Re: Magnets attract some objects, but not others, Why?

Juan, 

Maybe the reason nobody has answered your question for a few weeks is 
that it's a hard one!  But let me try...

Have you ever heard of a ferromagnetic metal?  That is a type of metal 
that responds strongly to magnetic fields.  There are only a few of 
them - some are common metals, like iron, cobalt, and nickel.  Others
are very rare, and have funny names like gadolinium, Terbium, Dysprosium,
and Holmium.  Anyway, almost all magnets are made out of these metals
or mixes (alloys) of them.  By the way, a nickel (the coin) is 
made out of 3/4 copper and only 1/4 nickel - this mixture is not very
magnetic at all, but a pure piece of nickel would be. 

So, why are ferromagnetic metals attracted by magnetic fields?  Like
all other material things, these metals are made up of atoms.  But 
the atoms are very special, because they have what is called a magnetic
dipole.  Atoms have "positive" charges at the center, called protons. 
They also have negative charges whizzing around in circles around the
positive charges - these negative ones are called "electrons."  When 
electrons go in a circle (really really fast, by the way!) they make
a magnetic field.  The important thing to understand is this:  most 
kinds of atoms have a bunch of electrons going around in different ways
and they make a bunch of little magnetic fields that point in different
directions and cancel each other out.  Metals like iron are different - 
they have electrons arranged so that the magnetic fields do NOT cancel
out.  Instead, each little iron atom has a magnetic field. 

There is ANOTHER important thing about iron.  A piece of iron has 
billions of billions of atoms, so many you could never count them all, 
even if you were as old as the earth!  If all of these atoms had their
little magnetic field pointed in different directions, they still 
would not amount to anything.  But with a piece of iron, the atoms 
arrange themselves into little groups called "domains."  Each iron
atom in a domain has its magnetic dipole field aligned perfectly with 
all the other atoms in that domain.  They just form that way when the 
iron is made. 

Now, if you bring an ordinary piece of iron close to another magnet, 
the magnetic field will try to spin these "domains" around so they 
are aligned with it.  Imagine this: you and all your classmates are
outside at recess, all running around and facing different directions. 
Then the teacher yells at you and tells you to listen.  Everyone turns
around and faces the teacher.  That's sort of what these magnetic domains
do. 

Anyway, if some of the domains turn around, the iron becomes magnetic. 
The more they align, the more magnetic the iron gets.  If you get it
magnetic enough, it stays that way EVEN when you take it away from the
first magnet.  That explains why you can use one magnet to make another
one out of an ordinary piece of iron.  (You can't do this forever, though, 
cause the new one you make is a lot weaker than the first one.)

You might think it is strange that you can actually get these domains 
to move - but they really move!  People have done experiments where they
hook up a REALLLY sensitive microphone to the iron - and you can HEAR
the domains turning.  (Sounds like popcorn). 

I hope that answers your question.  Because this is a long answer, 
let me repeat the main points:

1.  Atoms have protons at the center, with electrons zooming around them 
real fast. 

2.  These electrons are what makes an atom magnetic.  How many electrons
there are, and how they are moving determines whether the magnetic 
field adds up to something big or not. 

3.  Iron atoms are special, because they are magnetic (have magnetic 
dipoles) and their magnetic fields (their dipoles) line up perfectly 
with all of their neighbors in the same domain

4.  When a piece of iron comes near a magnetic field (like another magnet)
, these domains actually turn around around to try to align with that
field.  This gives the piece of iron a big magnetic field, too, and then
the two magnets attract. 

5.  If enough domains turn around and align, the iron will still stay
a magnet after the first magnet goes away. 

So now you might wonder about all the rules for the electrons, and 
how you can explain how they move and such.  That is a complicated 
question that requires a lot of math to answer, but the answer is 
VERY INTERESTING!  So, learn lots of math, take calculus in high school, 
and then write to me again!

Best of luck, 
Doug Finkbeiner
dfink@astro.berkeley.edu