Magnet Myths and Misconceptions

[itsu]

Itsu:

If you watch your clip at the end, the dip when you are doing the low scan is not replicated when you do the high scan.  On the high scan the 'dip' now becomes the peak.  So you can say it's 'opposite.'

That all makes sense relative to the standard field pattern.  The dip at the bottom is due to the flux density decreasing at the half-way point.  When you do the high scan, the probe is "entering the bubble" and then "exiting the bubble" with respect to the magnetic field there.  So you get a peak at the center of the bubble because at that height above the magnet you are in the densest flux stream in the middle.

Big Ben is still chiming!

MileHigh

I agree, it looks like that, but thats probably because my hasty video job on the upper flux, in reality its different, see this new short video:


https://www.youtube.com/watch?v=zShIcIsvBS4&feature=youtu.be


Regards Itsu

[allcanadian]

@TK
I downloaded a magnifier app for my smartphone and got the part number.


Allegro A1302, Continuous-Time Ratiometric Linear Hall Effect Sensor.
www.allegromicro.com/~/media/Files/.../A1301-2-Datasheet.ashx[/font][/size]


AC

[NoBull]

Storing energy by magnetizing  _permanent magnets_ and attempting to recover that stored energy is inefficient.

How would you do it, anyway?

Maybe cut a bar magnet along its axis in half while it is in an air solenoid?

xxxxxxxxxxx
   N-------S
   N-------S
ooooooooo

The two halves would counterrotate along their midpoint and the end result would be a pair of magnets pointing like this

xxxxxxxxxxx
   N-------S
   S-------N
ooooooooo

This pair of magnets would produce zero net flux inside the solenoid
...so the solenoid would experience large dΦ/dt during this counterrotation which would result in large induction according to Mr. Faraday.

Could this process be reversed by energizing the solenoid from outside?

[TinselKoala]

Itsu:

If you watch your clip at the end, the dip when you are doing the low scan is not replicated when you do the high scan.  On the high scan the 'dip' now becomes the peak.  So you can say it's 'opposite.'

That all makes sense relative to the standard field pattern.  The dip at the bottom is due to the flux density decreasing at the half-way point.  When you do the high scan, the probe is "entering the bubble" and then "exiting the bubble" with respect to the magnetic field there.  So you get a peak at the center of the bubble because at that height above the magnet you are in the densest flux stream in the middle.

Big Ben is still chiming!

MileHigh

Not exactly. In the scan with the sensor at right angles to the long axis of the magnet stack, the "dip" and the "peak" are indicating exactly or pretty much the same flux density, just different directions wrt the sensor plane. Recall that the center, or zero baseline, of the scope trace is indicating _zero flux_ through the plane of the sensor, and positive deflection indicates flux in one direction wrt the sensor plane, and negative deflection indicates flux in the opposite direction. The difference between "peak" and "dip" only indicates the direction of the flux through the plane of the sensor. This difference is because the sensor is flipped 180 degrees from one scan to the other, so the flux passes through it in the opposite direction relative to the sensor orientation. The "peak" and the "dip" in Itsu's video are showing the point where the field lines are the most parallel to the long axis of the magnet stack and have the least density, just as shown in the conventional picture of the field lines. If the "peanut waist" picture were true the data would be very different than what Itsu has demonstrated. All of the above paragraph refers to the scans where the sensor is held at right angles to the long axis of the magnet stack.

When the sensor is held so that the plane is parallel to the long axis of the magnet, the sensor output goes to zero or rather the zero baseline of the scope trace, indicating _no flux through the sensor_ at the center of the stack. This means that the field lines are parallel to the plane of the sensor at that point, fully confirming the conventional view of the field lines. If the "peanut waist" view were correct, there would be a maximum reading (well above the baseline) just to one side of this null point and a minimum reading (well below the baseline) just on the other side. But this is not what happens: the maximum reading occurs at one pole, grading smoothly to zero at the center, and grading smoothly to the minimum reading at the other pole. Very different from what is predicted by the "peanut waist" or "Bloch wall" picture.  The above paragraph refers to the scans where the sensor is held with the plane parallel to the long axis of the magnet stack.

I think Itsu's demonstration might be more stable if he used the wide part of the magnet stack instead of the narrow part. You can see how difficult it is to hold manually the proper orientation of the Hall sensor, and deviating slightly from the centerline of the narrow face of the stack causes fluctuations in the reading from the sensor. This effect would probably be less if he scanned the wider face of the stack.

Itsu's demonstration appears to me to be reporting the identical picture of the field line direction and density that my own demonstrations provide, and is refuting the "peanut waist" picture and confirming the conventional picture of the direction and density of the field lines around the stack of magnets.


Part 2, the parallel scan of my magnet stack, will be viewable in a few minutes:
http://youtu.be/OTe4rNwrZKY

http://www.youtube.com/watch?v=OTe4rNwrZKY

[MarkE]

Agreed, however I think tinman is saying that even with that orientation of the sensor, you will measure 0 net flux because the N-curl + the S-curl are opposite at the center and will cancel in the sensor.

tinman may correct me if I interpreted him incorrectly.

Tinman keeps conflating flux and flux density.    Take 100 little disc magnets like TK has in his video.  Scan them as one long magnet and get TK's results.  Pull one a foot away and scan again and they will look like two separate magnets.  Do the same thing but separating 10/90, and get the same qualitative results.  So, yes the vectors add.  But there is nothing special about how they add at the center versus closer to the ends.  The vectors all add up anywhere along the magnet, and that fact does not hide something unique happening near the middle of the dipole.