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granpa
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is an emf produced by a changing magnetic field or by a moving magnetic field?
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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 pagegranpa said:is an emf produced by a changing magnetic field or by a moving magnetic field?
pmb_phy said: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 said: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 said: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
Sure. It'd be my pleasure to. Here are the two of them -theperthvan said:Please will you upload them.
cheers
And in this case there would be no magnetic field generated either.granpa said: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.
pmb_phy said:And in this case there would be no magnetic field generated either.
Pete
f95toli said: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.
Actually I meant to saygranpa said: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.
pmb_phy said: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
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.granpa said:perhaps i am missing something but that website seems to have nothing to do with moving magnetic fields.
Why does it seem doubtful to you?... 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.
granpa said: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.
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.granpa said:because permanent magnetization is the result of spin orientation of the electrons. no current is involved.
pmb_phy said: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 said: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 said: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 said: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 said: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.
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 said: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.
pmb_phy said: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
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11-8 Find the electric field appearing around a uniformly magnetized sphere rotating at an angular frequency [itex]\omega[/itex] << r/a about the central axis parallal to the magnetization.
--------------------------------------------------
Please let me know if I can help with anything else.
Best regards
Pete
f95toli said:...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
A changing magnetic field induces an electric field according to Faraday's law of induction. This means that when a magnetic field changes in strength or direction, it creates a force that causes electrons to move, generating an electric field.
A changing magnetic field refers to a magnetic field that is increasing or decreasing in strength or direction. A moving magnetic field, on the other hand, refers to a magnetic field that is physically moving through space.
Electromagnetic induction is the process of generating an electric current by moving a conductor through a magnetic field or by changing the magnetic field around a conductor. This is directly related to changing magnetic fields because it is the changing magnetic field that induces the electric current.
A changing magnetic field can induce unwanted currents in electronic devices, causing interference and potentially damaging the device. This is why electronic devices often have shielding to protect them from external magnetic fields.
There is currently no scientific evidence to suggest that a changing magnetic field has any direct effect on human health. However, high levels of magnetic fields can be found near power lines and in certain occupations, so it is important to follow safety guidelines and limit exposure to strong magnetic fields.