Talk:Permeability (electromagnetism)

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[edit] From PNA/Physics

  • Permittivity, permeability - these should be explained more thoroughly; in particular, it's unclear what any of these mean. --[[User:Eequor|ᓛᖁ♀ᑐ]] 14:36, 6 Nov 2004 (UTC)

[edit] Numerical Value of Permeability in Free Space

Does somebody understand where 4 * pi * 10 ^ -7 comes from? 4 * pi is obviously the standard expansion factor of an eculidian universe, but the 10 ^ -7 is unexpected.


Here's my guess: the 4 * pi shows up whenever you try to do spherically or cylindrically symmetric surface integrals in 3 dimensions, and where it appears is a matter of convention. It shows up in the permeability so some other expression will appear cleaner. The 10^-7 arises through the old SI definitions which make the ampere be the amount of current such that two parallel wires carrying it, spaced at 1 meter, will feel a force of 1 newton/meter length. The ampere is inconviently large (because the coulomb is); the 10^-7 is needed to match up these large currents with the smallish magnetic force.
--Vortmeester
I saw this and had to comment on the phrase "has the exact value 4π×10−7 N·A−2." This is not true. The permeability of free space has the aproximate value 4π×10−7 H·m−1 (Henries per meter being equivalent to Newtons per square Ampere), but the exact value is 1.25663706143... H·m−1. Similarly the permittivity of free space is approximately (36π×109)−1, or exactly 8.854187... F·m−1. The convenient upshot of these approximations is 1) they are highly (but not exactly) accurate, 2) the intrinsic impedance of free space becomes approximately 120π, 3) the speed of light in a vacuum becomes 3×108 m·s−1. Obviously, as the meter is now defined so as to make most measurements of the twentieth century exactly accurate (that is, so that the vacuum speed of light is exactly 299,792,458 m·s−1), these values for permittivity and permeability in free space cannot be exact. They are, however, very convenient for engineering work. -- Charles Robertson; no User page on wikipedia. —The preceding unsigned comment was added by 72.177.69.176 (talk) 01:14, 3 April 2007 (UTC).
Well then according to your logic if we by definition make the meter, Newton, and Amp exactly these values than the given values for permittivity and permeability are by definition exact. -134.50.3.138 (talk) 00:19, 8 February 2008 (UTC)

[edit] History of the Ampere

You can find more information at the web-page

History of the Ampere


The factor of 10^-7 arises mainly because of the transition from CGS to SI units.

10^-5 going from a dyne to a newton of force.
10^-2 going from an abampere to an ampere (because it flows in both conductors).

Or something like that.


[edit] Definition of Absolute Permeability

I'm concerned about the definition of 'absolute permeability' being given synonymously with mu0. Absolute permeability is sometimes used in the SI, but not in the CGS system. All text books that I have read, eg Cheng's 'Fundamentals of Engineering Electromagnetics' define absolute permeability as the ratio of B to H in any medium you care to choose (a vacuum included). It's purpose is to reinforce the distinction with 'relative permeability' by which it differs by the factor 4 pi 10^-7.

In short, mu0 is an absolute permeability, but not all absolute permeabilities are mu0. Absolute permeability is a synonym for 'permeability', at least in the SI.

Bright Engineering > Physics


RAClarke 10:02, 2 October 2005 (UTC)

[edit] μ vs µ

I don’t know what difference it makes, but μ and µ are two different characters. The only difference I see (and only in some fonts) is that µ is slanted and μ isn’t. In the Unicode character palette, µ (0x00B5) was found under Symbols > Letterlike Symbols, and μ (0x03BC) under European Scripts > Greek. I changed all the mus in the article to the former, which is the one I can type (option-M on a Mac). If it makes any difference and I’m in the wrong, change them all to the other μ. —Frungi 06:58, 29 September 2006 (UTC)

U+00B5 is "MICRO SIGN", so I think when you say "μ = 1.26 µN A-2", the first μ should be U+03BC because it's the Greek letter being used as a variable, but the second µ should be U+00B5 because it stands for 10−6. —Keenan Pepper 06:31, 30 September 2006 (UTC)

[edit] Usage of the word "resulting" in the following

If one puts the ferromagnetic material into an externally applied magnetic field, the domains tend to line up, so that the sum of the fields from the ferromagnet and the resulting magnetic field is higher in magnitude than the applied magnetic field alone.

I suggest that the word "applied" would make more sense, otherwise it could be read that the ferromagnet field is included twice. Alternatively, it could be rewritten "so that the resulting magnetic field is higher in magnitude than the externally applied magnetic field alone." I have not made the change as this needs to be confirmed by an academic who should make the change. Just Maybe 23:27, 27 May 2007 (UTC)

[edit] Table values listed

Never mind, that was a stupid idea. Sorry.

[edit] No Estimation of Error

Just before the explanation for why there is no error in the figures comes reasons why there should be. I see nothing in SI units and mathematic definitions that keep error out of real-world experimental results. Brewhaha@edmc.net 06:17, 17 July 2007 (UTC)

[edit] Units problem

I am copying from the Wikipedia page on magnetic permeability:

According to the definition of the auxiliary field, H

B=μ(H+M)

where

μ is the material's permeability, measured in henries per meter. B is the magnetic field (also called the magnetic flux density or the magnetic induction) in the material, measured in teslas H is the auxiliary magnetic field, measured in amperes per metre M is the magnetic moment per unit of volume or magnetization, measured in teslas


How can you add H and M in the parenthesis if they have different units? (H has A/m, M has teslas).

This doesn't look right. —Preceding unsigned comment added by 77.49.235.14 (talk) 16:47, 30 October 2007 (UTC)

Most textbooks I have seen, and most web pages (e.g. http://www.ndt-ed.org/EducationResources/CommunityCollege/MagParticle/Physics/Quantifying.htm ) say that ‘Magnetization carries the same units as a magnetic field: amperes/meter.’

There was something called ‘Intensity of magnetization’ in the defunct Kennelly variant of the SI, which had units of teslas.

RAClarke 23:11, 4 November 2007 (UTC)