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This question is related to the book I'm reading (Edward M. Purcell, Electricity and Magnetism, third edition). In particular, here is the image from 6.2 chapter (Figure 6.7):

Magnetic fields around different materials

As you can see, the circuit consists of different materials (copper wire, water, vacuum). And here the author claims that

The line integral of B has precisely the same value around every part of this circuit

This is where I got confused.

Pure water has negative ions moving right and positive ions moving left. Copper wire has conduction electrons and protons. So, in these cases the presence of a magnetic field can be easily explained by relativistic effects.

But what about the vacuum tube? It's just the electrons - nothing more, nothing less. How can the relativity explain the magnetic field in this case?

To provide more context, here is how a magnetic interaction (mutual repulsion) of two conducting wires was explained earlier in the book:

Two wires carrying current in opposite directions

The electron distribution in a wire (from a corresponding frame of reference point) is Lorentz-contracted creating a negative net density. So the wires carrying currents in opposite directions repel each other.

In the case of the vacuum tube in the figure above the existence of the magnetic field seems confusing. What exactly is Lorentz-contracted when there are no other particles except of the electron beam?

I can't wrap my head around this confusion because it looks like a typical catch-22 situation. Is a magnetic field created only during an interaction between different currents? So it isn't really there when we are not measuring. Or does a magnetic field cause these interactions to occur in the first place?

Hope I clarified my doubts well.

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    $\begingroup$ Just forget special relativity for a moment and think Maxwell's equations.Does a moving charge produce a magnetic field around it? $\endgroup$
    – Jun Seo-He
    Commented Jan 9, 2022 at 17:52
  • $\begingroup$ The magnetic field is not a relativistic phenomenon. $\endgroup$
    – my2cts
    Commented Jan 9, 2022 at 18:21
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    $\begingroup$ "The magnetic field is not a relativistic phenomenon." Wow; that's controversial! Could one construct a set of Lorentz covariant equations that didn't contain a magnetic field as well as an electric field, to replace Maxwell's equations and the Lorentz force equation? But it all depends, I suppose, on what you mean by "a relativistic phenomenon". $\endgroup$ Commented Jan 9, 2022 at 19:29
  • $\begingroup$ @JunSeo-He, Thanks for your insight. Am I right in saying then that in the case of the vacuum tube in the first figure the magnetic field is just a mathematical abstraction? I mean, the equations do not tell us what is happening now in real world but what would happen if an electron beam in the tube interacted with other moving charges. Does it make sense? $\endgroup$
    – Stan Mots
    Commented Jan 9, 2022 at 20:12
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    $\begingroup$ Beautiful question. You would also have the same problem at the gap of a capacitor. Maybe both problems have a similiar solution, but I am currently drawing blanks here. It might be useful to dig up the first paper coming upnwith the relativistic wire solution. $\endgroup$
    – lalala
    Commented Jan 11, 2022 at 7:07

5 Answers 5

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The reason that Purcell uses the example of two attracting wires to illustrate the existence of magnetic fields is that they are electrically neutral. This means that in the lab frame, there cannot be an electric force between the wires. And yet, by looking at the charge densities in another frame, we can show that the wires will experience forces perpendicular to their length. In the lab frame, this force is explained by the magnetic force between the wires.

It would be possible to make a similar argument by looking only at two opposing beams of charge. In this case, the two beams would strongly repel each other in the lab frame, due to each other's electric fields; and we could calculate the expected repulsive force per unit of length. We could, however, also use the following method to calculate the force between the beams:

  • Go to one of the beams' rest frames.
  • Calculate the force per length of beam there (with the charge densities and the lengths transformed appropriately). Note that in this frame, the beam is at rest and so only experiences an electric force, which we know how to compute.
  • Transform the result back into the lab frame.

If we did this, we would find a different result from what we would expect by simply calculating the electric field in the lab frame. The discrepancy between these results is explained by the additional magnetic force between the beams.

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  • $\begingroup$ It is clear how it all works in the case of two opposing beams. But how can relativity explain the existence of the magnetic field around the tube in the first figure of the question? $\endgroup$
    – Stan Mots
    Commented Jan 10, 2022 at 22:04
  • $\begingroup$ @StanMots: By similar arguments applied to the force on a test charge, rather than on a beam of particles. Figure out what the electric force is on a moving charge in the lab frame; transform into the rest frame of the charge and figure out the electric force there; transform that the force calculated in the charge frame back into the lab frame; note the discrepancy and conclude there must be an additional force in the lab frame due to the motion of the charge. $\endgroup$ Commented Jan 10, 2022 at 22:08
  • $\begingroup$ Thanks, I understand the calculations. And I see how the magnetic field can be explained by Maxwell equations. The problem is how the magnetic field can be explained by relativity? Look, both the copper wire and the vacuum tube have the same magnetic field around them. So they would interact with the test charge the same way. In the case of the copper wire relativity states that the field is caused by Lorentz-contraction of positive ions. There are no positive ions inside the tube. This is where I'm confused. $\endgroup$
    – Stan Mots
    Commented Jan 10, 2022 at 23:27
  • $\begingroup$ @StanMots "There are no positive ions inside the tube" - exactly. In the vacuum tube case there is an electric field because the electrons are not matched with positive ions. That is the case in both the moving frame and the lab frame. That is in contrast with the "two wire" case, where there is no electric field in the lab frame. In the "two wire" case, the electric field is present in the moving frame because of Lorentz contraction of positive ions. In the "vacuum tube" case, the electric field is present because there are no positive ions. $\endgroup$
    – fishinear
    Commented Jan 11, 2022 at 11:55
  • $\begingroup$ Thank you @MichaelSeifert. Your clarifications helped me to better understand magnetism and relativity relationship. $\endgroup$
    – Stan Mots
    Commented Jan 22, 2022 at 12:18
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Compasses show that there is a magnetic field around a vacuum tube. Except for one compass, the one that co-moves with the electrons.

We can build a LC-circuit where the tube is the inductor, there is the problem that the tube is a capacitor too, because its charge varies. Normal wires have that problem too.

If we manage to build a very long tube and drive a very large current through it, we can use it as a energy storage device. The electrons behave like they had a small amount of extra mass, like they do in a normal wire. Coiling the tube would increase the extra mass greatly.

Oh yes, coiling the tube would make it impossible for any compass to co-move with the electrons.

A wire loop with current is one kind of compass, and relativity explains the turning of that loop near a tube with a current. Other types of compasses will agree with that compass.

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  • $\begingroup$ Thanks, that's an interesting point about different compasses. So, in the case of the compass co-moving with the electrons is it correct to say that there is no magnetic field in that frame of reference? $\endgroup$
    – Stan Mots
    Commented Jan 10, 2022 at 22:40
  • $\begingroup$ @StanMots Yes. Purcell means that in the rest frame of the pictured apparatus each segment produces same line integral of B. If the apparatus moves, then there are at least 2 different line integrals of B, probably 4. I mean if a compass went around the apparatus at constant speed, it would measure at least 2 different magnetic fields, probably 4. $\endgroup$
    – stuffu
    Commented Jan 11, 2022 at 3:00
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Maxwell's equations state that moving electric charges create a magnetic field around it. That is the case in all of the apparatus that you show, including the vacuum tube. The net result of all the moving charges in all of the different parts is the same current going around the whole apparatus. The same current gives rise to the same magnetic field.

As with all fields, they form a description of what influence a certain phenomenon has on the surrounding area. A single charge has a surrounding electric field. That field describes what would happen if we place another charge at a certain distance from the first charge. But the electric field is still there when there is only single charge. The same is true for the magnetic field. The field exists for a single moving charge, independent of whether any other charges are nearby.

The Relativity viewpoint can give insight into how the magnetic field and electric field are related to each other. The same phenomenon that is explained by the magnetic field in one reference frame, can be explained by the electric field in another reference frame. That does not mean that the magnetic field is somehow caused by Relativity.

We can take the vacuum tube as an example of that. Suppose we place a magnetic dipole (a compass) next to the tube. In the stationary reference frame, that dipole will align itself with the magnetic field generated by the moving electrons. Viewed from a reference frame moving with the electron(s), the electrons are stationary, and therefore don't generate a magnetic field. But in that reference frame the magnetic dipole is moving, and therefore generates an electric field. That electric field interacts with the electrons, causing the dipole to align itself in the same way.

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    $\begingroup$ "The same phenomenon that is explained by the magnetic field in one reference frame, can be explained by the electric field in another reference frame" - What frame of reference can be used to explain the magnetic field around the vacuum tube in the figure by the electric field? $\endgroup$
    – Stan Mots
    Commented Jan 10, 2022 at 22:10
  • $\begingroup$ "What frame of reference can be used to explain the magnetic field around the vacuum tube in the figure by the electric field?" - in the reference frame that is stationary to the vacuum tube, the flow of electrons are moving charges. Moving charges create a circular magnetic field around them, as described by Maxwell's equations. $\endgroup$
    – fishinear
    Commented Jan 10, 2022 at 23:02
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    $\begingroup$ But how can relativity explain this field? The author of the book states that a magnetic field is a consequence of relativity. That's why I'm asking. $\endgroup$
    – Stan Mots
    Commented Jan 10, 2022 at 23:06
  • $\begingroup$ @StanMots I have added in the text how the vacuum tube situation can be viewed with Relativity, as an example of what I said. You cannot state that the magnetic field is a "consequence of relativity". In some reference frames, the behaviour can be explained through a magnetic field, in others through an electric field, in yet others through a combination of the two. In all reference frames, Maxwell's equations are valid that describe that behaviour. $\endgroup$
    – fishinear
    Commented Jan 11, 2022 at 11:41
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Thanks to all the answers and comments. I think, I've finally resolved the problem of this question.

I see there are a lot of misconceptions around the web concerning magnetism and relativity relationship.

So, in the hope it will help others who are confused by the same issues, I'm gonna add a short FAQ below.

Q: Is an electric field a fundamental entity and can a magnetic field be understood as a pure relativistic effect?

A: Both an electric field and a magnetic field are fundamental phenomena. What relativity tells us is that everything depends on the chosen reference frame. From one viewpoint the magnetic field can be observed as a pure relativistic effect. From another viewpoint the observers see the world differently and, consequently, may even think that it's the magnetic field that's fundamental and the electric field is just a relativistic effect. Both viewpoints are correct at the same time. A good example of this is Moving magnet and conductor problem.

Q: How can a magnetic field be explained by relativity in a system where Lorentz contraction doesn't change charge densities? Take, for example, two charges moving parallel to each other.

A: According to relativity, when one frame moves relative to another one we should consider two effects: Lorentz contraction and time dilation. If Lorentz contraction is negligible for a particular system it doesn't mean relativity laws cannot be applied.

Take a look at the system of two charges moving with the same speed $\vec{v}$ and direction parallel to each other:

Two charges moving in the lab frame

In the reference frame where the charges are at rest there is a pure electric force between them which is repulsive (charges have the same sign). It can be calculated using Coulomb's Law.

In the lab frame where the charges are moving to the right the repulsive force is reduced because of time dilation. Lorentz contraction doesn't affect length in the direction perpendicular to the velocity, therefore, the force can be calculated using the following relativistic equation G.17 from Appendix G of Purcell's book: $$ \frac{dp^′_y}{dt^′} = \frac{f_y\Delta{t}}{\gamma\Delta{t}} = \frac{f_y}{\gamma} $$

where $ f_y $ is the force between the charges, $ \gamma $ is the Lorentz factor.

At the same time, for observers in the lab frame the force between the charges comprises both electric and magnetic components. So they may decide to calculate it using traditional equations (e.g. Lorentz force).

Both approaches are correct.

Q: Can relativity explain the existence of a magnetic field around a moving charge even when it doesn't interact with the other moving charges?

A: Yes. Relativistic laws apply even for a single charged particle moving in a vacuum tube.

From the reference frame where the particle is at rest there is just the electric field.

In the lab frame where the particle is moving with speed $\vec{v}$ its time slows down (time dilation) and its electric field $E$ concentrates in the direction perpendicular to the particle motion because of Lorentz contraction:

Electric field Lorentz contraction

These changes of the particle's electric field and its time coordinate can be explained from another viewpoint by magnetic effects. And again both viewpoints would be correct.

Q: Can there be the same magnetic field around different media with flowing charges in a circuit?

A: Yes. By looking at Ampère's circuital law we can conclude that the magnetic field is directly proportional to the current. The current $I$ can be defined as $$I=\frac{dQ}{dt}$$

So, it all comes down to measuring how much charge is transferred through the surface over a time $t$.

The charges are flowing with different speed inside the different media. However, we should also take into account the difference between the charge densities. If the charge density compensates the speed variation the current can still be the same. And, as a consequence, there will be the same magnetic field around.

The interaction force, on the contrary, will be different because of the different electric and magnetic field contributions around various conductor materials.

Q: In the example of two current-carrying wires repelling each other why aren't the electrons getting Lorentz contracted in the lab frame? Doesn't it violate the principle of equivalence?

A: In fact, the electrons do experience Lorentz contraction in the lab frame. It's just that we specifically chose such a system where the density $\sigma$ of positive ions equals the density $\sigma$ of negative ions in the lab frame.

In a system where $\sigma$ of electrons is increased by $\gamma$ a nearby stationary charge would experience the electric force. However, all relativistic laws would still hold good. As a result, in the rest frame of electrons their density will be $\frac{\gamma\cdot\sigma}{\gamma}=\sigma$ and the density of positive ions will be equal to $\gamma\sigma$.

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Maxwell's equations describe what has been found experimentally: Moving charges lead to a measurable magnetic field. Lorentz contractions and the consideration of different reference systems are named as cause.

stuffu writes in his answer: "The electrons behave like they had a small amount of extra mass, like they do in a normal wire. Coiling the tube would increase the extra mass greatly." Where does the extra energy come from that the electrons moving in a circle have? Definitely not from relativistic motion, because when the tube is straight, the electrons move at comparable speeds, but produce a smaller magnetic field.

The solution for the stronger magnetic field of the electric coil must lie in the acceleration of the charges (on their spiral path). At this point it should be pointed out that also a stationary electron has a magnetic field. In the standard model of the elementary particles for the electron not only a unique electric charge is noted, but also an intrinsic (independently of external influences existing) magnetic dipole.

We know that these magnetic dipoles in some materials (the so-called permanent magnets) stabilize each other and lead to a macroscopically noticeable magnetic field. And that at low temperatures the effect also occurs at room temperatures in non-magnetic materials - or is disturbed (destroyed) at higher temperatures (Curie point) because of the stronger thermal motion of the atoms.

To make a long story short, the cause of the collective magnetic field of moving electrons lies in the forced common alignment of their magnetic dipoles. The example of an electron flow through different materials given by Purcell is a perfect example of this. It is, after all, the property of an electric circuit that at each point the same number of electrons leave the unit per unit of time (this also applies to the liquid!). The moving electrons always generate the same magnetic field strength with their aligned magnetic dipoles.

In short, yes, moving electrons in a vacuum tube generate - because of the common alignment of their magnetic dipoles - a common magnetic field.

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