# The earths magnetic field.

97 views

If you heat up a magnet it stops being a magnet. So why is the molten metal in the earths core a magnet if it’s so hot?Also if you had a pure iron asteroid that was going to miss the earth but go really close nearby would the magnetic field have any pull on it not caused by gravity?

In: Planetary Science

Magnetic fields come from moving electric fields. There are a couple ways to create moving electric fields.

Permanent magnets get their field from the electrons bound to the atoms in the metal. Electrons have a property called *spin*, which is *kind of* like the electron is spinning like a top, but also entirely unlike that. Quantum mechanics is weird. Point is, electrons have angular momentum and, since they have an intrinsic electric charge, each electron has its own magnetic field.

For more quantum mechanics reasons, electrons tend to pair up around atoms, with one electron as spin *up* and the other as spin *down*. When paired this way, their magnetic fields flow in opposite directions and cancel each other out so that the atom has no net magnetic field. In certain metals, the electron is unpaired, so each atom has its own tiny magnetic field. However, the atoms themselves may not be aligned well in the metal crystal, so their magnetic fields cancel. If they do line up, you get magnetic *regions* within the metal, and those regions *also* need to be aligned. If all of these pieces are aligned well enough, each tiny magnetic field builds up to a field that creates a permanent magnet.

Heat is really the random movement of particles within the substance. Even if the temperature isn’t high enough for the metal to melt, it can still allow atoms to move around enough that they come out of alignment, so the small magnetic fields no longer add up and the permanent magnetism breaks.

The other way to make a magnet is an *electromagnet*, which works by moving electrons around through a substance (typically a metal wire). All electricity produces some degree of magnetism, as the electrons are pushed through the wires. A strong magnet can be made by coiling the wires so that many electrons move around next to each other and the magnetic fields add up, just like a permanent magnet. It doesn’t matter how how the substance is, because the electrons are still moving in the same direction. Plasma is a state of matter where there is so much energy that electrons *can’t* bind to atoms and you get a mixture of atomic nuclei and electrons flowing around each other. With all those charged particles flowing, plasma can create very strong magnetic fields.

The Earth is full of a *lot* of metal that is molten due to the intense heat of the core. That molten metal flows in huge [convection currents](https://azgs.arizona.edu/sites/default/files/MagmaZones.jpg), along with the spinning of the iron core that drags the molten metal around it. Due to the Coriolis effect, the convection currents tend to be very regular and predictable, all moving in about the same way. That creates a *huge* flow of metal, swirling around in the outer core. It’s the flow of that metal that creates the *dynamo* that creates the Earth’s magnetic field. The metal is all flowing in what are essentially giant “coils”, which makes the flow align just like electrons flowing through a metal wire. A little bit of magnetic field helps force the metal to follow the magnetic lines, which makes them line up even more, which makes the magnetic field stronger, which forces the metal to line up even *more*…and so on, so that the magnetic field created by each moving metal atom adds up with all the other atoms to create a very large magnetic field.

Only permanent magnets (the ones that get their magnetic field from the alignment of their molecules/atoms) lose their magnetism when heated. IIRC the Earth’s core mostly gets the magnetism from being a giant rotating mass of conductive material. You can have a fully liquid electromagnet if you pour red hot iron into a high melting temperature coil form and stick electrodes into the ends.

> pure iron asteroid that was going to miss the earth but go really close nearby would the magnetic field have any pull on it not caused by gravity?

Technically yes, practically you’d get a more significant reaction trying to push it around by hand. The earth’s magnetic field is honestly piss weak – hence why iron objects don’t randomly fly about the room and you need to suspend magnets on a string/pin to allow them to turn to north/south.

A solid permanent magnet basically comes from the iron (or other magnetic material) basically being lined up in a particular direction. Since when it melts, this is no longer the case, it isn’t a magnet anymore. In the Earth, the core is partly molten, and is spinning. The spinning iron causes a dynamo effect, which is what causes the magnetic field.

As for the asteroid, yes it could be affected, but the effect would be extremely tiny. The magnetic field is not particularly strong, and the strength drops off quickly the farther away you are. Something like a compass needle is affected only when there’s very little friction and if it has very little inertia (aka mass). Your heavy asteroid won’t be affected much at all

Earth’s magnetic field comes from the flow of electricity. It is not a permanent magnet, like you’re used to, but an electromagnet.

This magnetic field theoretically reaches off forever into space, but being a dipole, its strength drops off with the cube of distance, so very fast. At relatively short distances (on an interplanetary scale) the field drops to nearly zero. Even on Earth’s surface its magnetic field is not strong enough to noticeably increase the weight of an iron bar, so its influence on space projectiles is essentially zero.