Magnet Myths and Misconceptions

[NoBull]

This "figure-8" business is completely wrong.  Just work out what the magnetic field looks like in your head.

Yes, it is wrong when applied to the shape of magnetic flux lines.

...but he is not even referring to the shape of magnetic flux lines but to the force of attraction on some ferromagnetic blob in the vicinity of a magnet which he has felt with his fingers.  Such force really dips in the midpoint of a bar magnet, so it is no wonder that he came to the fig 8 conclusion.

the field between poles is NOT the same,and where this transition takes place is a null zone,a zone where there is no magnetic field potential. This zone dosnt attract feromagnetic material,it dosnt induce flux into an inductor,and it wont even close a reed switch

Look, he is talking about a "null zone" between poles and a lack of attraction force to a ferromagnetic blob there, he is not talking about magnetic flux density B there. He is conflating the two concepts.  The same mistake over and over...

MileHigh, you are advanced enough to point out where his mistake comes from.  This is much better than just trying to prove him wrong.

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And now for Tinman:  A tiny rotating loop of conductor rotating in the plane perpendicular to the bar magnet and crossing its midpoint will still have electric current induced in it, despite that a ferromagnetic object does not experience any axial force there.  A Hall sensor that has its sensing face perpendicular to the magnetization axis of the bar magnet will detect a B field there, too.

You are confusing the force of attraction with magnetic flux density B.  They are not the same!
Force of attraction is related to the gradient of the B field - not its magnitude.

P.S.
There is a point on the bar magnet's crossection, where the flux inside a magnet is equal but opposite to the parallel flux outside of the magnet.
It is very near the boundary condition - the surface of the magnet.  This oppositional equality has a radial relationship, though - not axial (between poles).

[TinselKoala]

Unless I am terribly mistaken you describe just as in my drawings, the Hall sensor being face flat against the side of the magnet.  Unless you've got a really long magnet, the only place where the flux perpendicular to the magnet and therefore through the Hall sensor falls to zero is half way between the north and south poles.  "Nearly constant" is not "nearly enough" to make the perpendicular component fall to zero.  Rotating the Hall sensor so that it is sensitive to flux density  parallel to the magnet changes things a lot.  There with a long magnet the parallel flux density can be relatively constant over significant distances.

Yes, MarkE, you seem to be terribly mistaken, since my entire set of hypotheses has to do with what I explicitly stated several times, including in the post to which you are replying here: The Hall sensor plane , or face, if you like, is PERPENDICULAR to the long axis of the magnet! Not "flat against the side of the magnet!"

5)So a Hall sensor held with its plane at right angles to the magnet's long axis, and scanned along the length of the magnet, would read very differently in the two cases. Right?

6)If the "peanut waist" picture is true, the sensor being held at right angles and scanned along the magnet will experience a changing angle of the field as the field dips toward the "Bloch wall" waist, and then an also changing angle as the field dips up out of the equator on the other side. This will cause changing readings as the sensor is scanned past the "Bloch wall equator".  Right?

7)But in the conventional case, with the field lines strictly parallel to the long axis except near the end poles, the sensor will experience the field lines straight through the plane of the sensor, and thus the sensor's reading will remain constant, and _at the maximum value_  as it is scanned along the magnet's long axis. Right?

Think about that again, please. The question referred to a Hall sensor whose plane is perpendicular to the long axis of the magnet.

Hence, if the field lines are parallel to the long axis of the magnet, as in the conventional view, the flux through the PERPENDICULAR PLANE of the Hall sensor will be nearly constant and will not change in polarity ("arrow direction") over a considerable span in the central region of the magnet. On the other hand, if the "peanut waist" picture is true, then there will be considerable change in the flux through the plane of the Hall sensor held PERPENDICULAR to the long axis of the magnet.
In fact, if one is careful and strives to maintain the Hall sensor plane AT RIGHT ANGLES to the imagined "field lines" from pole to pole, including around the curling portions and onto the pole faces, a nearly constant output (translating to field strength or flux) will be maintained throughout. This latter is of course somewhat dependent on the aspect ratio (length:width) of the magnet, since a broader pole face will have less concentration of flux.

http://www.youtube.com/watch?v=AB_xNARuJaA    But whatever, dude.

Long axis of magnet goes like this:   N--------------------------------------------------S
Perpendicular plane of Hall sensor scanning: ||||||||||||||||||||||||||||||||||||||
Hall sensor is held at RIGHT ANGLES, that is PERPENDICULAR, to the long axis of the magnet, as I have now stated about a dozen times.

[TinselKoala]

Yes, it is wrong when applied to the shape of magnetic flux lines.

...but he is not even referring to the shape of magnetic flux lines but to the force of attraction on some ferromagnetic blob in the vicinity of a magnet which he has felt with his fingers.  Such force really dips in the midpoint of a bar magnet, so it is no wonder that he came to the fig 8 conclusion.
Look, he is talking about a "null zone" between poles and a lack of attraction force to a ferromagnetic blob there, he is not talking about magnetic flux density B there. He is conflating the two concepts.  The same mistake over and over...

MileHigh, you are advanced enough to point out where his mistake comes from.  This is much better than just trying to prove him wrong.

------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
And now for Tinman:  A tiny rotating loop of conductor rotating in the plane perpendicular to the bar magnet and crossing its midpoint will still have electric current induced in it, despite that a ferromagnetic object does not experience any axial force there.  A Hall sensor that has its sensing face perpendicular to the magnetization axis of the bar magnet will detect a B field there, too.

You are confusing the force of attraction with magnetic flux density B.  They are not the same!
Force of attraction is related to the gradient of the B field - not its magnitude.

P.S.
There is a point on the bar magnet's crossection, where the flux inside a magnet is equal but opposite to the parallel flux outside of the magnet.
It is very near the boundary condition - the surface of the magnet.  This oppositional equality has a radial relationship, though - not axial (between poles).

Highlighted for emphasis. Yes, even though a ferromagnetic "blob" or probe here will experience equal forces in both directions, hence will "feel like" a no-force situation, the flux still exists and can still do work by inducing current in a moving conductor just as we expect it to nearer the poles. 

Take the bar magnet and bend it around into a C-shape or a nearly full circle. What magic is this! You can still find your "force neutral" position between the poles with a ferromagnetic probe particle or reed switch, of course. But also of course you will be able to do work by spinning a coil of wire in the same position as your "forceless" probe or non-acting reed switch: See "electric motor" in WIKI.

[tinman]

Yes, it is wrong when applied to the shape of magnetic flux lines.

...but he is not even referring to the shape of magnetic flux lines but to the force of attraction on some ferromagnetic blob in the vicinity of a magnet which he has felt with his fingers.  Such force really dips in the midpoint of a bar magnet, so it is no wonder that he came to the fig 8 conclusion.
Look, he is talking about a "null zone" between poles and a lack of attraction force to a ferromagnetic blob there, he is not talking about magnetic flux density B there. He is conflating the two concepts.  The same mistake over and over...


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You are confusing the force of attraction with magnetic flux density B.  They are not the same!
Force of attraction is related to the gradient of the B field - not its magnitude.

condition - the surface of the magnet.  This oppositional equality has a radial relationship, though - not axial (between poles).

I am not makeing the same mistake over and over again,i am,and have always been refering to magnetic field strength/polarity,and the shape that this field of strength and polarity is.The flux at this mid region that is the null point is a mixture of two sepperate flux form's,and they cancel one another out.
I can asure you that the figure 8/peanut shape is the shape that resembles the magnetic field area that is of a higher % of one polarity than the other.This is the magnetic field area that can do work when in motion,or act upon a magnetically active substance. This is the null zone,and is clear and apparent in any test that requires a magnetic field to do work or induce flux into a feromagnetic material.

This(like electrical flow) can be shown with water,pipes and pressure differential. And this very same test will show why a hall sensor will show the same reading across the magnet from pole to pole when used as TK showed on his video-although that seems to be in conflict with what Mark said-maybe just a misunderstanding.

[tinman]

Highlighted for emphasis. Yes, even though a ferromagnetic "blob" or probe here will experience equal forces in both directions, hence will "feel like" a no-force situation, the flux still exists and can still do work by inducing current in a moving conductor just as we expect it to nearer the poles. 

Take the bar magnet and bend it around into a C-shape or a nearly full circle. What magic is this! You can still find your "force neutral" position between the poles with a ferromagnetic probe particle or reed switch, of course. But also of course you will be able to do work by spinning a coil of wire in the same position as your "forceless" probe or non-acting reed switch: See "electric motor" in  WIKI.

Please feel free to show me an inductor producing a current when the mid point(between the dipole)of a magnet is passed across that inductor.
Quote: But also of course you will be able to do work by spinning a coil of wire in the same position as your "forceless" probe or non-acting reed switch: See "electric motor" in  WIKI

Yes,but now you are introducing two more magnetic fields into the system,and have opposites attracting.