Magnet Myths and Misconceptions

[tinman]

Tinman:

Look at the image for the magnetic field for a single loop of wire.

A coil is just a bunch of loops of wire in a row.  So what does the magnetic field look like?  It's just a bunch of magnetic fields from single loops of wire added together.  Just do the vector addition in your head, forget about all of the strategies for making physical measurements for a second.

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

MileHigh

No different than adding disk magnets together MH,the more you add ,the larger and stronger the field grow's(to a point) The field from the first loop is additive to the field of the next loop.

[tinman]

Note what they have to say about useing iron filings to show a magnetic field.
http://www.magnetage.com/The_Figure_Eight_WYWS.html

[MarkE]

author=tinman link=topic=14974.msg432056#msg432056 date=1420859540]
What a lot of crap Mark.Are you now telling TK how to run this test so as it shows your uniform field from one end to another?.

No, I am telling TK what to expect under two different hypotheses.

Quote:- If the figure eight idea is correct then there should be a big increase in flux density close to the dipole center.

Bullshit. There should be a decrease as the two opposite fields cancel each other out.

Ah, perhaps the light is coming on for you.  If there is no orthogonal flux, then there is no evidence that the flux forms a figure eight.  So let's extend this:  What is to say that there aren't dozens or hundreds of figure eights between the poles that all cancel out, leaving us with the consistent field distribution shown in countless experiments and established theory?  IOW, while we could consider any dipole to be the linear concatenation of many dipoles, once we do the vector math we get to the conventional view of net flux taking a contiguous path from pole to pole.  And in that either one must either adopt a representation of an infinite number of loops around an infinite number of dipoles which is of no practical value, or one must take the net sum which gives us what all reliable experiments show:  a contiguous field from pole to pole.

Quote:- A plot of the sensor output will be a non-linear, but monotonic line moving along from one end of the magnet to the other.

And why would it be non-linear if this field is suppose to be parallel to the dipole.

Because the field is developed by a dipole, the contours form a curve.  The curve comes dead parallel to the dipole at the center.  Everwhere else along and outside themagnet center the curve has a slope.

If this parallel field dose not change from one end to the other of the dipole,then neither should the readings from the hall sensor. If there is a change as i am saying,then the hall sensor will pick up that change near the center of the dipole.

If wishes were granted....  The field follows contours.  It is not a step value somewhere near each pole and some other fixed value in between.  The flux, and consequently the flux density perpendicular to the magnet comes to zero only at the midpoint.

@TK
I have said nothing in support of this bloch wall crap,and i am fully aware that a bloch wall dose NOT exist at the center of the dipole. If we face two north fields together from two magnet's,and place a steel bar between those two apposing fields,then you will have what might represent a bloch wall,as the domains now are facing opposite directions-are not aligned.

[MarkE]

Here is a really simple experiment. Lets just take a reed switch,LED and a 2.5 volt power source.Hook all in series,and then run the reed switch along the length of the magnet. If the LED stays on,then there is a magnetic field,if it go's out near the center of the dipole,then there is no magnetic field.

How dose a reed switch work-Quote wikipedia- The reed switch is an electrical switch operated by an applied magnetic field. It was invented at Bell Telephone Laboratories in 1936 by W. B. Ellwood. It consists of a pair of contacts on ferrous metal reeds in a hermetically sealed glass envelope. The contacts may be normally open, closing when a magnetic field is present,

You need to learn the difference between:  field, flux, and flux density.  A reed switch closes when sufficient flux at a high enough density is orthogonal to the reeds to overcome the spring force that holds them apart.   Reed switches use iron leads typically with Cu cladding.  This makes each reed into a pole shoe.  If you place a reed switch next to a little disk magnet, the reed closes when it is parallel to the magnetization axis because the leads act like pole shoes linking each one of the reeds to one pole or the other.

[MarkE]

Think about that again, please. The question referred to a Hall sensor whose plane is perpendicular to the long axis of the magnet. Perhaps you missed that part. If you look at Tinman's drawing above, you can see that in the central region the field lines are roughly parallel to each other and to the long axis of the magnet, and maintain a nearly constant density for some distance. The longer the magnet, the more parallel they are and the more constant the density along the mid portion of the magnet's length. Hence, a perpendicular Hall sensor will experience flux very nearly perpendicular to its plane and also with almost constant density while being scanned along this region. In fact, if the Hall sensor is rotated appropriately at the curly ends, it is possible to maintain a nearly constant reading from fully in contact with the pole, following around the curly corner, scanning along the length of the magnet, then curling around the corner again at the other pole. This action keeps the plane of the Hall sensor perpendicular to the field lines, as drawn in the diagram above, during the entire scan, and the density of the flux intercepted by the Hall sensor can also remain nearly constant throughout the scan including the curling portions, somewhat dependent on the actual geometry of the magnet. A fat "bar" magnet whose width is on the same order as the length will have less concentrated flux at the curling portions than will a long skinny magnet.

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.