Which material will stop magnetic attraction?

[songoku]

Homework Statement

Please see below

Relevant Equations

None

The answer key is (a) but I don't understand why. Option (a), (b), (c) are all non - magnetic material and (d) is magnetic material. Why (a) can stop the attraction while others can't?

Thanks

 

[berkeman]

Can you post a link to the online question? Obviously any magnetic shielding effect would require a ferrous material.

 

[songoku]

Can you post a link to the online question? Obviously any magnetic shielding effect would require a ferrous material.

Sorry I got the question in form of printed paper so I don't have the link. I think the answer is (d) but I am not sure how the distribution (shape) of magnetic field inside the steel sheet would be. Is there any possibility some part of the magnetic field is redistributed by the steel sheet towards the steel ball so it would still be attracted?

Thanks

 

[kuruman]

I will rephrase the question.

Which of the four materials exhibits stronger diamagnetism? A superconducting sheet would work even better.

 

[Steve4Physics]

The answer key is (a) but I don't understand why. Option (a), (b), (c) are all non - magnetic material and (d) is magnetic material. Why (a) can stop the attraction while others can't?

IMO the answer-key is wrong and the correct answer is d - the steel sheet.

Steel* has a high magnetic permeability. A large steel sheet will concentrate and redirect the field lines like this:

It’s worth noting that this wouldn’t apply with a narrow piece of steel. In that case, the local affect on the field would be more like this:

The steel would locally propagate the field in its original direction.

Diagrams from http://www.excelatphysics.com/magnetic-shielding.html

*Assuming this is not a non-magnetic variety of stainless steel!

 

[Gavran]

The answer key is (a) but I don't understand why. Option (a), (b), (c) are all non - magnetic material and (d) is magnetic material. Why (a) can stop the attraction while others can't?

Thanks

I think you are right, and the correct answer is d.
A copper shield can be better than a steel shield against a high-frequency magnetic field, but based on the picture from the original post, there is only a static magnetic field.

 

[kuruman]

I think that having (a) as the answer is based on the idea that the atomic orbitals in diamagnetic copper orient themselves to exclude (to some extent) the magnetic field lines. This is the physical mechanism behind diamagnetic levitation. Of course, the correct answer can be independently verified by reproducing Monica's experiment. Alas, I no longer have access to the necessary equipment.

Additional Comments
Magnetic shielding works best by using the shielding material to separate the two regions into an "outside" where the magnetic field is and an "inside" where the item to be shielded is placed. The boundary conditions at the air-metal external interface and then at the metal-air internal interface force the field lines to stay within the metal and go around the the "inside" region.

I am also bothered by the excelatphyscs.com drawing, specifically the "S" label in the middle of the plate. If it's meant to represent a south magnetic pole, then (more likely) it was arbitrarily pinned there as a magnetic monopole to prevent the field line along the axis of symmetry from coming out the other side of the plate. The reasoning is that if one line is allowed to emerge, there should be other, paraxial, lines emerging.

 

[Steve4Physics]

...the correct answer can be independently verified by reproducing Monica's experiment. Alas, I no longer have access to the necessary equipment.

YouTube is your friend (with music)! CLick here.

I am also bothered by the excelatphyscs.com drawing, specifically the "S" label in the middle of the plate. If it's meant to represent a south magnetic pole, then (more likely) it was arbitrarily pinned there as a magnetic monopole to prevent the field line along the axis of symmetry from coming out the other side of the plate. The reasoning is that if one line is allowed to emerge, there should be other, paraxial, lines emerging.

Interesting point. I think the diagram captures the main effect – the re-routing of most field lines so that they don’t enter the shielded region.

But shielding won't be 100%. The diagram could be improved by having:
- a downwards axial field line entering the magnet's S pole;
- a downwards axial field line leaving the magnet's N pole and passing, undeflected, through the steel sheet;
- omitting the (unhelpful) ‘N S N’ on the steel sheet.