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First, thanks to you both for taking time to reply. I really appreciate it.
gabbagabbahey I’m going to need to study your answer more to understand it. This is a magnetostatics situation. Actually the faces of the slab could be faces of a permanent magnet. If there is...
http://en.wikipedia.org/wiki/File:Magnetischer_Kreis.svg
The attached image shows a magnetic circuit with a small air gap. The magnetic material on each side of the gap is attracted to the other side. I've seen the magnitude of the force calculated based on principle of virtual work...