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 field at a distance - epxression

  1. Aug 30, 2007 #1

    I have a cylindrical Neodymium permanent magnet. I need to find the pull force of the magnet at various distances from the magnet. Is there an expression relating the pull force(or attractive force) to the magnetic flux density B and the distance from the cylindrical magnet? What is the expression that tells us how the B varies with distance?

    The parameters I have for the magnet are here

    http://www.kjmagnetics.com/proddetail.asp?prod=D2C .


  2. jcsd
  3. Aug 30, 2007 #2


    User Avatar

    Staff: Mentor

  4. Aug 30, 2007 #3
    The wiki doesn't seem to have an expression listed for calculating magnetic flux density at a distance for a cylindrical permanent magnet. There's no current involved, so I just want to be able to simulate and get field values at different points from the magnet.

    Seems there should be a straightforward expression to do this, maybe involving partial derivatives.

  5. Aug 31, 2007 #4
    Far enough away, most magnets can be approximated as a magnetic dipole moment m, which is a vector. For a magnetic dipole m at the origin and oriented in the z-direction, you get

    [tex] \vec{B} = {\mu_o m \over {4\pi r^3}} (2\cos\theta \hat{r} + \sin\theta\hat{\theta})[/tex]

    The trick for your problem will be to find out a good guess at m. As usual, Wikipedia has some good information.
  6. Aug 31, 2007 #5
    Thanks for that. What about points closer to the magnet - near the surface, for example, where the gradient is steeper?
  7. Aug 31, 2007 #6


    User Avatar
    Science Advisor
    Gold Member

    Maybe you can find an equation by experiment. Could you use some type of spring device (to messure weight) with a iron-mass on it and find the force at different points around the magnet? Say; leave the spring hanging and moving the magnet at different positions under the mass?
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Magnetic field at a distance - epxression