Register to reply 
Measuring Magnetic Field Intensity (H) and Magnetic Flux Density (B). 
Share this thread: 
#1
Nov2210, 06:15 PM

P: 32

Now here is a real world practical application.
Australia Post has a warning for shipping magnetic material in the post. Section D2.9.2 states... Any material that, when packed, has a magnetic flux density of 0.159 A/m or more at a distance of 2.1 meters from any point on the surface on the package (is deemed a dangerous good). They call it magnetic flux density but use A/m units, are they getting A/m confused with Tesla or am I confused? Under their definition, how would you measure 0.159 A/m because I don’t think a magnetometer will do the job. 


#2
Nov2410, 10:21 AM

Sci Advisor
PF Gold
P: 2,080

B is magnetic induction, aka magnetic flux density, and is measured (in SI units) in Tesla. It describes induction in a material. H is magnetic field (sometimes called magnetic intensity) in a vacuum, and is measured in A/m. So the Post got the labels confused, as you note. However it is easy to relate them. By definition,
[tex]\vec{B}=\mu_0\vec{H}+\vec{M}[/tex] Since M is zero in the air, you can use a magnetometer that measures either B or H and then convert to the desired unit. As a note, it is regrettable that the SI system (invented by engineers) uses different units for B and H. In the old Gaussian (cgs) system they have the same dimensions, making intuitive sense. 


#3
Nov2410, 11:00 AM

Sci Advisor
Thanks
PF Gold
P: 12,134

I guess you could call their bluff and ask them for the measurement system they would use.
ps no one should mock engineers!!! They can make life too hard for you. 


#4
Nov2410, 11:08 AM

P: 697

Measuring Magnetic Field Intensity (H) and Magnetic Flux Density (B).
Personally, I don't find it regrettable that SI units are used for EM. This system is very intuitive and convenient when doing experimental and design work. But then again, I'm an electrical engineer, so your comment is not out of line. Of course, your opinion is as valid as mine or anyone else's, but I can't help but want to quote an interesting comment by Richard Fitzpatick from his PHY387K course notes which are available online in PDF form. This just helps offer the OP alternative points of view. I actually was surprised to see this choice from a physics professor, and would have expected these exact words from an electrical engineering professor only. But, there it is. 


#5
Jun2612, 08:56 PM

P: 4

The originator of the SI units was Prof. Giovanni Giorgi who proposed the metre, kilogram, second, and ampere as the four fundamental units (since increased to 7). In particular, he originated the 'rationalised' concept of dealing with the pi (constant) in such a way that the pi appeared in 'circular' things and was absent from 'straight' things.
For a little more see here: http://en.wikipedia.org/wiki/Giovanni_Giorgi 


#6
Jun2612, 09:45 PM

P: 834

I don't mind nonSI units, but I have a heavily dislike of traditional electromagnetic cgs unitsnormalizing out the factors of [itex]4\pi[/itex] and shifting them to the sources of EM fields is massively backwards. Leave them in the Green's functions for the differential equations and just set [itex]\epsilon_0 = \mu_0 = 1[/itex].



#7
Jun2712, 04:20 PM

P: 4

Sorry  this is a thread hijack  but from where do you get the symbols for pi, epsilon sub zero, etc. and how do you insert them in a post?
Thanks. 


#8
Jul112, 01:28 PM

Sci Advisor
PF Gold
P: 2,080

The quotation in post #4 misses a key advantage of cgs units for electromagnetismthat E and M are actually relativistic manifestations of the same electric force, seen in different frames. Since relativity unites electricity and magnetism and shows that they are one, it is logical that they should be measured in the same units, not different ones as in SI.



#9
Jul212, 08:52 AM

P: 4

As I recall, though, the cgs units for electrostatic quantities are not the same as the units for electromagnetic quantities.



#10
Jul212, 10:12 AM

Sci Advisor
PF Gold
P: 2,080

It is true that there are several flavors of units based on centimeters/grams/seconds. The esu/emu systems you refer to were used, for instance, in the first edition of Smythe's book Static and Dynamic Electricity. I was thinking of the Gaussian system used by Jackson (2nd edition) and Schwartz (Principles of Electrodynamics), where E and B have the same underlying units. Even the emu system, however, has the advantage that B and H are identical in a vacuum (compare this to SI units where B in a vacuum is approximately 1e8 times larger than H).



#11
Jul212, 10:56 AM

P: 4

You are of course correct.
After a couple of hours reflecting on this I came to this understanding. The widespread use of SI units in general, and in electrical engineering in particular (electrical engineers being amongst the first to embrace SI units) lies in the observation that B = kH (where k is some constant, possibly unity) is only true in materials for which the BH relationship is linear. Since all practical electrical magnetic materials are highly nonlinear  usually by careful design  the simple relationship fails and it becomes less confusing to think of B as the magnetic induction and distinguish from H, the magnetic field. Interestingly, though, the current trend is towards operating power conversion circuitry at ever increasing frequencies (100kHz to 10MHz), permitting the use of low inductance components and sometimes air cores for chokes and transformers. 


#12
Feb1314, 04:48 PM

P: 1,040

Claude 


#13
Feb1814, 01:37 PM

Sci Advisor
PF Gold
P: 2,080

Electricity and magnetism are related through special relativityin fact, the magnetic force is exactly the Coulomb electric force as seen in a moving reference frame. Since E and M are the same at a fundamental level, it is intuitive to measure them with the same units. The widespread, indeed universal, use of SI units for electrical quantities makes this impractical to the point that Gaussian units have been abandonedbut the latter are relativitistically consistent and logical whereas the former are not.



Register to reply 
Related Discussions  
What's the difference between magnetic flux density,B and magnectic field density,H?  Classical Physics  18  
Magnetic fieldstrength, magnetic flux, magnetic flux density  General Physics  0  
Find k from magnetic field and magnetic flux.  Advanced Physics Homework  4  
Magnetic Flux & Magnetic Density  Introductory Physics Homework  4 