Attraction between a wall and a magnet

[sfink]

Hi All,

I was wondering: for a given size and strength magnet sticking to a wall of a given material and surface area, does the thickness of the wall affect the strength of the attraction? What are the governing equations at play?

I am trying to gain insight as to why or why not the thickness of the wall would matter. Any light shed on the situation would be appreciated.

 

[mfb]

Assuming that the wall is ferromagnetic: The field lines from the magnet will "try" to follow the wall outwards. However, the field strength in the wall is limited - if it is too thin, only parts of the field can follow the wall. This should reduce the attraction for thin walls. For thick walls, I think the attraction is nearly independent of the thickness.

 

[Meir Achuz]

For a bar magnet and an iron wall of finite thickness, the front wall attracts and the back wall repels. The force is zero for zero thickness, and gets larger as the thickness increases. Eventually, it will no longer depend on the thickness. This will start to happen when the thickness is larger than the diameter of the magnets face.
For a 'refrigerator magnet', there is very little dependence on the thickness of the wall.