# Diff. bet intensity and density

1. Oct 15, 2013

### kira506

I know that this question may sound silly , but I really can't get the difference between intensity and density in general ! And the whole magnetic field thing makes it worse , in my textbook , its written that magnetic field intensity is the same as magnetic flux density , but no matter how many times I research it on the internet , I find people saying that the two terms greatly vary ! How come is intensity the same as density ? then why isn't intensity a physical property as density ? Please help !

2. Oct 15, 2013

### mikeph

Burn the textbook that said that.

Magnetic flux density "B" is the standard magnetic field that we all learn about. It is THE physical field when we talk about magnetic fields. It's called a density because it is higher in regions where magnetic flux lines are more tightly bunched. So it is in some sense a density.

The magnetic field intensity "H" is an auxiliary field that greatly helps us analyse magnetic fields in materials. It's just B/mu0 - M, where M is the density of magnetic dipoles in the material. It's defined as this, so it's automatically different from B, even when M = 0. It has different units, too, just in case you aren't totally convinced!

3. Oct 16, 2013

### Andy Resnick

I understand your frustration. The large number of different units (Oersted, Maxwells, Weber, Gauss, Tesla, etc) used to quantify the magnetic field doesn't help the situation, either. Just look at http://www.kjmagnetics.com/proddetail.asp?prod=D101-N52 to see the variety of different units used to describe the magnetic properties of the same object.

As for the use of 'intensity' and 'density' specifically, the situation is analogous in radiometry when 'intensity' is used instead of 'irradiance' or even worse, substituted for 'flux'.

These websites may be of some help:

http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magfie.html
http://www.irm.umn.edu/hg2m/hg2m_a/hg2m_a.html

4. Oct 16, 2013

### kira506

XD I can't , I still need it .. But I think I'm kinda coninced , so the intensity of the field is "H" and "B" is the magnetic flux density :the whole magnetic field , so "H"'s presence depends on "B"'s presence which is the magnetic fild itself , thank you so much for explaining c:

5. Oct 16, 2013

### kira506

Thank you so much , sites helped a lot ! And yes , the whole numerous units thingie confused me a lot "H" is the strength of the magnetic field (its intensity) while B is the magnetic flux density , the field ,te image is now clearer in my mind , thanks !

6. Oct 16, 2013

### mikeph

Yes, my solution when confused is to always think of the B field as the true macroscopic field, this then causes a magnetisation M in certain materials (depending on their structure), but a neat trick allows us to express Maxwell's equations without having to worry about M (nor the electric analogue, polarisation P), just D and H.

We can then say "in this material, H is a function of B and D is a function of E", the functions, assuming we know them, take care of the fields within the media.

But you can never be too careful with conventions. Some people (most notably theoretical physicists) define H and D in a different way such that the units are the same as B and E. So it's a good idea to make sure you check before diving into a problem, what the letters are actually meant to represent!

7. Oct 18, 2013

### kira506

I will try my best to supress my A.D.H.D and check what the letters actually represent XD they play on that a lot in our exams ,letters and their meanings in the given context, Its the first time I actually got introduced to magnetic effects of electric current and its numerous equations , I'm still at the start so I don't full grasp what D and E stand for , (you see , our curriculum is divided such that we take all classical first then start modern) so I won't see Maxwell for a longtime , Thanks alot , I'm sure your explanation will help me in the future , your explanation really helped me visualize and understand the difference I couldn't from the textbook

8. Oct 18, 2013

### kira506

Silly me , totally mixed up Maxwell with Max Planck , Maxwell's equation is still to come and soon , must get a good grip on names