Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Question regarding units of some magnetic quantities

  1. Nov 23, 2004 #1
    In the Halliday-Resnick textbooks, there is only mention of strength of a magnetic field, which in SI units is the derived unit Tesla.

    However, on my TI-89 and on more modern sources on the internet (including a Fuji-film PDF!), there is mention of the unit of the Oersted for magnetic field strength. The units of Tesla (and the related unit, Gauss) are reserved for something called "Magnetic Flux Density," which for reasons unknown to me is considered seperate. The unit for Magnetic Flux is, as usual, the Weber.

    So...does anyone have any information regarding the distinctions between these two quantities?
  2. jcsd
  3. Nov 23, 2004 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    A good place to start is to write all of the quantities in terms of the fundamental units, Time, length, Mass and Charge. Now compare, this should make the differences obvious.
  4. Nov 23, 2004 #3
    i'm not sure what course you're doing but you will find such info in many books on electromagnetics. J.D. Jackson Classical Electrodynamics has the unit of magnetic field strength as a Henry (H) = 10^4 Oersted (Oe) Henry is in the SI unit system and Oersted is in the Gaussian unit system

    if you're actually asking what is the difference between a flux density and a field, then that is another matter; what exactly do you mean?
  5. Nov 24, 2004 #4
    i thought that the Henry was the unit for inductance. now i'm even more confused.

    i was also asking about what the difference is between magnetic field strength and magnetic flux density.

    i figured that the magnetic field strength would be the "B" in:

    F = qvB sin(theta)

    The unit for this, as far as I've seen in the textbooks I've come across (the most renowned of which being Halliday-Resnick), is the Tesla, where:

    1 T = 1 N-s/C-m.

    ...And magnetic flux is given by:

    phi = BA cos(theta)

    (these, of course, just being the equations for the magnitude.)

    so....what's the deal?
  6. Nov 24, 2004 #5
    oops sorry in Jackson's appendix (p. 783), the unit for inductance is given on the line two lines below that for magnetic field.

    as you rightly point out the SI unit for inductance is 'henry' or 'H' for short; whereas the SYMBOL for magnetic field is the symbol 'H' with its unit being 'Amp metre^(-1)'

    in summary

    magnetic flux density is symbol B SI unit tesla = 10^(-4) gauss
    magnetic field is symbol H SI unit A m^(-1) = 4*pi*10^(-3) oersted

    those last convertsion are from SI to the older Gauassian system of units
    ok, B is a SI magnetic flux density (tesla) A is area m^2 in SI, cos(theta) dimensionless in whatever units you choose.

    Last edited: Nov 24, 2004
  7. Nov 24, 2004 #6
    so lets answer you actual question: what is the difference between a magnetic flux density (B) and a magnetic field (H)?

    well in its simplest form, you get H by multiplying the B by the permeability of whatever medium you're working in; if it's free-space then its symbol is mu_0 ("mu nought") and it converts between the two: in SI units


    where mu_0 equals 1/(4*pi) * 10^(-7) H m^(-1)

    I might add that there's some confusion because Jackson the paragon of electromagnetic correctness points out that there IS confusion due to the nomenclature; further, Jackson calls magnetic flux density "magnetic induction" and defines magnetic flux in units of 'Weber' (with symbol phi, or F) just to REALLY confuse us both!!, so look up the answer and work backwards if you can (LOL)

    fun aint it!! so hence you see the need for standards committees, huh?

    finally, in maths, a flux usually is defined in terms of a quantity going through an area and so B would multiplied by the area A to give force is pretty much what your reference says also
    Last edited: Nov 24, 2004
  8. Nov 24, 2004 #7


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    You are correct Henry is the unit of inductance, not the Magnetic field, which is indeed Oresteds. Perhaps the confusion occurs because H is the commonly used sybol for the Magnetic field, and it is also the appreviation for Henrys, but they are not the same thing.
  9. Nov 24, 2004 #8


    User Avatar
    Science Advisor

    The unit of flux is the Weber, and 1 Tesla is 1 weber per square meter exactly, by definition. So Tesla, as units of B, is a flux density.
  10. Nov 27, 2004 #9
    thanks, tfleming. that clears it all up. so, the oersted isn't an SI unit after all but is instead gaussian.

    H = mu B.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Question regarding units of some magnetic quantities