Why do pennies become magnetic after being on a magnet?

[pattiecake]

So a non-physicist posed a question to me the other day. He said that he heard something once that if two pennies are within close proximity of a magnet for at least a few hours, they gain some magnetic charge. When removed, they will slightly attract or repel one another.

So I tried it. I left a penny on a magnet for 48 hours. When I removed the penny, it was able to deflect a magnet when it was held near it.

Why does this happen?

 

[kuengb]

Hm, I never held a US penny in my hands and I don't know of what material it consists (well, to be honest I don't know this of our own coins). But I assume they are partly made of nickel which is a ferromagnetic metal. It can be thought of as many little "elementary magnets", but since they point at random directions, the whole penny isn't magnetic. Now, if there is an outer magnetic field, some of the elem. magnets are turned in direction of that field, and after it is gone, the penny has become a (weak) magnet himself. And: There is no such thing as magnetic charge.

That process goes faster if the penny is hit with a hammer or something (no idea how you would do that); when the elem. magnets are shaken around, they can turn more easily. But hey! be careful, it's money after all :)

 

[2Pac]

The atoms of the metal contain dipole attraction. when they are left near a magnet the dipoles arange into groups creating a magnetic field.