BrightKidz Library
Subjects
The same bar of iron before and after a magnet is brought up to it Two bars of iron drawn one above the other, the same length, and they are the same bar. In the top bar twelve tiny arrows point in all directions, so their pulls cancel out and the bar behaves as though it were not magnetic at all. In the bottom bar a horseshoe magnet has been brought up to the left-hand end, and now all twelve arrows point the same way, so their pulls add up and the whole bar has become a magnet. No arrow was added or taken away, only turned.

Why a Magnet Picks Up Iron but Not Copper

About 12 minutes

When you sorted the tray, iron and steel stuck to the magnet and aluminum and copper did not. All four are metal. All four are hard, cold and shiny. Nothing you can see about a copper coin says it should behave differently from an iron nail.

There is a second thing in that experiment that should bother you, and it is stranger. You know that two north poles shove each other apart. Yet you picked up paper clips with either end of the magnet, and not once did a paper clip get pushed away. A magnet is fussy about which way round another magnet is, and completely relaxed about paper clips.

One idea answers both.

The iron was magnetic before you arrived

Inside a bar of iron, the atoms are not acting alone. They gather into patches, and every patch is already a magnet, with its own north end and its own south end. A patch like that is called a domain.

Domains are far too small to see. A single one holds billions of atoms, all agreeing with each other about which way to point.

In a fresh iron nail those domains point every which way. One patch pulls north-ish, its neighbour pulls sideways, the next pulls the other way entirely. Every pull is real. Added up, they come to nothing.

So an iron nail is not a thing with no magnetism in it. It is a thing with magnetism pointing in all directions at once, which is the same as none at all from outside.

What a magnet actually does

Bring a magnet close, and the domains turn.

Nothing new is made. Nothing flows in from the magnet. The patches were always there and were always magnetic; they simply stop disagreeing. As they swing into line their pulls stop cancelling and start adding, and the nail becomes a magnet while the real magnet is nearby.

Domains inside a bar of iron turning to line up with a magnet A magnet on the left with the end facing the iron marked N. Beside it a bar of iron holding twelve small arrows, each one a region called a domain. With the magnet far away the arrows point every which way and cancel out. As the magnet comes closer the arrows swing round until they all point the same way, and two labels appear on the iron: an S at the end nearest the magnet and an N at the far end. The end nearest the magnet always becomes the opposite pole, which is why iron is pulled in and never pushed away. S N magnet a bar of iron S N Each arrow is a domain: a patch already magnetic. None is created. They only stop disagreeing.

Slide the magnet closer and watch the arrows inside the iron.

Count the arrows at the start and count them at the end. There are the same twelve either way, and not one of them was created by the magnet. The only thing that changed is which way they face. That is the whole of what magnetising something means, and it is why a nail can be un-magnetised again — the domains just wander back out of line.

Why it is always a pull and never a push

Now the second puzzle, and the answer is hiding in the word turn.

The domains do not line up in a random direction. They swing round until the end of the nail nearest the magnet has become the opposite pole to the magnet's face. That is the position they are pulled into, so that is where they settle.

Opposite poles attract. So the nail is pulled in.

Turn the magnet round and present the other pole, and the domains simply turn round too, and the near end becomes the opposite of that instead. It is opposite again. A magnet cannot push a plain paper clip away, because the paper clip rearranges itself into whatever answer means "attract".

This is also why your paper clip chain worked. The first clip hangs from the magnet, and while it hangs there it is a magnet — so it turns the domains in the second clip, which turns the domains in the third. Let go of the magnet at the top and the whole chain falls apart, because the domains at the top have nothing left to keep them in line.

Copper has nothing to turn

Copper's atoms do not gather into domains. Neither do aluminum's, or gold's, or those in a plastic brick. There are no small magnets inside them lying about in a muddle, so there is nothing for an approaching magnet to tidy up, and nothing happens.

At ordinary room temperature only three plain metals do the domain trick: iron, nickel and cobalt. Steel is mostly iron, which is why a steel paper clip behaves like an iron one.

That is the real answer to "which metals are magnetic". It was never about being a metal. It was about whether there is anything inside to line up.

You cannot cut the north pole off

Try to imagine getting a north pole on its own. Take a bar magnet and cut it in half, and keep the north end.

You get two magnets. The piece you kept has grown a new south end.

Once you know about domains this stops being surprising. The magnetism is the domains, and every piece you cut still contains millions of them, all still lined up. Any lined-up piece of iron has an end where the arrows come out and an end where they go in. Cut again and again, down to a single domain, and it is still a small magnet with two ends. Nobody has ever found a north pole by itself.

A magnet picks up a steel paper clip with its north end. Turned round, it picks up the same clip with its south end. Why is it never pushed away?

Go back to your tray and pick up the copper coin and the iron nail, one in each hand. They feel like the same kind of thing. The difference is not on the outside at all: one of them is full of tiny magnets waiting to be told which way to face, and the other has nothing in it to tell.