I’ve never fully understood how compasses work, and how the magnetic North Pole can affect them. Like how aren’t other things affected by it? And apparently the magnetic North Pole also moves? Please help me make it make sense!
In: Physics
Do you understand magnetic poles in general just not the earths one? The earth has a molten core of iron that moves, so it can be at any spot, because its not related to the ground we stand on.
And everything magnetic or metallic is affected by this field its just not strong, sp you need a free mobing needle thats light and turns with no friction to measure it.
Have you noticed that the closer to a magnet you get something the stronger that magnet works on it? And that for a bigger magnet, it works from further away but it still needs to be close in order to be really strong?
Inside the earth is basically a big magnet. This magnet is HUGE and strong, which means that the effects of it can travel *very* far (from where it is in the earth to most of the way past the earth’s atmosphere). But because it is still a magnet, the strength of its effects falls off exponentially. This means that every single thing on the earth is feeling this magnet, but because we are most of the earth’s radius away from it we don’t feel it very strongly (and in fact have grown used to it enough to just not notice it most of the time).
So because everything on the earth’s surface is being affected by the magnet, if we have tools sensitive enough to them we can measure the magnet. And one of the important parts about a compass is that they are *very* easy to spin, so that even the small amount of magnet that they can feel will still push the needle around. And what that needle does is have one side that is attracted to the ‘north’ pole of the earth.
As for the poles moving? Much of our understanding of the magnet’s details comes from paying attention to that. We now know that this magnet is about at the center of the earth, we know that it’s spinning, and we know that it’s moving. We know that its ‘north’ pole pointing ‘actual north’ is a happy coincidence, and that throughout geological history this hasn’t always been true (though for the last thousands of years of civilization it’s always been at least northish). It’s the explanation that makes the most sense for how we’ve seen the poles move, and it is also what has allowed us to predict where the poles will move in the future (and have done that accurately so far).
Within the Earth there is a large amount of iron which slowly moves, that movement creates a magnetic field around the Earth, that field bulges around the area of the equator and curves down towards the pole, this means that a free turning magnet will align itself with this field and point towards the north until you get close to the north pole. Occasionally this movement can change direction meaning that the North pole eventually becomes the South pole. https://youtu.be/QGTPr3CG6GA
Magnetism is a weird, twisty effect.
The general rule is that if you have something that has a magnetic field around it (like the needle in a compass), and you put it inside another magnetic field (from some other object, like the Earth), the thing will twist or shift so that its magnetic field lines up as much as possible with the other one.
You might have seen [diagrams like these](https://en.wikipedia.org/wiki/File:VFPt_cylindrical_tightly-wound_coil-and-bar-magnet-comparison_stacked.svg) for showing what magnetic fields look like; magnetic field lines go in closed loops around magnetic things (the lines continue through the magnet).
So you have some of these lines around your magnetic needle, and some around the Earth, and so the needle will shift to line up with the Earth’s field (as it is much easier for the needle to shift than for the Earth do to so!).
Now it is important to note that the needle *doesn’t point towards the magnetic North pole*! It lines up with the local magnetic field. The Earth’s magnetic field [is a giant complicated mess](https://en.wikipedia.org/wiki/File:Geodynamo_Between_Reversals.gif) and doesn’t flow smoothly. For example, if you tried to use a magnetic compass in Moscow, it would point about 15 degrees off true North. In Japan it is around 10 degrees off. Parts of Brazil get over 22 degrees out. Wikipedia has [this helpful graphic](https://upload.wikimedia.org/wikipedia/commons/a/a1/World_Magnetic_Declination_2020.pdf) (from 2020) showing the difference between the local magnetic field and actual North. On the green “0” line a compass will point towards true north. Everywhere else it will be out a bit.
The Earth’s magnetic field is caused by the materials inside the Earth, and they shift around a bit. Which means the Earth’s magnetic field also shifts. [This neat little animation](https://en.wikipedia.org/wiki/File:Earth_Magnetic_Field_Declination_from_1590_to_1990.gif) shows how the last diagram has changed from 1590 to 1990. Slight changes in the Earth’s internal structure lead to changes in the field.
Finally, let’s talk about a magnetic pole. A magnetic pole is the point on an object where the magnetic field goes straight down into the object or straight up out of the object. But remember, magnetic field lines go in loops; they don’t start or end. So all pole is is the point where the field happens to cross the boundary of the object, while going up (or down). So the Earth’s north magnetic pole (which is actually a magnetic south pole) is the point where the magnetic field goes straight down into the Earth. If you stood there with a compass the needle would try to point down.
As the Earth’s magnetic field shifts, that point will also shift.
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