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About Magnetism
Magnetism is often taught in upper elementary science classes, but it is a
challenging concept to wrap your head around.
The NRC content standard for K-4 states simply “Magnets attract and repel each
other and certain kinds of other materials.” The NRC does not have a content standard
for magnetism for 5-8. For 9-12, the standard states, “Electricity and magnetism are two
aspects of a single electromagnetic force. Moving electric charges produce magnetic
forces and moving magnets produce electric forces. These effects help students to
understand electric motors and generators.”
So, the standards say that students should understand what magnets DO, but not
WHY. But curious minds will always wonder, why do magnets attract some materials
and not others? What causes magnetism? Can magnetism run out? For the most part,
the answers to these questions are abstract and difficult to grasp even for many adults, let
alone 5th graders! Below is my attempt to explain things.
Electricity and magnetism are intimately related like the heads and tails of the
same coin. You can’t get one without the other. They run perpendicular to each other
(90° angle). So what this means, is that if I can make one run in a circle, as with a coil of
wire, the other go through the middle of the circle, like the handle of a top.
But what is electricity? Electricity comes from moving electrons. In metal wires
the electrons jump from atom to atom along the wire. In lightening, the electrons hop
from water droplet to water droplet across the air, or to the ground. This is the origin of
the term “ground” in reference to electricity. The Earth is huge compared to everything
on it, so it can absorb a lot of electrons and let them spread out. Something that is
“grounded” is usually physically connected to the ground, or another “sink” for electrons.
But I digress.
Electrons, along with anything else, can move in a couple of ways. They can
translate (move from place to place) or they can rotate, or spin. Either movement causes
electricity, and therefore magnetism. In many (though not all) metals, the electrons can
move from atom to atom fairly readily. But in most other materials (insulators) the
electrons are stuck on one atom (localized). Even localized electrons spin. In general we
say they can spin “up” or “down” (or you can think of this as clock-wise and counter
clock-wise). The two types of spins create magnetic fields in opposite directions. If
there are as many up electrons as down electrons, they cancel each other out. These
materials are “diamagnetic”, or for our purposes, non-magnetic. (This is slightly untrue,
but that’s a lesson for another day). When there are more of one kind(up) than the
other(down), the material is “paramagnetic”. This doesn’t mean that it is a magnet, only
that it will be attracted to a magnet. Most metals fit in this category.
So what causes electrons to exactly cancel or to have leftovers of one type of
spin?
For that, we will use an analogy. Think of an atom as an apartment. All of the
electrons are the tenants. This apartment building has different floors and hallways.
Each apartment can hold only two electrons (one spin up and one spin down). There are
rules for living in the apartment. First, the electrons have to fill the bottom floors first.
This causes a lot of pairing of the electrons as the floors fill up. Remember, paired
electrons cancel each other out.
Now let’s consider the top floor with electrons (the valence floor). The next rule
says that in each hallway the electrons should have their own apartment if possible. For
the atoms, “hallways” (sub-shells for the chemistry minded) can have 1, 3, or 5
apartments. (Transition metals, in the middle of the periodic table have 5 d orbitals or
“apartments”, while most non-metals, on the right side of the periodic table, have 3 p
orbitals or “apartments.”) That means, depending on the number of electrons, we could
have a hallway with 5 electrons in their own apartments without the spins cancelled. This
is how many metals are. Manganese and chromium have 5 unpaired electrons, iron 4.
Most of the time the spins of these electrons are completely random, some up and some
down, so the metals are not magnetic. But if I put them in a magnetic field, it’s as if a
drill sergeant came by and all the electrons stand at attention, spinning the same way. If
they are all spinning the same way, their magnetic fields re-enforce each other, and the
material is attracted to the magnet. Now if there are one or two unpaired electrons they
relax pretty quickly once the drill sergeant (magnetic field) goes away. But for some
materials, like iron, the electrons can get stuck. This is because all those unpaired
electrons are creating their own magnetic field that reinforces itself. These become
“permanent magnets”. [We fudge a bit on permanent too, because over time, most
permanent magnets can lose their magnetic fields, which is why refrigerator magnets stop
working after a few years.] In honor of iron, these materials are called “ferromagnetic”
(fer is Latin for iron).
Most materials are not pure substances, but rather compounds made of more than
one element. While metals can handle being fairly pure, most non-metals prefer to bond
to other atoms. Bonds are made of pairs of electrons, which necessarily have one
electron spin up, the other down. Because of this, almost all non-metals are diamagnetic.
However, there are a few, because of unusual bonding circumstances, that are
paramagnetic, oxygen for example. You can find video online of liquid oxygen being
poured between the poles of a magnet, and it is in fact attracted to the magnet! There are
also many salts that are paramagnetic. A salt is a compound of a metal and a non-metal.
Some of the electrons get transferred from the metal to the non-metal, so not all salts are
magnetic. However, manganese, iron and cobalt have so many unpaired electrons that
there are enough left over to maintain paramagnetism!
So why are materials magnetic? They have unpaired electrons. These electrons
are attracted to a magnetic field AND create their own magnetic field. The more
unpaired electrons, the stronger the magnet. When things become “demagnetized” it
usually because the electrons start spinning in random directions rather than aligning.