Magnet Myths and Misconceptions

[NoBull]

There is one caveat here that I think Itsu has stumbled upon.

When a cylindrical bar magnet is mapped by a Hall sensor whose face is perpendicular to the cylinder's axis (which is also the magnetization axis) and scanned along the radius of the cylinder that crosses its axial midpoint, then this sensor would indicate the B magnetic field in the opposite direction outside the magnet than inside the magnet (if it could penetrate inside the magnet).

This means that at one point somewhere along this radius, the Hall sensor would indicate zero.

[tinman]

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!"

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.

Can you run the test with the hall sensor face flat on the magnet as Mark says to TK?.

[tinman]

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. 

OK,who's test method is correct? your's or TK's,as TK has the hall sensor at right angles to the plane of the magnet,and you say face flat on the magnet

[MarkE]

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!"

That resolves that discrepancy.  However I think that if one wants to know whether flux turns back into the magnet half way along, the direct way to measure that is with the face of the sensor right up against the magnet.  If flux at a detectable density curls back into the magnet then that orientation will detect it.  Perpendicular as you propose should show a double camel hump distribution.

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.

Perpendicular you should see a double camel hump or  "peanut" waist because near each pole much of the flux is curled towards the pole face, and near the middle the flux density is lower overall.

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.

I quite agree that the magnet aspect ratio is a significant factor to the aspect ratio of the camel humps.

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.

[MarkE]

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.

As is the flux at any slice.  There is nothing special about the vector addition of the left 1/100th of the magnet with the right 99/100ths of the magnet.  The net field is the vector sum of all the infinitesimal slices of the magnet.  Consider what the fields would look like if we took five or fifty or five hundred magnets all aligned, but first well spaced from each other and then brought together in different groupings.

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.

Again it looks like you are mixing up:  flux, flux density, and in the case of induction the vector orientation of the flux.