# Direction of External vs Internal Magnetic Fields

I'm a bit confused about the direction of external magnetic fields vs internal magnetic fields of an object.

If I understand correctly, the direction of an object's external magnetic field lines go from North to South. Its internal magnetic field lines go from South to North.

So, if a particle were to be placed between the two poles of an object, would the magnetic field acting on it be to the North pole?

I'm just starting to learn about electromagnetism, so preferably a simple explanation is better.

• What do you mean by "magnetic field"? – Rob Jeffries Apr 8 '17 at 16:35
• – Yashas Apr 8 '17 at 16:36

## 1 Answer

An external magnetic field is something that by definition is extraneuous to your object and does not depend on it. This could be a magnet, or the Earth's field itself, to which your sample is exposed to.
It is usually defined by H.

Not sure what exactly your "internal" magnetic field would mean.

The correct terminology to use would be the magnetisation M, of the object, which is the object's own magnetic field (e.g. if it's a magnet), in the absence of any external field.

When you place your magnetic sample in your external field, then the two contributions add up vectorially and you usually use the letter B to describe the outcome

Some objects may have a magnetisation M that emerges from being exposed to an external field H: e.g. a ferromagnetic substance may have all its magnetic domains oriented in random directions therefore adding to 0 net M, but an external field could line them all up in one direction giving a net M $\neq 0$.
In this case, M and H would have the same direction so B would also point in the same axis. Being just a sum of the two, B could end up being > H (ferromagnets, paramagnets), < H (diamagnets) or $=0$ if M cancels H (superconductors <=> Meissner effect).

H and M are off by a factor of $\mu_0$ wrt to B, because of reasons:

$$\mathbf{B} = \mu_0(\mathbf{H} + \mathbf{M}).$$

--

Usually B is used for magnetic fields everywhere, the distinction betwen B, H and M only really becomes relevant when you have magnetised samples placed in external magnetic fields. The Earth's field is so weak that you usually don't care about it.

• Thanks for your answer. What I meant was the external magnetic field lines of an object. I had seen in diagrams of magnetic objects that its magnetic field lines originated from the North pole and went down to the South Pole around the planet. Then at the South Pole, the lines went back up to the North Pole through the object's center. I was just wondering if this was the case with magnetic field lines. – Inertial Ignorance Apr 8 '17 at 23:17