Null point on a bar magnet

First, I want to be sure of something.

You seem to be saying that a bar magnet has no null point. Am I correct in perceiving that you are saying that?

If you affirm that, then, great! I will let you know!

 

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But, i read in some book somewhere that a bar magnet has places where the field lines are mathematically discontinuous.

Don't this mean that where the field lines are discontinuous there is a null point?

Wish i could think of what book it was.

What about it. Don't a mathematically discontinuous field line mean a null point?

 

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So the distance between the North Pole and South Pole get closer and closer as the magnet gets smaller and smaller, but will never "touch" in the same way that you'll never get to zero when dividing something in half over and over, correct?

 

Any thorough classical E/M textbook.

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B lines are 'refracted' at the surface, remember. Same goes for E lines for dielectrics.

 

Hello all

Is the op referring to the Bloch wall?

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Possibly. The fact that the magnetic field is discontinuous doesn't mean that there's a null point. I can even draw a picture:

Code:

 1                         -----------------------
 0                         *
-1  -----------------------

At the point *, the function is discontinuous---it jumps from a value of -1 to +1. But you'll notice the function is never equal to zero.

 

What about if I do remember book! And what about if book actually does exist!

I have book! It is sitting beside me right now!

Now book is my hand as i write!

The Electromagnetic Field
by Prof. Albert Shadowitz
out of Dover Publications Inc.
ISBN 0-486-65660-8 (pbk.)

First, FIGURE 6-13, just to get warmed up on modeling of E field field lines associated with electrets. Be sure to look at pretty pictures but also take time to read pertinent text. FIGURE 6-13 on page 275.

Then, FIGURE 7-8 to see modeling of H field field lines associated with bar magnets. Be sure to look at pretty pictures but also take sufficient time to thoughtfully read pertinent text. FIGURE 7-8 on page 317.

According to pictures and text there is a location at each pole of a bar magnet where the H field field lines, both in the interior and exterior of the magnet, terminate. The field lines terminate at the surface of the pole. Within the width of the surface of the pole, since there are no H field field lines, the H magnetic field must be acknowledged to be zero. The acknowledgment of a zero magnetic field must mean that there is, in some terminology, a null point. Each pole of a bar magnet must be acknowledged to have a null point.

 

The "null point" you are talking about comes from the discontinuity in magnetic constant---that is, the constant in the bar magnet is \(\mu\), and the constant outside the bar magnet is \(\mu_0\). This sudden difference at the edge of the magnet causes edge effects, such as discontinuities in the magnetic field, along the lines of the drawing that I made above.

This is completely different than what the OP was asking about, as this effect only pops up AT one of the poles, as opposed to the middle of the bar magnet. You seem to be confusing terminologies.

 

LOL!

Ben is very, very lucky that he was able to get a science degree of some sort rather than having to make a living as a technical writer and/ or technical illustrator.

Ben's diagram, inscrutable as it is, seems to show either one or two null points for a pole.

Ben is kindly urged to get my cited book and read it. Including the pretty pictures. Twice. Or however many times it takes to secure a correct and logical understanding of field lines and what it means when a field line terminates at a barrier and an antiparallel field line exists on the other side of the barrier.

When Ben summons his chief butler to bring round both his gold plated Rolls Royce and his platinum plated Rolls Royce, and park one in front of his manse, then park the other one nose to nose, bumpers touching, is there not a plane, though perhaps of exquisitely diminished thickness, betwixt Rollers, in which there is a null point of Rollers? Yes, Virginia, there is a plane betwixt Rollers in which there are no Rollers, and therefore is a Roller null point.

And, of course, JamesR, who seems to have gone into seclusion in a secret location, is welcome to beg, borrow, or

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buy as many copies as needed, and to read them as much as needed to secure missing information about electromagnetic fields.

As for me, i am going fishing. See ya when!

 

He's fundamentally asking the following.

If there's a bar magnet made of let's say...99 atoms in a straight line.

Will the case be that 49 will form a closed loop with 49, and what will be of the remaining? Will it form its own loop? Will it have no charge?
Or
Is the field independent of individual atoms?


I am not sure if a legitimate answer can be given.

 

Exactly. In mathematics, \(\oint_S {\mathbf B}\cdot d{\mathbf A}= 0\) (the integral form of Gauss's law for magnetism, \({\mathbf{\nabla}}\cdot {\mathbf B} = 0\)). The net magnetic flux across the boundary of any 3D volume is zero.

 

I think what the article is referring to are edge effects, where the permeability of the bar magnet is different from that of free space. The boundary conditions should cause kinks in the field lines (as H -> B). This may possibly produce some regions of net zero magnetic field, apparently. CANGAS is getting confused.