# Relationship between magnetic field lines and magnetic field

• a1234
In summary: However, a three-dimensional model should be able to account for the conservation of flux, and hence provide a more accurate depiction of the field.
a1234
Homework Statement
Show that the density of magnetic field lines can be used as a measure of the size of the magnetic field.
Relevant Equations
psi = BAcos(theta)
As stated in the problem, I want to demonstrate mathematically that field line density is directly related to the magnitude of B. How would I be able to do this, other than simply using the flux equation and showing that for a higher flux in the same area, the magnetic field must be rise accordingly. Or would this be sufficient?

The density of flux lines in 3 dimensions is proportional to the magnetic field strength. Drawings that are two dimensional don't show the inverse square law properly that occurs from a magnetic pole, etc. I would need to think about exactly why a 3 dimensional model gives exactly what is needed for the conservation of flux, etc, but in any case it does.

Edit: I think it probably can be demonstrated by saying that the flux lines into any box are the same as the flux lines out of the box=if they go in, they must emerge, and by Gauss' law, they then obey ## \nabla \cdot B=0 ##, just like the magnetic field, but maybe someone else can comment on this.

Last edited:
dlgoff
Edit: I think it probably can be demonstrated by saying that the flux lines into any box are the same as the flux lines out of the box=if they go in, they must emerge, and by Gauss' law, they then obey ## \nabla \cdot B=0 ##, just like the magnetic field, but maybe someone else can comment on this.
How can we show that the individual field lines obey Gauss' law? How would this be different from showing that Gauss' law is satisfied for the field?

a1234 said:
As stated in the problem, I want to demonstrate mathematically that field line density is directly related to the magnitude of B. How would I be able to do this, other than simply using the flux equation and showing that for a higher flux in the same area, the magnetic field must be rise accordingly. Or would this be sufficient?
It may help to start with the definition of a vector field line. If parametric curve ##\vec x(s)## is a field line, it obeys
$$\vec x'(s) = \vec B(\vec x).$$ Perhaps someone else can explain how to define field line density mathematically.

vanhees71
I learnt physics using the centimetre-gram-second (cgs) system of units, where Gauss specified that 4 pi lines of force originate from a unit magnetic pole, and the magnetic flux density is expressed in lines per cm2. So I find it hard to put my heart into the question!

Replies
1
Views
959
• Other Physics Topics
Replies
1
Views
407
• Classical Physics
Replies
24
Views
1K
• Electromagnetism
Replies
27
Views
1K
• Electromagnetism
Replies
28
Views
1K
Replies
2
Views
1K
• Classical Physics
Replies
22
Views
985
• Electromagnetism
Replies
35
Views
3K