Can aluminum alter the B field in a dielectric?

[cragar]

Why do materials like aluminum cut down the value of the B field?
Like suppose we had an infinite current carrying wire wrapped with a cylinder of aluminum.
To find the B field we would use amperes law and the magnetic susceptibility of aluminum.
For example, when a superconductor is exposed to a static B field it creates surface currents to counter the B field and to make sure B is zero on the inside. But in the case of aluminum I don't think there are surface currents. And according to the Lorentz force a charged particle has to be moving to be affected by a static B field. So really the only thing that could be altered are the electron orbits. So are the electron orbits being affected and this in turn reduces the B field inside the aluminum.

 

[netheril96]

...But in the case of aluminum I don't think there are surface currents...

You think wrong.

 

[cragar]

So there are surface currents, I thought it takes a changing B field to produce a current.
Faraday's law tells us we need a changing B field to induce a current.

 

[Born2bwire]

So there are surface currents, I thought it takes a changing B field to produce a current.
Faraday's law tells us we need a changing B field to induce a current.

Well, don't forget reciprocity here. For example, we can make a magnetostatic field using a DC current (Biot-Savart Law). So obviously we do not need a time-varying magnetic field to induce a current. A rough classical idea for why physical currents arise is as follows: The bulk of your material is going to have randomly oriented microscopic loop currents (in terms of the classical picture they can be thought of as the electron orbitals). Randomly oriented there is no net magnetic field produced by these loop currents. However, an applied magentic field will align some of these loop currents in accordance to the material's permeability. This is akin to how an applied electric field induces electric dipole moments, the magnetic field induces magnetic dipole moments.

So the alignment of the magnetic dipoles means that these loop currents will align and we can think of them as flowing in the same direction. So loop currents that are adjacent will have cancellation and we can sum up the adjacent loop currents into a larger single loop current.

For example, see the attached image in this post: https://www.physicsforums.com/showpost.php?p=2474064&postcount=69

The whole of it is that in the end, alignment of these microscopic currents creates a net macroscopic loop current that flows on the surface of the object.

As for what you stated in the OP, I guess the key point is that there is an assumption of existing currents in the material already.

 

[cragar]

thanks for your answer it cleared it up.