Magnetic lines of force

"Jeff Root" wrote in message
om...
George Dishman replied to Jeff Root:

The shape of the filings is obviously significant. Do you have
any suggestions for getting filings which are nearly spherical,
and which are not themselves magnetized? Maybe a particular
kind of iron or steel?

No, but I would use thin pointy ones anyway so they show the
direction of the field.

Without an enormous effort to sort them out, I will take what
I get from filing. The filings are too small to make out very
well with a 30-power magnifier, but they appear to be fairly
uniform in size, and random in shape. No long, thin, pointy
ones at all, though. But little random globs clinging together
give the appearance of long, thin, pointy needles.

It seems I misunderstood slightly, I thought you felt a
spherical filings were necessary. Certainly use them all,
asymmetry in the shape will help the demonstration but is
not necessary.

Just now I filed a nail with two different sizes of file. The
bigger, slightly coarser file gave bigger filings, but it isn't
clear that the size and shape distribution is different.

That was an editing error. It isn't clear that the shape
distribution is different, because my magnifier isn't powerful
enough for me to see the shapes easily. The finer file did
definitely give smaller filings, and the coarser file gave
larger filings.

No, problem, use the finer ones. Another trick that will
help is to place a loudspeaker somewhere nearby and play
some music at the whole thing. Keep the speaker far enough
away that its magnet doesn't drag the filings away of course.
The vibration of the paper in response to the sound will
overcome static friction and the force from the magnet only
needs to introduce a slight asymmetry in the dynamic friction
to let the filings move. Turn the volume down slowly and see
where they settle.

I made the filings onto a hard plastic tray from the kitchen.
It seems to have lower friction than some paper. When I apply
a magnet, the itty-bitty little filings climb up on each other's
shoulders to do a flea-circus stunt where they make relatively
huge needles (compared to the size of the filings) which stand
up from the tray surface.

With just a single filing, is there any evidence of
preferred locations that it wants to go to?

That has turned out to be a very hard question to answer.
I think a proper answer might require more thorough examination
of the setup and careful measurement of the observations than I
want to bother with.

Filings in water in a thin, transparent, styrene tray are
noticeably retarded by friction with the tray surface. Filings
floating on the surface of that water move freely. A floating
filing goes to a point over the magnet. When I move the magnet,
careful to keep the orientation unchanged, the filing stays in
the same position relative to the magnet. That is what I would
expect, lines or no lines. However, the coupling between the
magnet and filing is loose, not tight. the filing will slosh
back and forth with the water while being held by the magnet.

What you have seen is a good indicator. The filing just
moves to the magnet's pole, and the motion should be
smooth and regular with no indication of little spurts
as it goes from line to line.

Some factors seen to affect motion or positioning of filings:

- Number of filings
- Friction between the filing(s) and the surface
- Shape of the magnet
- Orientation of the magnet
- Changing the orientation of the magnet
- Changing the direction of motion of the magnet

The most remarkable effect is clumping of the filings into
well-separated stacks. When filings stand up on the surface,
they may all be in one clump, or in many separate clumps which
are fairly evenly-spaced. Larger clumps tend to be spaced
farther apart from each other. Moving the magnet away tends
to make clumps move apart.

You have to remember that the field is really 3D. Scattering
the filings thinly on the paper is an attempt to take a 2D
slice through the field.

Some orientations and directions of motion of the magnet lead
to a single large clump; other orientations and motions lead
to many small clumps which often look like a hedgehog.

Most tellingly, perhaps, some orientations and directions of
motion of the magnet give a really neat circulation pattern of
the clumps, rather than a simple linear motion.

This tries to explain why the lines form (without glue!)

http://home.clara.net/george.dishman/Jeff/Filing_1.htm

In answer to the final question, consider what you say above:
"Filings floating on the surface of that water move freely.
A floating filing goes to a point over the magnet."

These is perhaps one of the better web pages I have found:

http://www.ndt-ed.org/EducationResou...cFieldChar.htm

This lists the rules for "lines":

http://www.tpub.com/neets/book1/chapter1/1i.htm

This is a kit sold as a teaching aid. Note in particular
the comment headed "Suggestions for Presentation"

http://www.physics.purdue.edu/demo/6...d_magnets.html

whereas this site treats the lines as if they
were real:

http://wow.osu.edu/Magnetism/magfieldlines.htm

If you have a cow handy:

http://www.exploratorium.edu/snacks/magnetic_lines.html

There is another way to look at this too. The field
at any point near the dipole can be seen as the vector
sum of two components from the two end "monopoles". At
any point you can calculate that and draw an arrow to
represent the sum vector. If you then take the end of
that arrow and calculate the vector at that point and
then repeat the process, you will get a line of little
arrows that ends at the pole.

However, the calculation of the field is valid
everywhere, not just on lines. Start at any point and
you will end up at the pole. Any starting point gives
a single path to the pole because the field has a
specific value at every point.

Lines cannot cross because from the point where they
did, you would have two paths to the pole but you
cannot have two values for the direction of the field
at any given point.

George

Jeff Root wrote in message
om...
George Dishman replied to "greywolf42":

[Jeff root wrote:]
Could you apply this same kind of wonderfully clear exegesis to
the term "magnetic lines of force"? I keep seeing intimations
that they have some kind of reality. Thank you!

I understand him to be asking 'are the lines formed in
the filings indicative of real lines in the field'.
...
I think you haven't grasped the essence of Jeff's question:
are the lines formed in the filings indicative of 'real'
lines in the field? My answer was no, the lines are formed
as a consequence of the distortion produced by the presence
of the filings themselves.

"greywolf42" replied to George Dishman:

And I understand him to be asking 'are the lines formed in the
filings real?' A 'field' is not 'real'. It is a mathematical
device. Useful, but not physical -- or 'real.' If you try to
make 'reality' in a fiction (the field), you are going to fail.

The magnetic field is obviously real. I see its effect on iron
filings. I see its effect on a magnetic compass. I feel its
effect when I hold a magnet near a large piece of iron or steel.

A field is still not real (physical). You see the effect of something
'physical'. We can map that physicality with a mathematical field. And we
can use Poisson's method to make calculations. But the field cannot itself
be observed. FORCE is observed. The 'field' is a mathematical bookkeeping
method to track forces.

My question had nothing to do with filings, only with "magnetic
lines of force", whatever those are supposed to be. I know what
magnets are; I have a pretty good idea what magnetism is; I know
what lines are; I know what forces are. I'm not certain what
"magnetic lines of force" are.

That is why I directed you to the works of Faraday and Maxwell. 'Lines of
force' were experimentally identified, defined, and quantified by Faraday.
Maxwell theoretically derived them. That *IS* the exegesis of the term.

My question was specifically directed to George Dishman.

It was directed to a newsgroup. And thus fair game. You don't have to read
my replies, but don't get huffy if someone other than George replies. If
you didn't want anyone else in the game, send an e-mail.

I was
sure he would understand my question as I worded it, and he did.
However, it was worded in such a way that one pretty much needed
to know the answer in order to understand the question. It is
hard to word it in such a way as to be clear to someone who
doesn't already know what I mean, and how I have been confused
by other peoples' use of the term "magnetic lines of force".

"Exegesis: Critical explanation or analysis; especially, interpretation of
the scriptures." And Faraday and Maxwell are the closest to 'scriptures'
that we can get for "Lines of Force"

I have read descriptions of "magnetic lines of force" in which
an analogy was drawn to lines of latitude or longitude, or to
lines of elevation on a topographic map. Those struck me as
awfully poor analogies.

The directions of those lines are
nothing like the directions of the force vectors in a magnetic
field. If anything, lines of elevation are at a right angle
to the direction that analogous lines of force should be.

Analogies are always poor substitutes for understanding the fundamental
physical causality.

But I sometimes come across a text in which the "lines" in a
magnetic field are talked about in a way which implies that
they are more than just lines showing the direction of the
force vector. I don't understand that, and that is what I
was asking about.

And that is why I posted sources where you could find out what you didn't
understand.

George Dishman wrote:

What is important is whether there is evidence for "lines"
in the plot of force versus distance

Not knowing what is supposed to be meant by the expression
"magnetic lines of force", I didn't assume that the force
measured on a line would be different from the force measured
between lines. The lines could be there, perfectly real, but
not make any difference in the force, for all I knew.

Also, I wouldn't assume that individual lines, if they existed,
could be detected by *any* means, just as it is not possible to
detect individual photons of radio waves.

But there's no need to assume. That's the beauty of the scientific method!
Go get a piece of paper, a magnet and some iron filings! If the "lines of
force" are real, you won't need glue.

greywolf42
ubi dubium ibi libertas

Jeff Root wrote in message
om...
"greywolf42" replied to George Dishman:

In my original post I used two filings to illustrate how
lines could be formed in a field that has no inherent
line structure.

Unfortunately, you were responding to a post about the
*question* whether there *was* an 'inherent line structure.'
Your post about 'gluing' a filing to the paper before bringing
in the magnet simply confused the issue.

It didn't confuse me. However, I interpreted the proceedure
George described to be that the filing was glued down after it
had settled into alignment with the magnet, not before. His
post was not explicit, but the implication was so obvious that
it hadn't occurred to me to interpret it the way you did.

George's post *WAS* explicit. And it included gluing the filing down
BECAUSE it would 'fly off to' and 'attach itself to' the magnet if he
didn't. This physically requires the magnet and the filings to be on the
same side of the paper:

"Place a single filing on the paper and glue it down so it doesn't fly off
to the magnet. " (His first post in this thread)

"If you drop a single filing it will always attach itself to the magnet if
it is free enough to move" (2nd post)

"Yes, my reply was misleading. I meant it would always get as close as
possible to the magnet, i.e. above the pole on the other side of the paper."
(3rd post)


But it is not 'illustrative' of the issue under discussion.
People can do the experiment without glue and without placing
the magnet on top of the paper.

The fact that you thought George was saying to put the magnet
on the same side of the paper as the filings-- and you repeated
that error in another post-- shows that your comprehension
isn't necessarily as good as you think it is.

As George clarified, above, it was his writing error. ("Fly off to the
magnet", "attach to the magnet").

The demonstration with a glued-down filing could be very
useful if carefully done. It would show exactly what George
intended it to show: The effect of one filing on another.
That wouldn't be possible if one filing were not glued down.

True. But what George was trying to show had nothing to do with the
question of the physicality of magnetic lines of force. The latter has no
need of glue.

greywolf42
ubi dubium ibi libertas

"greywolf42" wrote in message
...

Jeff Root wrote in message
om...
It didn't confuse me. However, I interpreted the proceedure
George described to be that the filing was glued down after it
had settled into alignment with the magnet, not before. His
post was not explicit, but the implication was so obvious that
it hadn't occurred to me to interpret it the way you did.

George's post *WAS* explicit.

Jef is right, my post was not explicit about whether it should
be allowed to align first or not. He correctly guessed what I
meant.

And it included gluing the filing down
BECAUSE it would 'fly off to' and 'attach itself to' the magnet if he
didn't. This physically requires the magnet and the filings to be on the
same side of the paper:

"Place a single filing on the paper and glue it down so it doesn't fly off
to the magnet. " (His first post in this thread)

My mistake, "towards the magnet" would have been clearer.

"If you drop a single filing it will always attach itself to the magnet if
it is free enough to move" (2nd post)

"Yes, my reply was misleading. I meant it would always get as close as
possible to the magnet, i.e. above the pole on the other side of the
paper."
(3rd post)

Agreed, it was sloppy writing.

But it is not 'illustrative' of the issue under discussion.
People can do the experiment without glue and without placing
the magnet on top of the paper.

The fact that you thought George was saying to put the magnet
on the same side of the paper as the filings-- and you repeated
that error in another post-- shows that your comprehension
isn't necessarily as good as you think it is.

As George clarified, above, it was his writing error. ("Fly off to the
magnet", "attach to the magnet").

The demonstration with a glued-down filing could be very
useful if carefully done. It would show exactly what George
intended it to show: The effect of one filing on another.
That wouldn't be possible if one filing were not glued down.

True. But what George was trying to show had nothing to do with the
question of the physicality of magnetic lines of force. The latter has no
need of glue.

What I was trying to show was that the filings themselves
behave in such a way as to produce lines therefore their
existence cannot be taken for evidence of thin regions of
greater force created by the magnet.

Now chaps, since the confusion was all my fault and I have
apologised for that, can we drop the nitpicking and return
to the physics please.

George

George Dishman wrote in message
...

"greywolf42" wrote in message
...

Jeff Root wrote in message
om...

{snip discussion over wording}

What I was trying to show was that the filings themselves
behave in such a way as to produce lines therefore their
existence cannot be taken for evidence of thin regions of
greater force created by the magnet.

Now chaps, since the confusion was all my fault and I have
apologised for that, can we drop the nitpicking and return
to the physics please.

Certainly. Grab some paper, a magnet, and some filings.

greywolf42
ubi dubium ibi libertas