Eli5: How do compasses work and what are magnetic fields?



Eli5: How do compasses work and what are magnetic fields?

In: Physics

Magnetic fields are best understood in a mathematical way, so please bear with some math. I’m going to try to keep this as ELI-mathematically-disinclined as possible, but there’s only so much I can do.

At every point in space there is a local magnetic field. That is to say that magnetic fields are everywhere; they can get stronger or weaker or change direction but they are a fundamental property of space; you can’t pick one up and move it. It’s like how every pixel in a photograph has some RGB value even if it’s zero.

Magnetic fields only interact with magnets and moving electric charges (like flowing electrons). In fact, these two things are somewhat the same.

So, in the area around a bar magnet, the magnetic field at any point will be directed along a curve. [Visual aid.](https://images.app.goo.gl/jubYapbvQhJQkV1y5) The direction of the local magnetic field is the direction that the north pole of a magnet is pulled in, and opposite to the direction that the south pole is pulled in. As such, a bar magnet will align itself with the local field. [Visual aid.](https://img.brainkart.com/extra3/dd1qDHz.jpg)

Compasses are tiny bar magnets, held up by water, thus allowing them to rotate with almost zero friction. They will align themselves with the local direction of the magnetic field.

If you’re away from magnetic things, then the local magnetic field is almost exclusively generated by the Earth. The Earth’s magnetic field runs from the south pole to the north pole (for geological reasons that I don’t understand), and so the needle’s north end will point towards the north pole.

If you know what magnetism is skip the first part.

First the basic of all this is magnetism. Magnetism itself is a quantum-mechanical effect and very complicated. It belongs to one of the “four fundamental interactions” of physics.
It basically constitutes of negatively charged parts and positively charged parts, which attract each other. This effect can be realized through electricity, where electrons charge something negatively and the “lack” of electrons charges positive. That’s called electromagnetism.

Every magnet projects his attractionits out of its own body to it’s surroundings, like gravity. The further away you are from the body the weaker the attraction gets though. That is the magnetic field. You can’t see magnetic force, but when we lay down metal shavings they align themselves along the magetic field making it visible.

The earth itself has an iron core that acts like a gigantic magnet, projecting it’s magnetic force as far out as space. So the whole earth is covered in a weak, but constant magnetic field. Like any magnet earth has a positive side and a negative side, attracting the opposite. A compass uses that by charging on side positive, which then points towards the negative side of earthand the other side negative, which then points to the positive side of earth.