Physics/Permanent magnet near magnetic shield
I am interested in magnetic shielding and I was wondering if you could tell me what would happen in the following scenario. A permanent magnet is placed under/inside a magnetic shield which absorbs/redirects part of the magnetic field and allows part of the field to leak through. (The permanent magnet attracts the magnetic shield, but the objects are fixed in place by a plastic housing). Above the magnetic shield is an electromagnet. What would happen when the electromagnet is turned on? (generating a magnetic field flowing in the same direction as the field of the permanent magnet) How might this change the magnetic field of the permanent magnet? Is it possible that the electromagnet's field could reduce the magnetic shielding and cause more of the permanent magnet's field to leak out? Could the electromagnet redirect the field of the (semi) shielded permanent magnet?
Thanks and best regards,
ANSWER: The following is only a reasonable guess. I've thought about my answer for several days, and fully admit I'm not 100% certain I'm correct. If I'm wrong, sorry.
ANYWAY, first thing to be aware is that a magnetic shield doesn't absorb or weaken the B-field coming from the outside or the inside. Quite the opposite -- a proper shield is a magnetically "soft" (the technical name is "permeable"), ferromagnetic substance that bends the B-field lines so much that the B-field doesn't go past the shield.
The B-field actually is STRONGER within the shield, but it's weaker than it would be beyond the shield.
If the shield lacks enough permeability to re-direct ALL of the B-field from the permanent magnet, then some of the B-field lines from the permanent magnet will go outside the shield. They will be weaker than they would be without the shield, but they'll still be there.
The B-field that will result when the electromagnet is turned on will the sum of the two B-fields: the B-field that escapes from the permanent magnet PLUS the B-field from the electromagnet. Depending on the polarity of the electromagnet's B-field, the resultant field could be stronger or weaker than what would be without that former B-field. But no more of the B-field from the permanent magnet would leak out; that's determined simply by the permeability of the shield material. The field from inside the shield wouldn't be "re-directed," it would simply be added to the same B-field coming out.
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QUESTION: Thanks very much for your answer.
I have been thinking about what you said about the magnetic shield permeability determining how much of the permanent magnet's B-field would leak out of the magnetic shield.
I have been doing some research into what changes the permeability of the shield material and I found this website
which contained the graph I have attached, which shows the permeability of a shield material decreasing as the frequency increases. Can you help me understand this? Does the frequency refer to an electromagnet, rather than a permanent magnet?
What would happen if an electric current was run through the magnetic shield of a permanent magnet? If it was an Alternating current, would an increase in frequency cause the shield to lose permeability and allow more of the permanent magnet's B-field to leak out of the shield?
Thanks again for your answers.
ANSWER: > permeability of a shield material decreasing as the frequency increases.
> Does the frequency refer to an electromagnet, rather than a permanent magnet?
"Frequency" of the magnetic field (aka "B-field") refers to how rapidly that B-field is changing. Think of the "frequency" of a radio signal -- a 100 kHz signal means that every second there are 100,000 complete cyclings in the direction of the E-field (and the B-field)
A 200 kHz signal means that every second there are 200,000 such cyclings -- twice the frequency, twice how rapidly the signal changes.
Thus, if the field is not changing at all, the frequency is zero. A permanent magnet, for which there is NO signal change, the frequency is zero.
> Can you help me understand this?
In a ferromagnetic substance (which, from here on, I'll simply call "iron" in order to save my fingers some keystrokes), the atoms are like soldiers, waiting for an order to all point in the same direction. The internal B-fields of each of these atoms points in random directions, like this
The external B-field is like a commander giving orders for the atoms to all point in the same directions, like this
A magnetic shield works because the iron atoms react to the external B-field by lining up such that the total B-field is re-directed such that it exists only within the iron. The field is completely consumed with lining up the iron atoms, and none escapes beyond the iron.
A change in the direction of the external B-field is like a commander giving orders like this: "All of you, face left. All of you, face right. All of you, face left. ..." A commander giving orders TOO rapidly will result in some of the soldiers facing one way, and some of them the other way -- eventually the "direction" they are facing is completely random. Similarly, a B-field changing too rapidly for the iron atoms to change their direction will result in the same thing -- they no longer line up well enough to re-direct the external B-field.
> What would happen if an electric current was run through the magnetic shield of a permanent magnet?
> would an increase in frequency cause the shield to lose permeability?
No, because a current is basically an E-field. This is like a commander that has no authority to tell the iron atoms which way to face. Thus, they "ignore" any command* from this E-field. However, a strong enough current, even a DC one, might cause enough heating to disrupt the ferro-magnetism of the iron in the shield. Do a search for
Also, not that a strong B-field from the electromagnet, if IT is changing rapidly and continuously, might get the effect you are looking for. I cannot answer the question of how strong and how high a frequency. If the field from the electromagnet disrupts the ferromagnetic properties of the iron, then the shielding ability goes down as well.
* There is SOME change in their direction from this external E-field, but very little compared to the "line-up" command from the permanent magnet.
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QUESTION: Thanks for your answer again. I think understand more now, but I also have some questions about what you told me.
I was particularly interested to read the statement near the end of your answer - "not that a strong B-field from the electromagnet, if IT is changing rapidly and continuously, might get the effect you are looking for." and I would like to ask you more about this. Am I understanding correctly to think that a weak B-field may achieve a change (decrease) in the magnetic shield's permeability that a stronger field is unable/less able to do? How would that work? Does the rapidly changing field need to be weak in order to effect a change in the shield's permeability? I am not sure if I have understood this correctly, so my grasp on the issue is tenuous...
I am also wondering if I could confirm where the electromagnet would be positioned in relation to the magnetic shield and permanent magnet in order to achieve this effect. I have included an image with three possible configurations of permanent magnet, magnetic shield and electromagnet. (Type C is a permanent magnet inside a magnetic shield. The magnetic shield is also an electromagnet of sorts, with copper coils around the magnetic shield, which acts as a metal core for the solenoid - I think!) Are any of these configurations going to work where the effect of the "field from the electromagnet disrupts the ferromagnetic properties of the iron, then the shielding ability goes down as well."?
Thanks again for all your answers; they were in depth, clear and easy to understand, and really helped further my understanding.
> Does the rapidly changing field need to be weak in order to effect a change in the shield's permeability?
> Am I understanding correctly to think that a weak B-field may achieve a change (decrease)
> in the magnetic shield's permeability that a stronger field is unable/less able to do?
You seem to be focusing on the word "weak" instead of the important words: "rapidly and continuously," as well as my word "might." You seem to understand that a rapidly changing external B-field gets through certain types of iron, when a static B-field (ie, a permanent magnet) does not. It is POSSIBLE (please do not overlook that word) that a rapidly changing B-field might change the permeability of the iron, even for the B-field of a permanent magnet. If this is, indeed, the case, then a STRONGER field, as well as one with higher frequency, would do a better job of changing the iron's permeability. Once again, using the commander analogy, a strong field is like a commander with a louder voice -- the louder the voice, the more soldiers will respond to the command.
I emphasize, like I did the last time, that I do not know if this would even work; and, even if it did, I can not calculate HOW strong or HOW rapid a change or WHICH configuration would accomplish what you are hoping for.