All right, let’s look at that nail that you made into a magnet. Before you sent an electric
current through the wire wrapped around the nail, the nail didn’t act like a magnet. After
you connected the wire to the battery, though, the nail could pick up a paper clip or two.
To understand this, remember that we are assuming that a circular electric current is a
magnet. Therefore, the coil of wire wrapped around the nail creates a magnetic field that
looks like Figure 4.13.
Figure 4.13: Magnetic field lines from a coil carrying an electrical current
When you removed the coil of wire from the nail, the nail was still slightly magnetic.
That’s because once lined up, the magnetic moments of the atoms in the nail don’t head
back to a random orientation as easily as do the magnetic moments of the atoms in a
paper clip. The magnetic moments in the nail are still somewhat lined up, and still make
the nail act like a mag- net.
The magnetic moments of the atoms in the nail see this magnetic field from the coil of
wire and line up with it. By lining up with the magnetic field of the wire, these magnetic
moments make for a stronger magnetic field (Figure 4.14). Thus, a coil of currentcarrying wire with a nail inside is a stronger magnet than just the coil of wire alone.
Figure 4.14 Magnetic field lines from a nail wrapped in a coil carrying an electrical
current
How to get those magnetic moments randomly oriented again? Simple. Bang the heck
out of the nail! That shakes things up inside the nail and gets those magnetic moments
into a more random orientation. Of course, that method for demagnetizing something
works not just for nails, but also for “permanent” magnets. Whack a magnet around
enough and it won’t be a magnet for long. You can also ruin a magnet by heating it, as
high temperatures make atoms move around more, messing up the orientation of their
magnetic moments. Of course, it is possible to realign the magnetic moments in a magnet
by placing it in a strong magnetic field. So, all is not lost if you have bar magnets that
have been banged around and messed up. You can buy a device that will “recharge” bar
magnets and get them back to their original condition.
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If each individual atom is itself a magnet, then no matter how small a piece you get from
a magnet, that tiny piece is still a collection of tinier magnets (the atoms them- selves). In
fact, since each atom is a magnet, you would have to begin cutting atoms apart in your
search for a magnetic monopole. Physicists never have been able to isolate a magnetic
monopole, even by breaking atoms apart, so if you do manage to find one, call your local
physicist—your Nobel Prize is waiting.
A recap is in order. Electric currents exert forces on other electric currents. Electric
currents that move in a circle act like permanent magnets. We use those facts to create a
model of magnetism that includes individual atoms acting like they are circular electric
currents, which we call magnetic moments. The orientation or lack of orientation of the
magnetic moments of atoms explains why permanent mag- nets do what they do, why
magnets attract some materials and not others, and why you can ruin magnets by hitting
them or heating them.