Magnet Myths and Misconceptions

[NoBull]

The field at the center (and outside) of the dipole is parallel to the dipole

Correct

and does not change direction.

Wrong.
The flux in the radial center of a cylindrical bar magnet has an opposite direction to the flux outside of the magnet.
Somewhere along the radius of the magnet, these two fluxes must sum to zero by simple vector addition.

[poynt99]

Wrong.
The flux in the radial center of a cylindrical bar magnet has an opposite direction to the flux outside of the magnet.
Somewhere along the radius of the magnet, these two fluxes must sum to zero by simple vector addition.

I specified the field outside the magnet. Perhaps you missed that important piece of information?

[allcanadian]

@Poynt99

If so, then that is not a correct statement. The field at the center (and outside) of the dipole is parallel to the dipole and does not change direction. The flux density outside the magnet at the center is however weaker compared to the ends. But if you were able to insert your Hall probe into the middle of the magnet material at the center, you would find the flux density just as high, if not higher than what you might measure at either end of the magnet. Agreed?

Some interesting experiments this morning with the hall probe, more like confusing,lol.
First I would like to say when taking measurements my intent is not to prove anything one way or another but to understand what I see.


1)First my thinking was that if Mark is correct then logically I should be able to point the hall sensor face at the North pole and rotate the sensor one way or the other to "follow" the maximum field density of a given polarity. That is I pretend to follow a "loop" from the N to S pole and if there is a max value of constant polarity it should show me the way...in fact it does and Mark is right. I should note I am not following a loop I am trying to maintain a max reading of same polarity within any given space which happens to follow what appears to be a loop from pole to pole.


2)However, and this is a big however if I start with the hall face pointing towards the N pole and follow the magnet contour around the corner to the side the max reading and polarity is also maintained, the same hall face is always facing inwards towards the magnet. In fact I can follow a line from the N pole center around the edge along the side of the magnet and the reading is constant at max value and same polarity until a very small region near the side/center of the magnet at which point it reverses. The polarity remains constant on both ends/sides of the magnet as I move away from it as does the zero value at the center. This measure suggest there are no loops but straight lines acting away from the magnet center outwards in all directions not unlike rays from the Sun. The measures suggest two half spheres of different polarity with a zero boundary condition between the two. No loops but lines moving away from the magnet center in all directions.


Now measure (1 )suggests Mark is correct and it would seem I can follow imaginary lines looping from pole to pole by rotating the sensor. However measure (2) with the hall face always pointing towards a point in the center of the magnet suggests there are no loops but lines, two half spheres of differing polarity acting outwards in all directions.


The confusion would seem to lie in the fact that in between the two planes of the poles the sensor shows a constant max value and polarity if the sensor face is pointing in the same direction as the poles N-S axis. However if sensor is always facing towards the magnet center we see two half spheres of different polarity.
Another note, just above or below the side center region the hall sensor can be rotated 90 deg and the value and polarity do not change in any way, very strange.


This was unexpected and I will leave it with everyone here to make sense of it.


AC

[TinselKoala]

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.

Simply by bending a bar magnet into a C-shape I have introduced NO new magnetic fields! I have simply redirected the flux so that instead of "flowing" along the long axis of the bar magnet from pole to pole,it is now flowing from pole to pole out in a space away from the body of the magnet. This takes the flux that formerly was surrounding the bar magnet and concentrates it in the region between the poles or pole pieces. It is the _same flux_ that formerly flowed along the sides of the magnet from pole to pole. The opposites attract just as they did in the straight magnet. The center region between the poles has just been moved out to the side, and instead of being spread around the body of the magnet it is now concentrated mostly to the area defined by the width of the poles or pole pieces. Just as when you hold a hose or water nozzle perfectly vertical, the water flows down all around the outside of the nozzle and would have a hard time driving a turbine wheel, but if you bend the nozzle over, now the water flows in a stream away from the hose body, making it easier to drive a water wheel. Nothing new has been introduced, the position of the flow (water) or magnetic flux (magnet) has just been relocated away from the magnet body to make it easier to use.  Now I think you are really grasping at straws.

The Hall sensor test in my video definitely shows that there is a flux at the midpoint of the magnet, and since it is giving almost the same reading there as on the pole itself, it shows that the flux is going mostly straight through the plane of the sensor, which is held perpendicular to the long axis of the magnet. Please refer to my "7 questions". This _fact_, which can be observed in my video, shows the same thing that a moving wire would show in the same position. Recall that current is induced in a wire that "cuts" across magnetic field lines. This means at the exact center region of the magnet, the wire would have to be moving radially towards and away from the body of the magnet to "cut" the field lines perpendicularly. This is kind of hard to arrange experimentally with actual straight magnets and wires, but the Hall sensor shows the same thing: see "hall effect" in your favorite reference to see how the Hall voltage is produced. Also it is demonstrated by 2-piece homopolar dynamos, where a strong current is produced by moving a conductor, at right angles to the flux, through the midpoint of the field between the poles of a U or C shaped magnet.

[MileHigh]

I think a few simple drawings would aid in people's descriptions from time to time.

Anyway, I can see that Tinman is close to getting a better understanding.  I note it looks like he has a notion of 'north flux' and 'south flux' originating at each pole and where they meet half way they 'cancel each other out' and then there is no 'useful field' there.

NoBull mentioned the field gradient which is very important.  A perfectly straight set of magnetic field lines has no gradient and therefore no net pull in any direction on a piece of ferrous metal.  Although it will still line up the metal in the field like an iron filing.  That's a clue for Tinman right there.  There is no pull in either direction but there is a torque on the metal piece half-way between the poles if it is rectangular in shape.   Same thing for the reed switch.  There is torque on the reed switch at the half-way point if it is not lined up with the magnetic field.  You just don't feel it when you are handling the reed switch which is connected to wires, etc.  Likewise a rotating loop of wire at the half-way point will generate EMF as long as it cuts the field lines.

The gradient in simple terms just means that the magnetic field is stronger on one side of ferrous object than the other side.  That will then 'suck you into the pole.'  No field strength gradient then no 'sucking.'  The half-way point between poles has an infinitesimally thin plane of zero gradient.  On either side of the plane the gradient is extremely low and therefore it's very difficult to feel any attraction there with your hands.

Tinman deserves tons of credit because he wants to defend his points and actually debate.  That can lead to learning.  Meanwhile Chris/EMJunkie played an aloof character and as time went on I think he started to realize how little he really knew and he had to try to bluff his way through.  That's not the way to learn about this stuff.  It's a classic case of putting on blinders.

So much of a tempest in a teapot over the same old drawing of the magnetic field around a bar magnet that we have all seen since we were children.  But it's actually a good exercise because the more you learn and understand the more your mind is able to 'cut through the crap' when you look at somebody's free energy proposition, as an example.

And it's interesting how some people will intentionally stick to their bad ways just the same.

MileHigh