1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Magnetic susceptibility

  1. Oct 2, 2014 #1
    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. jcsd
  3. Oct 3, 2014 #2

    rude man

    User Avatar
    Homework Helper
    Gold Member

    B = μ0H + μ0M = μ0H(1 + Xm).
    B cannot be negative!
     
  4. Oct 3, 2014 #3
    We're dealing with vectors though - I'm not sure what that has to do with anything?
     
  5. Oct 3, 2014 #4
    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
  6. Oct 3, 2014 #5
  7. Oct 3, 2014 #6

    rude man

    User Avatar
    Homework Helper
    Gold Member


    Not only possibly - definitely!
    What do vectors have to do with it? B nd H are always collinear.
     
  8. Oct 3, 2014 #7
    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...
     
  9. Oct 3, 2014 #8

    rude man

    User Avatar
    Homework Helper
    Gold Member

    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).).
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Magnetic susceptibility
Loading...