# Effect of Iron on Magnet

Q:
Hi, If a magnet is "partially" attracted to a piece of iron, lets say a square magnet with a diameter of 1" has 10% of it's surface attracted to the iron piece. The maximum attraction force this magnet can generated is about 4lb, will the rest of the "un-attracted" surface of the magnet still be able to attract other objects? Possibly with the same force? Or less by a bit like 3.6lb? Is 10% of the magnetic field attracted to that piece of iron? Generally, what happens when a magnet is on the edge of a ferromagnetic object, and most of the magnet's surface is out of the edge, while 10% of it's surface is still attracted to it.
- Anonymous
A:

The detailed effect depends on the detailed geometry. Let's say it's the typical field arrangement, with the field coming out of the magnet face. You can picture the iron as pulling field lines into itself. It's true that leaves the field somewhat weaker on the other parts of the magnet face, although in this case it wouldn't be precisely 10% weaker. (I can't do the exact calculation.) It also leaves the field just past the iron, away from the magnet, stronger than it would have been. That's why you can pick up a chain of paper clips with a magnet- each guides the field lines on to the next clip. So although magnets are sometimes given ratings in "lbs", that's not really a very good specification except for some particular geometry, such as for a piece of iron covering the whole face. It doesn't quite work to think of a certain number of lbs of strength, distributed among various pieces.

Mike W.

(published on 09/16/2013)

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