# I Magnetic attraction

1. Nov 15, 2016

### thetexan

If I have a bar magnet is it correct to assume that the bar is symmetrically magnetic? In other words is one pole just as likely to attract as the other end?

Here's where I'm going with this. A compass needle lines up north and south. Is this because one end is attracted to north AND the other end is attracted to south?

Then, if that's true is the end that is attracted to north MORE attracted to the north magnetic pole than the end that is attracted to south is to the south magnetic pole because we in the US are closer to the North Pole?

I know the needle will align with the flux lines but does proximity to one closer pole cause a greater pull than the farther pole.

It seems the there are two forces acting on a compass needle...the magnetic force and gravity. If the magnetic force is greater on one end than the other then there would be a center of magnetic force...the point where the force is balanced. Likewise there is a center of gravity for the needle which should be near the center of the needle. If these two do not coincide then there would be a torque moment which would account for magnetic compass dip, acceleration and turning errors on an airplane.

This is how I think it works...at least to the best I can understand it. Is any of this correct?

tex

2. Nov 16, 2016

### Simon Bridge

Attract what?
Commercial bar magnets are usually built so that both poles have the same strength... but define "pole". The "north pole" is just the face where magnetic field lines mostly emerge from.

... however, this is not what you are talking about when it comes to compass needles below...

No - it is because the magnetic field of the compass needle aligns with the external magnetic field.

There is a model which treats magnetic poles separately - for convenience - so, in that sense, the closer two poles are to each other the stronger their attraction/repulsion. In this model, the Earth's north pole attracts the north-seeking pole of the needle, and repells the south-seeking pole. Similarly the Earth's south pole attracts the south-seeking pole of the needle and repells the north seeking pole of the needle.

The Earth's magnetic field is not parallel to the surface. The field lines point mostly down near the poles, so the effect on a compass needle is weaker there.
I suspect this is the main thing you are interested in.

The Earth's magnetic field is not uniform across the surface either, and compass needles are often finely balanced so they will register small changes in the momentum of the aircraft (say) which may not register on other instruments. This would account for errors due to various accelerations.

3. Nov 16, 2016

### Cutter Ketch

Magnets do not have to be symmetric in field strength at the two poles, but they do have to have the same net field. In other words if one pole is pointed and the other is bulbous the pointy end will concentrate the field while the field lines will be more spread out on the bulbous end.

However for a bar magnet where the two ends look about the same you can take the field to be

RE compass points, they are suspended on a needle at their CG and you are supposed to hold the compass flat. This is intended to make gravity irrelevant. Like all magnets they have a north and south poles. And they line with the field line

4. Nov 16, 2016

### Cutter Ketch

Regarding the North Pole being more or less attractive, it is true the field lines do converge near the poles. However I don't think that there is enough convergence on the scale of the length of a compass needle to be worth considering.