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'[EE]: Magnetic fields and forces'
2003\10\19@224623 by

I recently realised that didn't know the formula for the effect of one
magnetic field on another magnetic field source.

I now imagine that this is not the simple concept I originally believed it
to be.  Really what I'm looking at to start with is a formula for say, the
force that a solenoid of field B1 exerts on another solenoid of field B2
with a radius of separation of R in a one-dimensional system.

Initially, I expected something along the lines of the law of universal
gravitation, but I still haven't been able to find confirmation of this,
and furthermore, I found two conflicting sources, one claiming that the
relationship is inverse square, and the other that it is inverse fourth.

Anyway, I was hoping that someone on this list might have some idea what
the relationship is for the one dimensional case.  I am, of course,
interested in the two and three dimensional cases, but those I expect to be
far more complex.

--Brendan

--

> Initially, I expected something along the lines of the law of universal
> gravitation, but I still haven't been able to find confirmation of this,
> and furthermore, I found two conflicting sources, one claiming that the
> relationship is inverse square, and the other that it is inverse fourth.

Just to confuse, it's inverse cube ! :-)

Just to confuse further

http://www.conformity.com/0102reflections.html

Maxwell is your friend, allthough not always a nice one.

RM

--

>Just to confuse, it's inverse cube ! :-)
>
>Just to confuse further
>
>         http://www.conformity.com/0102reflections.html

This seems to me to be primarily in terms of electromagnetic radiation,
whereas I'm looking for magnetostatic configurations.  However, it is
probably still a useful read--which I will commence now.

>Maxwell is your friend, allthough not always a nice one.

I should have known it would come back to Maxwell's equations.  Silly
me.  Too bad I don't really know how to apply them yet.

--Brendan

--

Well, I have succeeded in refining the phrasing of my question.  It should be:

"I need an equation for the forces experienced by two colinear magnetic
dipoles of magnetic field B1 and B2, be they parallel or anti-parallel,
with a radius of separation R."

And, of course, an equation for *non* colinear magnetic dipoles would be
good too.

--Brendan

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What 'magnetic dipoles' ? You can use F=B*I*l (Laplace magnetic force)
where B/H=Mu for isotropic media (i.e. air coils and no armature).
H=I/(2*PI*d) at a distance d from an infinitely long wire carrying a
current I, and H=2*I/r in the center of a single turn coil of radius r,
H=N*I/l inside a long and narrow solenoid (the last formula is empirical
afaik - the exact version is very hairy). If the coil(s) have cores with
Mu different from air then B will be non-linear and probably change with
the inverse square of the distance if the distance between the poles is
reasonably large (larger than the diameter of the pole pieces) so
alignment errors do not play a major role. So F ~= N*I*B/(l*r*d^2) for d
>= 2r where B is the induction from a nearby magnet or other coil. Try a
book near you ?

If you have trouble with the maths you can use a scale with a nonmagnetic
table to get real life values for coils you make or obtain and magnets you
own (remember F=BIl, and F is proportional to Ampere-turns). You can even
calibrate your coils using a compass needle to get a fairly accurate
(+/-10%)  reading on B at some distance from the magnets/coils (earth
field ~= 45uT +/-5uT but not horizontal in most places, and NOT inside or
near armored concrete buildings).

Peter

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At 08:19 PM 21/10/2003 +0200, you wrote:
>What 'magnetic dipoles' ? You can use F=B*I*l (Laplace magnetic force)
>where B/H=Mu for isotropic media (i.e. air coils and no armature).
>H=I/(2*PI*d) at a distance d from an infinitely long wire carrying a
>current I, and H=2*I/r in the center of a single turn coil of radius r,
>H=N*I/l inside a long and narrow solenoid (the last formula is empirical
>afaik - the exact version is very hairy). If the coil(s) have cores with
>Mu different from air then B will be non-linear and probably change with
>the inverse square of the distance if the distance between the poles is
>reasonably large (larger than the diameter of the pole pieces) so
>alignment errors do not play a major role. So F ~= N*I*B/(l*r*d^2) for d
> >= 2r where B is the induction from a nearby magnet or other coil. Try a
>book near you ?

The very first thing I did was look in my old physics text book.  It didn't
seem to be any help, which is why I came here.  Perhaps I should have tried
a little harder to develop my own equation, I don't know.

This all looks very helpful, but let me make sure I've got the variables
right.  (sure would throw a wrench in the works if I had that wrong)

F: Force
N: number of turns in the coil
i: current in the coil
B: you stated
L: length of the coil
d: radius of separation between the coil and the other source of magnetic
field.

Now all I have to do is try to apply the Mu of a ferrous material (like a
steel bolt) to this equation.

Well, anyway, for a better idea of what I'm trying to accomplish, have a
look at http://members.shaw.ca/annirak/index.htm, and have a look at the
"[EE]: Active maglev with permanent magnets & solenoids" thread

Thanks for the help, that's pretty well does it.
--Brendan

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An interesting project.

In theory you should be able to build a proof of concept with permanent
magnets for your electromagnets. It looks like you are trying to create a
magnetic valley between the two electromagnets on each corner.
Since you are trying to use repulsive forces, I would suggest making them
point outwards from within the vertices of the platform,
rather than inwards as you currently do. That way the system is self balancing.

Think about the individual force vectors you are creating with the corner
magnets, what happens as the spacing changes, and what you need them to do for you.

What kind of loads are you planning to put on this platform? What size in
real life?

Robert

Brendan Moran wrote:
{Quote hidden}

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At 10:23 PM 22/10/2003 -0600, Robert Rolf wrote:
>An interesting project.
>
>In theory you should be able to build a proof of concept with permanent

At 09:03 AM 17/10/2003 +1300, Russell McMahon wrote:
{Quote hidden}

Couldn't have said it better myself.

At 10:23 PM 22/10/2003 -0600, Robert Rolf wrote:
>Think about the individual force vectors you are creating with the corner
>magnets, what happens as the spacing changes, and what you need them to do
>for you.

That's why I aimed the coils inwards.  Consider... Getting the most bang
for your buck.  I'm trying to create a semi-mobile, battery powered
system.  It needs to be energy efficient.  The larger the force vector I
can get out of a little electromagnet, the better.

>What kind of loads are you planning to put on this platform? What size in
>real life?

Something about the size of a smallish model tank.  A little bigger.  It
measures (non-uniformly) 6 1/4" x 4 1/2" x 2 1/4"  And that's pretty
light.  ~5oz+magnets all told.

--Brendan

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> >It can be shown mathematically (Earnshaw, 1842) that it is impossible to
> >produce STABLE static levitation using "normal" magnets.

Define "normal".

Is not the magnet levitating above a superconductor 'normal'?
Seems that all you need is superconducting corner supports and
it's a 'no brainer'.

Yes, I am aware that the Meissner effect is responsible
www.ornl.gov/reports/m/ornlm3063r1/demo1.html
but the magnet IS normal and it IS levitating.

> >Good basic introduction  at    http://www.physics.ucla.edu/marty/diamag/

Good info there. Thanks.

Have you considered using graphite in your configuration to make
a 'no power' solution?

> Something about the size of a smallish model tank.  A little bigger.  It
> measures (non-uniformly) 6 1/4" x 4 1/2" x 2 1/4"  And that's pretty
> light.  ~5oz+magnets all told.

So what exactly is the point of the project? Could you not float the triangle
by having the electromagnets above the corners and use the standard
'levitating globe at science fair' control code.

Robert

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>Is not the magnet levitating above a superconductor 'normal'?
>Seems that all you need is superconducting corner supports and
>it's a 'no brainer'.

Using cryogenic cooling units for a model seems a little excessive to me...

>Have you considered using graphite in your configuration to make
>a 'no power' solution?

If you can show me a geometry that looks workable, then I'm up for it.

> > Something about the size of a smallish model tank.  A little bigger.  It
> > measures (non-uniformly) 6 1/4" x 4 1/2" x 2 1/4"  And that's pretty
> > light.  ~5oz+magnets all told.
>
>So what exactly is the point of the project? Could you not float the triangle
>by having the electromagnets above the corners and use the standard
>'levitating globe at science fair' control code.

I thought that I had made the point of the project clear.  Apparently
not.  I want to levitate a futuristic model "flying" tank for a table top
war game/hobby.  To this end, a frame over it would be best
avoided.  Having it spin is also to be avoided.

Let me set the record straight here.  This is not a "school project"  this
is not a "term project"  this has nothing to do with school, and little to
do with my past areas of study (electronics and electronic control systems).

I got the impression that some people had begun thinking that this was a
science fair type project, and that I was of the age to be making
such.  That is not the state of things.  I graduated from my tech school a
while back.  I simply do not have the physics background required to know
the equations for magnetic repulsion/attraction.  I suppose I should have
been able to derive them, but when I tried a while back I didn't get very
far.  I guess I took the wrong line of reasoning, or didn't draw it out far
enough.

If and when I get this thing workable, and operating, I'll post a link to
pictures of the system.  It's unlikely to be soon, since there will be a
largeish amount of painting after the control system is done, then packaged
in some kind of base.

I've updated the site I previously linked (http://members.shaw.ca/annirak)
of the models that I'm looking at trying to levitate.  If you want to show
me visually some idea for a configuration, then a POV-Ray file will work
fine for me if you don't want to link or email a diagram.

Thanks for all the help so far from all the people who have contributed.

--Brendan

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So you want to let some futuristic models fly above a table?
There's no way to do this with supercondutors or some kind of graphit!
You have to think different.
Use the repulsive forces of two equal magnet poles to let the model fly,
and then use the attracting force of opposite magnet poles to hold
the model in position! You'll have to use some more magnets for the
fly than for holding.
I think there's no other possibility to do this, unless you have  the money
for an ultrasonics solution.

MfG,
Do.Pe.

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On a similar issue, please see:

http://www.cable.net.nz/ou/coils%20page.html

http://www.cable.net.nz/ou/

This guy is challenging Lenz's law...

On 23 Oct 2003 at 15:22, desertFOL.DYNDNS.ORG wrote:

{Quote hidden}

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