Physics/Magnetic powder in a solenoid
QUESTION: Hello Steve,
I saw an interesting video recently about magnetic powder and I was wondering if I could ask your opinion on how it might behave. ( https://www.youtube.com/watch?v=ZazPBWQexgk )
According to the video, the powder is made up of tiny magnets. When the man in the video applies an external magnetic field from a permanent magnet (at 1:10), he says "each of these little particles is orienting themselves relative to the north south field"
Suppose I have a plastic tube about 70% full with this magnetic powder, with a copper coil solenoid around the cylinder.
What would happen when we turn on the solenoid? How would the magnetic powder react? Would the orientation of resulting magnetic field resemble that of a standard iron core electromagnet?
Your opinion would be most useful, as I am really unsure whether my understanding of the situation is at all correct.
Thank you and regards,
ANSWER: Hello Eddie,
Yes, that is an interesting video. As usual I have to say that I have no experience directly applicable to your question. But I will give my opinion, or prediction, of what would happen in the situation you asked about. I would expect that the particles in the powder would orient themselves in an organized manner when the solenoid is energized. So yes, I would expect the orientation of resulting magnetic field to resemble that of a standard iron core electromagnet. Perhaps the affect would be stronger than if the same solenoid had a standard iron core. (However, there would be a minimum field strength that would be required for this to happen.) And I think they would tend to retain that organization after the solenoid power is turned off.
But I don't know how long lasting that organization would be. They might tend to reorient themselves with the Earth's magnetic field. Perhaps they would reorient themselves only when experiencing vibration or a change in orientation of the plastic tube. My best guess would be that they would retain the orientation if left undisturbed - a change in temperature might be enough disturbance. When disturbed, the powder might disorganize and become what the man in the video called a "clump".
I hope this helps,
---------- FOLLOW-UP ----------
QUESTION: Thanks Steve,
I really appreciate your answer - it was a big help to me. It got me thinking a lot, and now I am wondering whether you could tell me if you think I am on the right track.
You raised the issue of the minimum field strength the solenoid is required to produce in order to cause the combined magnetic fields to follow the orientation of a standard iron core solenoid, and I have been pondering how to calculate what that minimum field strength might be.
It seems like a very complicated issue, but as a first step I was wondering if you could give me your opinion on the following idea.
In the brief moment after the electromagnet is turned on, but before the tiny magnets in the powder all align and add their fields to the field of the electromagnet, is the strength of the magnetic field produced by the electromagnet going to be closer to that of an air core solenoid, or an iron core electromagnet? My gut feeling tells me it is going to be closer to the iron core solenoid, given that the core is ferrous. Does this seem like a reasonable assumption?
If the magnetic powder in the plastic tube is of a density, say 70% of the iron core, can I assume the magnetic field will be 70% of the denser iron core electromagnet (assuming here that the other material characteristics eg permeability are the same in both cases).
I understand that after the granules in the powder all align, the field will be stronger than the standard iron core electromagnet, but in the split second before this happens, is the electromagnet going to produce a field as if it had a standard ferrous core?
Thank you again for your answers - it is great to have someone tell me whether my thinking is reasonable, because sometimes I am really not sure!
Your followup has caused me to think more deeply about this. You asked about "the brief moment after the electromagnet is turned on, but before the tiny magnets in the powder all align. That forced me to think about what happens in an iron core. This is a good discussion about what we now need to consider.
We don't need to grasp all the sections in this webpage, but we need to talk about magnetic domains which are introduced in the section titled Ferromagnetism. In the section titled Long Range Order in Ferromagnets, it says "Sizes of domains range from a 0.1 mm to a few mm." What size are the grains of the "magnetic powder"?
When I thought about what happens in an iron core when you turn on an electromagnet, I realized that the domains have to orient themselves with the applied magnetic field before the iron core boosts the magnetic field above what an air core would give. It is because of the domains getting themselves oriented with the external field that the iron core boosts the external field above what it would otherwise be. (Explanatory note: In the previous comment I assume the iron core has no memory of previous cycles and the domains have random orientation. Read the paragraph beginning "Ferromagnets will tend to stay magnetized" in the section titled Ferromagnetism.) So when the electromagnet is turned on, much the same thing would happen to the core whether it is iron or magnetic powder. The domains and the grains of powder are very similar; both have to reorient themselves to the sudden application of the magnetic field. I don't know which to expect to react more quickly.
In my answer to your previous question, I said "Perhaps the affect would be stronger than if the same solenoid had a standard iron core." I now caution you not to expect that. I now think that 70% of the iron core's magnetic field might be the best you should expect from a core of 70% density magnetic powder.
I hope this helps,