A magnetic shield material that blocks magnetic fields from the outside but allows fields from the inside to pass. I was curious whether such a material existed. See below example.
i don't know,but it would be a wierd one :) maybe something like that is possible in casimir cavity.
Hello broli
Maybe an aluminum sheet attached to a vibrating piezoelectric crystal (aluminum will get warm though from eddy currents). I don't know of any room temperature magnetic shielding materials.
:)
Hello, Broli,
I will try to answer your question. For ordinary materials at room temperature, the answer is simple: NO!
Magnetic shields normally work by having such a high permiability that magnetic field lines are trapped in the material. At high flux, the material will saturate and lose its ability to shield. Mu metal is such a material.
If you have a box made of mu metal, say, the material will take up field lines equally well from inside or outside.
There is an interesting counter-example for electric charges. If you surround a charge by a conducting shell, and provided that the shell is ungrounded with no net charge on it, then the electric field passes through the shell, but with some distortion. If the charge is put at the center of an ungrounded conducting sphere, then the field outside looks just as if the conductor weren't there. But, if you ground the conductor, then the field is stopped at the conductor by the induced charge.
On the other hand, if the charge is outside of the conducting shell, then no field is allowed to enter whether the shell is grounded or not.
Now, let's talk about a really bizarre situation. Suppose you have a "hard" superconductor, but it is above the transition temperature. Suppose you put a shell of that around a magnet. The magnetic field will pass through the shell. Now, lower the temperature so that the material goes to the superconducting state. Will the magnetic field change as the material goes to superconducting?-- I don't think so, since energy would be required and I don't see a source for that. (I could be wrong on this point--somebody should check.) However, once the material is superconducting, it will exclude magnetic fields, or at least it will prevent any change in its internal magnetic field, since CURL (E) = - dB/dt, and no electric field can exist inside a superconductor.
So, you have created a condition in which the field from the internal magnet does pass through the superconductor shell, but no external magnet can put a field through the shell because it is brought near after the shell was made superconducting.
Ernie Rogers
I don't exactly understand how the magnetic field passes through the super conductor. That the field is not allowed in it I get, this is also called the Meissner effect.
I will tell why I'm exactly asking for this after some reply.
Well, Broli,
I'll start off by saying I was wrong when I said--
QuoteWill the magnetic field change as the material goes to superconducting?-- I don't think so, since energy would be required and I don't see a source for that. (I could be wrong on this point--somebody should check.)
Yep, I was wrong. Checking on the Meissner effect, the magnetic field inside the material is "expelled" as it transitions to superconducting. With this correction, it appears that I am incapable of thinking of a situation where a magnetic shield would pass a magnetic field originating inside the shell but not one from outside.
Wait a minute, maybe I was wrong again! Let's look at that situation again--
Consider the magnet inside the shell above the transition, and the field passes through the shell. Now, cool down and transition to the superconducting state. The Meissner effect as stated only requires that the field be exluded from inside the material. The material generates surface currents that cancel out the field within. IF that's all it does, then the field outside didn't collapse, but is being maintained by the superconductor.
Okay, we know I am wrong at least once here.
Ernie Rogers