shifted by the external impedance of the transformer windings.
 
  • #25
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There are multiple paths. One through the air outside the iron core, the others at different depths through the iron. The result will depend on the sum of the signals through those two parallel paths, and that will depend on the frequency of the AC.
As I thought. And would I be correct in assuming that the one traveling through the iron would be stronger?
 

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