Magnetic susceptibility

1. Oct 2, 2014

fayled

This is defined by M=XmH.

Using H=(B/u0)-M to eliminate M gives us M=1/u0(Xm/1+Xm)B, where B is the total magnetic field.

Now my problem is, my book states that for paramagnetic media, Xm is positive, and for diamagnetic media Xm is negative. Now for paramagnetism, we expect M and B to have the same directions, i.e the constant of proportionality above should be positive - Xm>0 achieves this so it is fine. For diamagnetism however, where M and B have opposite directions, we expect the constant to be negative. If we write the constant as 1/1+1/(Xm) (ignoring the positive u0), we see that Xm<0 achieves this, but only for Xm between 0 and -1. So what the book is saying doesn't seem to be true all the time. The only thing I can see that could save this is that apparently Xm values are typically of the order of around 10-5 so this would be correct - but I don't like how the book doesn't mention something that could theoretically happen so would be grateful if somebody could tell me if I'm right or not, thankyou :)

2. Oct 3, 2014

rude man

B = μ0H + μ0M = μ0H(1 + Xm).
B cannot be negative!

3. Oct 3, 2014

fayled

We're dealing with vectors though - I'm not sure what that has to do with anything?

4. Oct 3, 2014

fayled

We're dealing with vectors though - I'm not sure what that has to do with anything?

Edit: are you possiby claiming this is why Xm is limited between 0 and -1 in terms of the negative values it can take?

Last edited: Oct 3, 2014
5. Oct 3, 2014

6. Oct 3, 2014

rude man

Not only possibly - definitely!
What do vectors have to do with it? B nd H are always collinear.

7. Oct 3, 2014

fayled

Why must B and H be collinear though, I'm struggling to see this - it would solve a few other issues I'm having with this topic too. And It's most likely very obvious...

8. Oct 3, 2014

rude man

B = μH.
B and H are vectors. μ is a scalar.

H is a function of current. The current sets up the H field per Ampere's law or more generally by del x H = j (in the absence of time-varying electric fields). j is current density. (In permanent magnets the currents are "amperian" currents not subject to resistive dissipation).

B is the magnetic field as defined by F = qv x B. B is "generated" by H. In a vacuum, the relation is B = μ0H. If there is magnetic material present, individual domains will align with the H field (what else could they do? They either align with the H field or stay put, or anti-align in the case of predominantly diamagnetic materials. The domains that stay put average to zero net susceptibility. If most of them line up the susceptibility is high (can be > 1000 in certain paramagnetic substances, like iron).).