Changing or moving magnetic field

[granpa]

is an emf produced by a changing magnetic field or by a moving magnetic field?

 

[ranger]

The induced emf will be produced if the magnetic flux is changing through the element. Whether it field is moving wrt to the element or the field is changing, it does not matter. Just the flux density has to change.

 

[granpa]

what if the magnetic field is moving but the flux density does not change?

 

[ranger]

You should read up on Faraday's Law.
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/farlaw.html

 

[granpa]

http://en.wikipedia.org/wiki/Moving_magnet_and_conductor_problem

Einstein's 1905 paper that introduced the world to relativity opens with a description of the magnet/conductor problem.[1]

“ It is known that Maxwell's electrodynamics--as usually understood at the present time--when applied to moving bodies, leads to asymmetries which do not appear to be inherent in the phenomena. Take, for example, the reciprocal electrodynamic action of a magnet and a conductor. The observable phenomenon here depends only on the relative motion of the conductor and the magnet, whereas the customary view draws a sharp distinction between the two cases in which either the one or the other of these bodies is in motion. For if the magnet is in motion and the conductor at rest, there arises in the neighbourhood of the magnet an electric field with a certain definite energy, producing a current at the places where parts of the conductor are situated. But if the magnet is stationary and the conductor in motion, no electric field arises in the neighbourhood of the magnet. In the conductor, however, we find an electromotive force, to which in itself there is no corresponding energy, but which gives rise--assuming equality of relative motion in the two cases discussed--to electric currents of the same path and intensity as those produced by the electric forces in the former case.

At the time of Einstein in 1905, the field equations as represented by Maxwell's equations were properly consistent. The Lorentz force equation, however, had to be modified in order to provide unique particle trajectories upon which all observers could agree.

 

[pmb_phy]

is an emf produced by a changing magnetic field or by a moving magnetic field?

It is an error to think that field lines move. It can lead to calulations which are wrong. For example: Consider a rotating magnet like the one in this page

http://www.geocities.com/physics_world/em/rotating_magnet.htm

If one were to claim that the field lines are rotating with the magnet then calculated the force per unit charge and called that an "electric field" then when you take the curl you will not get zero as is reqwuired by Maxwell's equations. I have two articles on the idea of moving field lines and how it is meaningless to speak of them. Say the word and I'll make them available.

Best wishes

Pete

 

[granpa]

rotating magnet

It is an error to think that field lines move. It can lead to calulations which are wrong. For example: Consider a rotating magnet like the one in this page

http://www.geocities.com/physics_world/em/rotating_magnet.htm

If one were to claim that the field lines are rotating with the magnet then calculated the force per unit charge and called that an "electric field" then when you take the curl you will not get zero as is reqwuired by Maxwell's equations. I have two articles on the idea of moving field lines and how it is meaningless to speak of them. Say the word and I'll make them available.

Best wishes

Pete

http://www.geocities.com/physics_world/em/image_gif/em11-im-01.gif

considering how magnetic fields are produced i am not sure i would expect the field of a rotating magnet like that in the picture to be moving anyway.

 

[jostpuur]

At the time of Einstein in 1905, the field equations as represented by Maxwell's equations were properly consistent. The Lorentz force equation, however, had to be modified in order to provide unique particle trajectories upon which all observers could agree.

Wait a minute... The Lorentz force equation is Lorentz invariant too.

 

[theperthvan]

It is an error to think that field lines move. It can lead to calulations which are wrong. For example: Consider a rotating magnet like the one in this page

http://www.geocities.com/physics_world/em/rotating_magnet.htm

If one were to claim that the field lines are rotating with the magnet then calculated the force per unit charge and called that an "electric field" then when you take the curl you will not get zero as is reqwuired by Maxwell's equations. I have two articles on the idea of moving field lines and how it is meaningless to speak of them. Say the word and I'll make them available.

Best wishes

Pete

Please will you upload them.
cheers

 

[pmb_phy]

Please will you upload them.
cheers

Sure. It'd be my pleasure to. Here are the two of them -

http://www.geocities.com/pmb_phy/Webster_1963.pdf

http://www.geocities.com/pmb_phy/ajp_apr1961_webst.pdf

This one I just found now and is straight to the point.

http://plasma.colorado.edu/phys7810/articles/Falthammar_MovingFieldLines_2007.pdf

As such it appears like a good article. The author states
--------------------------------------------------
Another example of how the concept of
moving magnetic field lines can be deceptive
is that of a homogeneously magnetized conducting
sphere surrounded by vacuum and
rotating around its axis. For someone thinking
of magnetic field lines as entities that
can ‘move,’ it is a not an uncommon fallacy
to believe that the magnetic field lines outside
the spherical magnet ‘rotate with the magnet’
and that this rotating field is capable of exerting
a force on a test charge at rest, due to the
percieved ‘relative’ motion between the test
charge and the magnetic field.
What really happens is very different. As
the spherical magnet is conducting, a charge
on its surface will be subject to a magnetic
force. This causes a redistribution of charges
until the electric field from these charges
gives a force that precisely balances the
magnetic force. The result is an electrostatic
potential on the surface of the sphere with
the equator having a potential opposite to
that of the poles.

--------------------------------------------------



I have no others so if someone knows of any which are related to this topic please let me know. I'm seeking such articles right now. There is mention of this in the problem section of Chapter11 in Classical Electromagnetic Theory, by Jack Vanderlinde, John Wiley & Sons, Inc., (1993). See page 316 problem 11-8
--------------------------------------------------
11-8 Find the electric field appearing around a uniformly magnetized sphere rotating at an angular frequency $\omega$ << r/a about the central axis parallal to the magnetization.
--------------------------------------------------
Please let me know if I can help with anything else.

Best regards

Pete

 

[granpa]

imagine a superconducting ring with a current flowing around it and therefore a magnetic field surrounding it. if i spin the ring around an axis passing through its center then i see no reason to think that the magnetic field will rotate.

furthermore if the magnecic field extends to infinity then wouldn't the velocity of the field increase without limit as one went further from the magnet?

 

[pmb_phy]

imagine a superconducting ring with a current flowing around it and therefore a magnetic field surrounding it. if i spin the ring around an axis passing through its center then i see no reason to think that the magnetic field will rotate.

And in this case there would be no magnetic field generated either.

Pete

 

[f95toli]

You need to be a bit carefull when talking about superconducting rings in this case. Quite a few people have probably seen demonstrations where a YBCO "tablet" is levitating above a permanent magnet (cooled by LN2).
The "problem" with YBCO is that it is a type II supeconductor meaning it will be penetrated by flux lines when cooled in a magnetic field. This is the reason why the YBCO is "stuck" on top of the magnet (if you try the same experiment with a zero-field cooled piece of YBCO there is no stable position).

Anyway, my point is that the the field in this configuration is NOT symmetric and you can actually rotate the magnet my rotating the piece YBCO even if the latter is perfectly round; but this is ONLY due to the fact that the flux lines are pinned in the YBCO.
I.e. what granpa writes is correct in MOST cases, but not always.

I used to do this demonstration when I was teaching a few years ago and while it is a neat experiment it can be quite confusing for the students since YBCO does not behave like the superconductors they read about in their textbooks.

 

[granpa]

And in this case there would be no magnetic field generated either.

Pete

i can only assume that you didnt understand what i said. the ring has a current therefore has a magnetic field. spinning would have no effect on that field at all.

 

[granpa]

You need to be a bit carefull when talking about superconducting rings in this case. Quite a few people have probably seen demonstrations where a YBCO "tablet" is levitating above a permanent magnet (cooled by LN2).
The "problem" with YBCO is that it is a type II supeconductor meaning it will be penetrated by flux lines when cooled in a magnetic field. This is the reason why the YBCO is "stuck" on top of the magnet (if you try the same experiment with a zero-field cooled piece of YBCO there is no stable position).

Anyway, my point is that the the field in this configuration is NOT symmetric and you can actually rotate the magnet my rotating the piece YBCO even if the latter is perfectly round; but this is ONLY due to the fact that the flux lines are pinned in the YBCO.
I.e. what granpa writes is correct in MOST cases, but not always.

I used to do this demonstration when I was teaching a few years ago and while it is a neat experiment it can be quite confusing for the students since YBCO does not behave like the superconductors they read about in their textbooks.

superconductivity has nothing to do with my point. forget superconductivity. imagine that its a normal ring of metal with a battery producing a current in it. spinning the ring will not effect the magnetic field. there is no reason to think that the magnetic field would rotate.

 

[pmb_phy]

i can only assume that you didnt understand what i said. the ring has a current therefore has a magnetic field. spinning would have no effect on that field at all.

Actually I meant to say

And in this case there would be no magnetic field generated either.

That should have read

And in this case there would be no electric field generated either.

Consider the example I gave here
http://www.geocities.com/physics_world/em/rotating_magnet.htm

Read from Eq. (14) on to see what the volume charge density and surface charge density are non-zero and yet the total charge is zero. This configuration of charge will produce an electric field. This can't happen with a ring of current.

Pete

 

[granpa]

Actually I meant to say

And in this case there would be no magnetic field generated either.

That should have read

And in this case there would be no electric field generated either.

Consider the example I gave here
http://www.geocities.com/physics_world/em/rotating_magnet.htm

Read from Eq. (14) on to see what the volume charge density and surface charge density are non-zero and yet the total charge is zero. This configuration of charge will produce an electric field. This can't happen with a ring of current.

Pete

perhaps i am missing something but that website seems to have nothing to do with moving magnetic fields. its about the effects of relativity on moving charges in a conductor. further it assumes that there are moving charges in a permanent magnet, which seems very doubtful to me.

 

[pmb_phy]

perhaps i am missing something but that website seems to have nothing to do with moving magnetic fields.

The magnet has a magentic field. It used to be thought that the rotating magnet carried its field with it, i.e. that the magnetic field rotated with the magnet. In fact that's part of the subject matter in one of the articles I posted the URL to.

... its about the effects of relativity on moving charges in a conductor. further it assumes that there are moving charges in a permanent magnet, which seems very doubtful to me.

Why does it seem doubtful to you?

Besides, that is only a model of a magnet that I'm employing in order to describe the mechanism of the charge distribution. If the current loops bother you that much then ignore them. Focus on the equations for the polarization instead.


Pete

 

[f95toli]

superconductivity has nothing to do with my point. forget superconductivity. imagine that its a normal ring of metal with a battery producing a current in it. spinning the ring will not effect the magnetic field. there is no reason to think that the magnetic field would rotate.

Yes, I understood that. But my point is that you need to be carefull about usings superconductors as an EXAMPLE when you talk about magnetic fields since quite a few people have seen the YBCO demonstration and that does not quite agree with the explanation you gave.
It is probably less confusing to talk about e.g. a perfect conductor instead of a superconductor.

 

[granpa]

Why does it seem doubtful to you?


because permanent magnetization is the result of spin orientation of the electrons. no current is involved.

 

[meopemuk]

Perhaps the best proof that magnetic field lines do not rotate with the rotating magnet can be found in the device known as "homopolar generator" or "Faraday disk" http://en.wikipedia.org/wiki/Homopolar_generator

In this experiment a rotating magnet fails to induce currents in a metal disk next to it.

Eugene.

 

[granpa]

http://en.wikipedia.org/wiki/Faraday_paradox

but this only indicates that the magnetic field of a rotating magnet does not move (considering how the field is produced, i wouldn't expect it to move anyway). it does not prove that all magnetic fields don't move.

 

[pmb_phy]

because permanent magnetization is the result of spin orientation of the electrons. no current is involved.

First off that was a page under classical physics so no quantum properties were used to explain the mechanical properties. There are two components of the magnetic moment of an atom. One is the magnetic moment of the orbiting electron, which provides a small current, while the other is the spin magnetic moment. While very small, almost neglegible, its still there. Often in EM we use current loops to create models for calculations and the one of modeling a bar magnet, while never used in real life, can make a classical explanation of something very useful as in my case.



Pete

 

[granpa]

First off that was a page under classical physics so no quantum properties were used to explain the mechanical properties. There are two components of the magnetic moment of an atom. One is the magnetic moment of the orbiting electron, which provides a small current, while the other is the spin magnetic moment. While very small, almost neglegible, its still there. Often in EM we use current loops to create models for calculations and the one of modeling a bar magnet, while never used in real life, can make a classical explanation of something very useful as in my case.



Pete

1 the electrons don't actually orbit the nucleus.
2 if the magnetic field can be produced by any method any method that doesn't require a currrent then that casts doubt on the conclusions of that website.
3 even if the magnetic field is produced by a current i never did see any reason to expect that the field would rotate with the magnet.
4 the website is only about rotating magnetic fields, not moving magnecic fields in general.

rotating the magnet does not produce any current in the apparatus BUT moving the magnet will produce a current. if one still insists that the field doesn't move then the only explanation for the emf that i can imagine is that the emf is produced because the strength of the field is changing.

constant magnetic fields are easy to produce. it should be rather easy to construct an experiment in which a constant magnetic field is moved over a conductor and see whether any emf is produced.

 

[meopemuk]

constant magnetic fields are easy to produce. it should be rather easy to construct an experiment in which a constant magnetic field is moved over a conductor and see whether any emf is produced.

Take a bar magnet and rapidly insert it into a solenoid. There will be a current through the solenoid. Does it answer your question, or I misunderstood it?

Eugene.

 

[granpa]

Take a bar magnet and rapidly insert it into a solenoid. There will be a current through the solenoid. Does it answer your question, or I misunderstood it?

Eugene.

the field around the bar magnet is irregular therefore the stregth of the magnetic field at anyone point on the solenoid would be changing as the bar magnet moved.
the strength of the field must be constant, like that inside a very long solenoid.

 

[meopemuk]

the field around the bar magnet is irregular therefore the stregth of the magnetic field at anyone point on the solenoid would be changing as the bar magnet moved.
the strength of the field must be constant, like that inside a very long solenoid.

So, suppose I put a wire loop inside a long solenoid and then start to move the solenoid along its axis (or perpendicular to its axis?). You are asking whether there will be current in the loop? Do I understand you correctly?

Eugene.

 

[granpa]

So, suppose I put a wire loop inside a long solenoid and then start to move the solenoid along its axis (or perpendicular to its axis?). You are asking whether there will be current in the loop? Do I understand you correctly?

Eugene.

yeah.
the solenoid would have to be moved perpendicular to its field lines (perpendicular to its axis).

 

[pmb_phy]

1 the electrons don't actually orbit the nucleus.
2 if the magnetic field can be produced by any method any method that doesn't require a currrent then that casts doubt on the conclusions of that website.
3 even if the magnetic field is produced by a current i never did see any reason to expect that the field would rotate with the magnet.
4 the website is only about rotating magnetic fields, not moving magnecic fields in general.

rotating the magnet does not produce any current in the apparatus BUT moving the magnet will produce a current. if one still insists that the field doesn't move then the only explanation for the emf that i can imagine is that the emf is produced because the strength of the field is changing.

constant magnetic fields are easy to produce. it should be rather easy to construct an experiment in which a constant magnetic field is moved over a conductor and see whether any emf is produced.

This is getting way off track and now has nothing to do with the original thread. Since you're refusing to acknowledge that it was but a mere model, a very common model at that, I will no longer respond to this line of discussion. Especially since after all these posts you are unable to figure out that a rotating magnet has mistakenly been interpreted as a magnetic field rotating with the magnet and that you don't graps that this means that the field is moving according to this view. Sorry but this is very tiring and of no use to me or the OP.

granpa: Please don't take those as personal criticisms. I am criticizing what I read and not who wrote it.

Pete

 

[theperthvan]

Thanks mate.

Sure. It'd be my pleasure to. Here are the two of them -

http://www.geocities.com/pmb_phy/Webster_1963.pdf

http://www.geocities.com/pmb_phy/ajp_apr1961_webst.pdf

This one I just found now and is straight to the point.

http://plasma.colorado.edu/phys7810/articles/Falthammar_MovingFieldLines_2007.pdf

As such it appears like a good article. The author states
--------------------------------------------------
Another example of how the concept of
moving magnetic field lines can be deceptive
is that of a homogeneously magnetized conducting
sphere surrounded by vacuum and
rotating around its axis. For someone thinking
of magnetic field lines as entities that
can ‘move,’ it is a not an uncommon fallacy
to believe that the magnetic field lines outside
the spherical magnet ‘rotate with the magnet’
and that this rotating field is capable of exerting
a force on a test charge at rest, due to the
percieved ‘relative’ motion between the test
charge and the magnetic field.
What really happens is very different. As
the spherical magnet is conducting, a charge
on its surface will be subject to a magnetic
force. This causes a redistribution of charges
until the electric field from these charges
gives a force that precisely balances the
magnetic force. The result is an electrostatic
potential on the surface of the sphere with
the equator having a potential opposite to
that of the poles.

--------------------------------------------------



I have no others so if someone knows of any which are related to this topic please let me know. I'm seeking such articles right now. There is mention of this in the problem section of Chapter11 in Classical Electromagnetic Theory, by Jack Vanderlinde, John Wiley & Sons, Inc., (1993). See page 316 problem 11-8
--------------------------------------------------
11-8 Find the electric field appearing around a uniformly magnetized sphere rotating at an angular frequency $\omega$ << r/a about the central axis parallal to the magnetization.
--------------------------------------------------
Please let me know if I can help with anything else.

Best regards

Pete

 

[Mike S.]

...you can actually rotate the magnet my rotating the piece YBCO even if the latter is perfectly round; but this is ONLY due to the fact that the flux lines are pinned in the YBCO

You've really made me wish I had one of these in hand to play with. How far can the effect be taken? If you turn it 360 degrees that it won't turn 360 degrees back... will it? If you have several pieces like this, do they interact like ordinary magnets with one another? I'm daydreaming you could use this as an analog computer to solve complex "knots" in magnetic fields but I think that is simple misunderstanding. ;)

 

[Robin07]

My two cents worth.

To my surprise, through experimentation, I have found that the "Flux tubes"/lines of force, do not move when the magnet is turned on its' axis but the field lines do move with the magnet when one flips it from N to S and/or back again. This would lead me to conclude that the N/S area are a property of the poles and not the magnet it self. Am I interpreting my visual confirmation correctly?

I've constructed a 3D magnifying glass, much like the iron fillings on a sheet of paper but in
3D (I'm not prepared to discribe the tool since I would like to put it on the market first) This tool shows clearly the findings above. Additionally I have found that lines of force can be shaped, meaning that they are able to be tightly spaced and as a result, finding stronger gauss readings, larger spacing give way to lower gauss readings. Can this be or are my readings awry?

And if I may pose one more question, or finding. In-order to shield magnetic influence, the mesh that is used will consist of weaves that intersect each other at right angles, effectively re-directing the flux along the plane of the mesh material or does it disapate the field and its' influence?