Magnetism seems absolute despite being relativistic effect of electrostatics

[harrylin]

Thanks harrylin and DrGreg you’re a great help.

Is this field absolute because we also have none moving +ve charges in the wire’s rest frame? Would it still be absolute if those charges were not present?

I don't think that the positive charges are important for the discussion. The magnetic field is here "absolute" in the sense that the magnetic field of a current loop can't be transformed away in SR. This is simply because it's impossible to transform all the velocities away in SR. Similarly, length contraction and time dilation can't be transformed away completely in such situations (see Ehrenfest paradox and twin paradox).

 

[Dale]

Is this field absolute because we also have none moving +ve charges in the wire’s rest frame? Would it still be absolute if those charges were not present?

I don't think that the positive charges are important for the discussion. The magnetic field is here "absolute" in the sense that the magnetic field of a current loop can't be transformed away in SR.

The word "absolute" doesn't merely mean that it can't be transformed away. By that definition time and length would also be absolute.

The magnetic field is relative to a given reference frame, not absolute. Just like time and length and energy and momentum and velocity and all of the other relative quantities we are familiar with.

 

[harrylin]

The word "absolute" doesn't merely mean that it can't be transformed away. By that definition time and length would also be absolute. [..]

You misunderstood: the magnetic field is an effect that is not compared with length or time, but with length contraction and time dilation. Such effects can be transformed away in special cases.

 

[Dale]

I was primarily objecting to the use of the term "absolute". Whether you are comparing it to length or to length contraction, the magnetic field is not absolute. It makes no sense to use that word to describe it.

 

[universal_101]

This it should perhaps not surprise that the same type of "absolute" vs. "relative" discussions can arise about magnetic fields as with for example the twin paradox.

This is very nice analysis, how can time (from twin paradox) be relative when effects are totally absolute !

 

[universal_101]

As I stated above, I mis-read your OP. The scenarios that I described using the LT correspond to your scenario (1) and to the LT of (1). They do not correspond to your scenario (2). I identified the modification that you would need to make to (2) in order to make it physically equivalent to (1).

They are different.

It doesn't. The link I provided uses the LT to analyze the same scenario from two different reference frames. The scenario analyzed in the link is not the same as your (1) or (2).

But if you apply charge-symmetry, then I think we should be able to transform the two scenarios.

That is, scenario(1) is exactly in conjugation with the scenario(2) according to C-symmetry.

Do you still believe the two scenarios are different and we cannot transform one to other.

 

[Dale]

But if you apply charge-symmetry, then I think we should be able to transform the two scenarios.

That is, scenario(1) is exactly in conjugation with the scenario(2) according to C-symmetry.

Do you still believe the two scenarios are different and we cannot transform one to other.

Sure, but that is not a Lorentz transform. The Lorentz transform preserves charge.

Also, you would have to have completely symmetric charge carriers, i.e. no high-mass fixed "lattice" and no low-mass "free current" charge carriers. For real wires and currents you cannot transform (1) into (2) even including both charge conjugation and a boost.

EDIT: Actually, even with symmetric charge carriers you cannot change (1) into (2) because in one the test charge is at rest wrt the same polarity charge carriers and in the other the test charge is at rest wrt the opposite polarity charge carriers. Changing charge conjugation doesn't change that discrepancy.

While (1) and (2) are both perfectly valid scenarios, you cannot simply Lorentz transform from one to the other. So relativity is not going to explain them.

 

[Per Oni]

This is simply because it's impossible to transform all the velocities away in SR.

I go along with that.
But that would still be true even when your traveling particles are not charged. Therefore the properties you attached to the magnetic field really should be attached to a different property of physics.

Therefore, and for other reasons as well, I go along with Dale’s point of view in that magnetism is not absolute but is totally and completely dependent on the frame of reference we wish to chose. (Hoping he is happy with the way put it).

 

[Dale]

Therefore, and for other reasons as well, I go along with Dale’s point of view in that magnetism is not absolute but is totally and completely dependent on the frame of reference we wish to chose. (Hoping he is happy with the way put it).

I am happy with that. I think "magnetism" refers both to the "magnetic force" and the "magnetic field", and your comment applies to both.

 

[chingel]

I haven't understood and followed the entire thread and I'm sorry if this has already been answered, but if two parallel wires have current going in the same direction and then from the electrons frame the wires are positively charged and they feel a electrostatic force from the other wire, why don't the electrons feel the electrostatic force from their own wire, equalizing the electron and proton ratio?

 

[Dale]

I haven't understood and followed the entire thread and I'm sorry if this has already been answered, but if two parallel wires have current going in the same direction and then from the electrons frame the wires are positively charged and they feel a electrostatic force from the other wire, why don't the electrons feel the electrostatic force from their own wire

They do, that is the basis of the Hall effect as described in the electron's frame. The electrostatic force from the other wire is balanced by the electrostatic force within their own wire.

equalizing the electron and proton ratio?

It is pointing in the wrong direction to do that.

 

[chingel]

They do, that is the basis of the Hall effect as described in the electron's frame. The electrostatic force from the other wire is balanced by the electrostatic force within their own wire.

How does the other wire's electrostatic force do that? What I mean is that in the electrons point of view, there are more protons between electrons in it's own wire than if it were electrically neutral, so why don't the protons in it's own wire pull the electrons until the charge of the wire is neutral?

 

[chingel]

Does anyone know why doesn't the electrostatic force caused by the increased density of protons in its own wire in the electron's frame pull the electrons closer until the electron/proton ratio is equal in the electron's frame in its own wire? I read in this or some other thread that the experiment is set up such that the wire is neutral in the lab frame and thus the electrons cannot be pulled together, but the electrons don't know that, there must be a force on them or an explanation to make them act as such.

 

[Dale]

Does anyone know why doesn't the electrostatic force caused by the increased density of protons in its own wire in the electron's frame pull the electrons closer until the electron/proton ratio is equal in the electron's frame in its own wire?

I am not 100% sure what you are asking, but you seem to be ascribing to the electrostatic force something that it cannot do. The electrostatic force can only move charges around, it cannot make charges appear or disappear. If the wire is charged then it is charged and there is no amount of electrostatic force that can make it otherwise.

Also, the charges are not static, so you need to think in terms of electrodynamics, not electrostatics. Fundamentally it is Maxwell's equations and the Lorentz force law that must be satisfied, not Coulomb's law except as an approximation to Maxwell and Lorentz.

 

[Subplotsville]

Has the case of a single charge been looked at yet? For example, a solitary electron is moving past an (uncharged) piece of iron and so causes a current in it due to the electron's magnetic field. How would this interaction be described from the point of view of the electron?

 

[Dale]

How would this interaction be described from the point of view of the electron?

Just standard electrostatic induction.
http://en.wikipedia.org/wiki/Electrostatic_induction

 

[Subplotsville]

Just standard electrostatic induction.
http://en.wikipedia.org/wiki/Electrostatic_induction

Care to explain what you mean? The forces caused by the electric and magnetic fields of the moving electron are in different directions. They are at right angles to each other. How does the electron, which sees itself at rest and therefore without a magnetic field, account for its magnetically directed effect on the (incidentally electrically neutral) piece of iron?

 

[Dale]

Care to explain what you mean? The forces caused by the electric and magnetic fields of the moving electron are in different directions. They are at right angles to each other. How does the electron, which sees itself at rest and therefore without a magnetic field, account for its magnetically directed effect on the (incidentally electrically neutral) piece of iron?

All that is important is the total force on the charge. The fact that the electric and magnetic forces are in different directions in one frame is not important. In the frame of the electron there is no magnetic force and the total force is just the electrostatic force. The total force in one frame maps to the total force in the other frame, even if the individual electric and magnetic forces do not.

 

[Subplotsville]

All that is important is the total force on the charge. The fact that the electric and magnetic forces are in different directions in one frame is not important. In the frame of the electron there is no magnetic force and the total force is just the electrostatic force. The total force in one frame maps to the total force in the other frame, even if the individual electric and magnetic forces do not.

I gathered that you meant something like this. What I'm asking is for you to show your work, if you feel up to it. If not, maybe someone else would like to. How does the moving electron explain its electric field producing a force of any sort on the electrically neutral piece of iron, much less a magnetic force? The fact that electric and magnetic effects occur in different directions is only one of several discrepancies.

 

[Dale]

I gathered that you meant something like this. What I'm asking is for you to show your work, if you feel up to it.

I don't feel up to it. It seems like a lot of effort for little benefit.

How does the moving electron explain its electric field producing a force of any sort on the electrically neutral piece of iron, much less a magnetic force?

I already said that in the electrons frame it doesn't produce a magnetic force, just an electric force. As you said, the iron is electrically neutral so there is no net force on the iron, but there are net forces on the free conduction electrons, so they move so as to maintain a 0 E field inside (neglecting resistance).

 

[Subplotsville]

I don't feel up to it. It seems like a lot of effort for little benefit.

That's up to you, of course. The thing is, without some sort of explanation in terms of a physical dynamic, you might as well claim the Great Pumpkin causes it.

I already said that in the electrons frame it doesn't produce a magnetic force, just an electric force. As you said, the iron is electrically neutral so there is no net force on the iron, but there are net forces on the free conduction electrons, so they move so as to maintain a 0 E field inside (neglecting resistance).

Well yeah, this is what an electric field is expected to do. What needs explanation is how this amounts to a magnetic-like effect just because the source is motion, despite the formal differences between the two fields. Thanks anyway.

 

[lugita15]

Well yeah, this is what an electric field is expected to do. What needs explanation is how this amounts to a magnetic-like effect just because the source is motion, despite the formal differences between the two fields. Thanks anyway.

A good electrodynamics book or special relativity book should explain how the magnetic field arises from the electric field and relativity. I think Purcell has a good treatment of that.

 

[Dale]

That's up to you, of course. The thing is, without some sort of explanation in terms of a physical dynamic, you might as well claim the Great Pumpkin causes it.

The physical dynamic is Maxwells equations and the Lorentz force law, as always.

Well yeah, this is what an electric field is expected to do. What needs explanation is how this amounts to a magnetic-like effect just because the source is motion, despite the formal differences between the two fields. Thanks anyway.

there is no "magnetic like effect" in the electrons frame.

 

[Subplotsville]

A good electrodynamics book or special relativity book should explain how the magnetic field arises from the electric field and relativity. I think Purcell has a good treatment of that.

Thanks, though I'm looking for an in-thread explanation, if possible. More forum readers will benefit from it that way.

The physical dynamic is Maxwells equations and the Lorentz force law, as always.

These are general principles. How do they lead us to expect the claimed result in this particular instance? Again, an electron moves past an electrically neutral piece of iron and so causes a current in it. What is the step-by-step description of how this happens through the electric field alone in the electron's frame of reference?

there is no "magnetic like effect" in the electrons frame.

Electromagnetic induction seems pretty magnetic-like to me. If you are repeating your claim that this is reducible to electric forces alone, I am repeating my request for a physical description of how this takes place.

 

[lugita15]

Thanks, though I'm looking for an in-thread explanation, if possible. More forum readers will benefit from it that way.

OK, attached is an excerpt from Purcell describing how the magnetic force arises.

 

[Subplotsville]

I can't open PDFs on this computer. Maybe you could copy and paste the part where he addresses electromagnetic induction by an isolated charge.

 

[chingel]

I am not 100% sure what you are asking, but you seem to be ascribing to the electrostatic force something that it cannot do. The electrostatic force can only move charges around, it cannot make charges appear or disappear. If the wire is charged then it is charged and there is no amount of electrostatic force that can make it otherwise.

Also, the charges are not static, so you need to think in terms of electrodynamics, not electrostatics. Fundamentally it is Maxwell's equations and the Lorentz force law that must be satisfied, not Coulomb's law except as an approximation to Maxwell and Lorentz.

I guess what I mean is that considering only one wire, what forces the wire to be charged in the electron's frame? Since the wire has no resistance, the voltage, charge and the overall electric field is zero at least in the lab frame, but why couldn't the electron say the same thing in its own frame? What forces the electrons to be spaced wider, shouldn't they feel an electrostatic force from the increased density of positively charged particles between them? Why don't more electrons come in from the source of the electrons, since the electrostatic force of the protons should be pulling them?

Is it that the electric field of the protons is contracted in the electron's frame and allows more proton density while the electrons don't feel an extra force from it?

 

[Dale]

Again, an electron moves past an electrically neutral piece of iron and so causes a current in it. What is the step-by-step description of how this happens through the electric field alone in the electron's frame of reference?

As I said, back in post 106, there is only an E-field and therefore you get only electrostatic induction:
http://en.wikipedia.org/wiki/Electrostatic_induction

Electromagnetic induction seems pretty magnetic-like to me.

No, electroSTATIC induction. That is why I even included a link, so that there would be no confusion as to which induction concept I was referring to.

 

[Dale]

I guess what I mean is that considering only one wire, what forces the wire to be charged in the electron's frame?

It has a different number of electrons and protons on it at any given time. That is what it means to be charged.

Since the wire has no resistance, the voltage, charge and the overall electric field is zero at least in the lab frame, but why couldn't the electron say the same thing in its own frame?

Because that is not self-consistent. Those values are all frame-dependent, so they cannot be the same in the electron's frame. In particular, any wire (even one with no resistance) has some capacitance wrt ground. The same field that drives the current through the wire in the lab frame also charges the capacitance of the wire in the electron's frame. I.e. it is not just a potential across the wire, but the whole wire is at an elevated potential.

Is it that the electric field of the protons is contracted in the electron's frame and allows more proton density while the electrons don't feel an extra force from it?

Hmm, that makes sense, I will have to think about that a bit.

 

[Subplotsville]

As I said, back in post 106, there is only an E-field and therefore you get only electrostatic induction:
http://en.wikipedia.org/wiki/Electrostatic_induction

Then, in post 107, I asked you to explain how that works. Namely how "only an E-field" could be responsible for what is otherwise known as electromagnetic induction in a piece of iron by a moving electron. I'm still waiting for an explanation.

No, electroSTATIC induction. That is why I even included a link, so that there would be no confusion as to which induction concept I was referring to.

Yes, you are referring to electrostatic induction. The problem is, the question is about electromagnetic induction. You claim they are equivalent. Okay, demonstrate the equivalence. Merely claiming this or that and calling a question resolved is not satisfactory in science.