What's the derivation in a moving magnet & conductor problem?

AI Thread Summary
The discussion centers on the derivation of the equation E' = v x B in the context of a moving magnet and conductor problem, referencing the Maxwell-Faraday equation. Participants express concerns about the circularity of deriving the Lorentz force from observations and emphasize the need for clarity in the derivation process. There is debate over the appropriateness of using low-velocity approximations versus full relativistic treatments, with some arguing that classical concepts should not complicate understanding. The directional derivative notation (v · ∇) is clarified, emphasizing its role in expressing changes in the magnetic field due to motion. The conversation highlights the complexity of electromagnetic theory and the importance of accurate transformations in understanding these phenomena.
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In the Wikipedia page of the moving magnet and conductor problem, it asserts "This results in: E' = v x B", but does not elaborate why.

What's the full derivation?
 
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A derivation of the Lorentz force would be somewhat circular: the expression for the LF is derived from observations. That's the way it is ...

If you find this view unsatisfactory, I can understand. But digging deeper doesn't really change this situation, I think. Would be interested in a theoretician's opinion.
@Orodruin , @vela, @Nugatory ?
 
What is being referred to is the Maxwell-Faraday equation
$$
\nabla \times \vec E' = - \frac{\partial \vec B'}{\partial t}
$$
leading to the given relation. The behaviour of ##\vec B'## is given in the equation before, but that equation seems incomplete. Approximately (for small velocities),
$$
\vec B'(\vec r', t) = \vec B(\vec r'+\vec v t).
$$
This leads to
$$
\partial_t \vec B' = \partial_t \vec B(\vec r+ \vec v t) = (\vec v \cdot \nabla)\vec B.
$$
For the left-hand side with ##\vec E' = \vec v \times \vec B## you would have
$$
\nabla \times \vec E' = \nabla \times (\vec v \times \vec B) = \vec v (\nabla \cdot \vec B) - (\vec v \cdot \nabla) \vec B = - (\vec v \cdot \nabla) \vec B,
$$
because the magnetic field is divergence free. Thus ##\vec E' = \vec v \times \vec B## solves the Maxwell-Faraday equation.

The "better" way I think is to use the Lorentz transformation properties of the electromagnetic field.
 
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Orodruin said:
What is being referred to is the Maxwell-Faraday equation
$$
\nabla \times \vec E' = - \frac{\partial \vec B'}{\partial t}
$$
leading to the given relation. The behaviour of ##\vec B'## is given in the equation before, but that equation seems incomplete. Approximately (for small velocities),
$$
\vec B'(\vec r', t) = \vec B(\vec r'+\vec v t).
$$
This leads to
$$
\partial_t \vec B' = \partial_t \vec B(\vec r+ \vec v t) = (\vec v \cdot \nabla)\vec B.
$$
For the left-hand side with ##\vec E' = \vec v \times \vec B## you would have
$$
\nabla \times \vec E' = \nabla \times (\vec v \times \vec B) = \vec v (\nabla \cdot \vec B) - (\vec v \cdot \nabla) \vec B = - (\vec v \cdot \nabla) \vec B,
$$
because the magnetic field is divergence free. Thus ##\vec E' = \vec v \times \vec B## solves the Maxwell-Faraday equation.

The "better" way I think is to use the Lorentz transformation properties of the electromagnetic field.
Thanks, though sorry, could you explain the meaning of this notation: $$(\vec v \cdot \nabla)\vec B$$

And also the intermediate steps required to go from: $$\partial_t \vec B(\vec r+ \vec v t)$$ to: $$(\vec v \cdot \nabla)\vec B.$$

Hope its not too much of a hassle with a lot of TeX
 
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BvU said:
A derivation of the Lorentz force would be somewhat circular: the expression for the LF is derived from observations. That's the way it is ...

If you find this view unsatisfactory, I can understand. But digging deeper doesn't really change this situation, I think.

I would like the derivation so I can understand how the two formulas give the same numerical results.
 
Lorentz force is ##\vec F_L = q (\vec v\times \vec B) ## and with ##\vec F = q \vec E## you are back at the 'result' expression.
 
BvU said:
Lorentz force is ##\vec F_L = q (\vec v\times \vec B) ## and with ##\vec F = q \vec E## you are back at the 'result' expression.
As Oroduin mentioned, it is: $$\partial_t \vec B(\vec r+ \vec v t)=(\vec v \cdot \nabla)\vec B$$ with some intermediate steps, and I'd like to figure out the details
 
I'm a bit worried about the fact that the OP is mislead by using utterly wrong transformation properties of the em. field. Here Lorentz transformations rather than Galilei transformations have to be applied. BTW it's the very problem Einstein used in his famous 1905 paper on special relativity as a motivation for the reformulation of the space-time model!
 
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vanhees71 said:
I'm a bit worried about the fact that the OP is mislead by using utterly wrong transformation properties of the em. field. Here Lorentz transformations rather than Galilei transformations have to be applied. BTW it's the very problem Einstein used in his famous 1905 paper on special relativity as a motivation for the reformulation of the space-time model!

oh no, which part is wrong? and what are the steps of the correct derivation? thanks
 
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  • #10
tade said:
oh no, which part is wrong?
The transformation rules are being given in the low-velocity approximation, not in its full relativistically invariant glory.

tade said:
and what are the steps of the correct derivation?
Orodruin said:
The "better" way I think is to use the Lorentz transformation properties of the electromagnetic field.
 
  • #11
Orodruin said:
The transformation rules are being given in the low-velocity approximation, not in its full relativistically invariant glory.
I see, I guess that's not "utterly wrong" though.Back to my previous question, as the low-velocity approximation is fine for me, I'd like to know the notation's meaning and the intermediate steps, thanks :rocket:
tade said:
Thanks, though sorry, could you explain the meaning of this notation: $$(\vec v \cdot \nabla)\vec B$$

And also the intermediate steps required to go from: $$\partial_t \vec B(\vec r+ \vec v t)$$ to: $$(\vec v \cdot \nabla)\vec B.$$

Hope its not too much of a hassle with a lot of TeX
 
  • #12
tade said:
vanhees71 said:
I'm a bit worried about the fact that the OP is mislead by using utterly wrong transformation properties of the em. field. Here Lorentz transformations rather than Galilei transformations have to be applied. BTW it's the very problem Einstein used in his famous 1905 paper on special relativity as a motivation for the reformulation of the space-time model!
oh no, which part is wrong? and what are the steps of the correct derivation? thanks

@vanhees71 I'm good with the low-velocity approximations, keeping it simple :)
 
  • #13
It's utterly wrong not only in a physical sense but also in a didactical. Classical electrodynamics is among the most difficult subjects in the undergraduate curriculum. It's not necessary to make it even more complicated by using oldfashioned concepts which have been solved more than 110 years ago by the development of special relativity, finalized by Minkowski in 1908.
 
  • #14
vanhees71 said:
It's utterly wrong not only in a physical sense but also in a didactical. Classical electrodynamics is among the most difficult subjects in the undergraduate curriculum. It's not necessary to make it even more complicated by using oldfashioned concepts which have been solved more than 110 years ago by the development of special relativity, finalized by Minkowski in 1908.
I do not think it needs to be didactically wrong as long as one is clear about being in the low velocity limit. To the contrary, keeping only the leading order terms in a small parameter expansion is an important tool in phenomenology.
 
  • #15
There is not one low-velocity limit in the sense of Galilei invariant electromagnetics but (at least) two, i.e., the electro-quasi-static and the magneto-quasi-static approximation. Neither is complete. Already for the most important application of these approximations in engineering, i.e., AC circuit theory, you need both. I'm not sure, but isn't precisely this "moving-magnet problem" the paradigmatic example for the fact that neither works, and that's why Einstein put it in the introductory paragraphs of his 1905 paper on the subject?

Another example is the homopolar generator, which is describable only with the correct relativistic version of the constituent equations a la Minkowski (i.e., taking into account the Hall effect in Ohm's Law, which is the correct relativistic form of it) although no large velocities are involved, see

https://th.physik.uni-frankfurt.de/~hees/pf-faq/homopolar.pdf
 
  • #16
@vanhees71 seems to forget that there are millions and millions for whom Ampere law, Faraday law etc are well beyond their zenith in abstraction. Yet they make their living with everyday applications of the Maxwell equations that they design and realize. A curriculum as he proposes is indeed ideal for a very, very select group.
 
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  • #17
  • #18
I didn't get where the problem is. The operator is self-explaining by notation. Maybe it helps to write it down in the concrete Ricci calculus for Carstesian coordinates ##x^j##:
$$(\vec{v} \cdot \vec{\nabla}) B^j=v^k \partial_k B^j.$$
 
  • #19
Also, the only thing necessary to reach that expression is the chain rule for derivatives.
 
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  • #20
vanhees71 said:
I didn't get where the problem is. The operator is self-explaining by notation. Maybe it helps to write it down in the concrete Ricci calculus for Carstesian coordinates ##x^j##:
$$(\vec{v} \cdot \vec{\nabla}) B^j=v^k \partial_k B^j.$$

Orodruin said:
Also, the only thing necessary to reach that expression is the chain rule for derivatives.

tade said:
Thanks, though sorry, could you explain the meaning of this notation: $$(\vec v \cdot \nabla)\vec B$$

And also the intermediate steps required to go from: $$\partial_t \vec B(\vec r+ \vec v t)$$ to: $$(\vec v \cdot \nabla)\vec B.$$

Hope its not too much of a hassle with a lot of TeX

sorry, I’m not clear about what the operator $$(\vec v \cdot \nabla)$$ means
 
  • #21
It is a directional derivative in the direction of ##\vec v##. It is written explicitly how it is expressed in post #18. I am sorry, but I do not see how it can be any clearer than that ...
 
  • #22
Orodruin said:
It is a directional derivative in the direction of ##\vec v##. It is written explicitly how it is expressed in post #18. I am sorry, but I do not see how it can be any clearer than that ...
I’m not familiar with Ricci calculus sadly
 
  • #23
Then just look at it as a directional derivative
$$
(\vec v \cdot \nabla) f(\vec x) = \lim_{\epsilon\to 0}\left(\frac{f(\vec x + \epsilon \vec v) - f(\vec x)}{\epsilon}\right),
$$
where ##f## is any field (scalar, vector, tensor, etc).
 
  • #24
tade said:
I’m not familiar with Ricci calculus sadly
@vanhees71
 
  • #25
Orodruin said:
Then just look at it as a directional derivative
$$
(\vec v \cdot \nabla) f(\vec x) = \lim_{\epsilon\to 0}\left(\frac{f(\vec x + \epsilon \vec v) - f(\vec x)}{\epsilon}\right),
$$
where ##f## is any field (scalar, vector, tensor, etc).

ε is the quantity of time right?
 
  • #26
tade said:
ε is the quantity of time right?
No. This is just the standard definition of a derivative, it has nothing to do with any physical quantity. As I already said, you will also need the chain rule for derivatives.
 
  • #27
Again, in Cartesian components it reads
$$\vec{v} \cdot \vec{\nabla}) \vec{B} = \vec{e}_j v^k \partial_k B^j.$$
 
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  • #28
Orodruin said:
No. This is just the standard definition of a derivative, it has nothing to do with any physical quantity. As I already said, you will also need the chain rule for derivatives.
As in, in this specific case of the moving magnet, ε represents t?
 
  • #29
vanhees71 said:
Again, in Cartesian components it reads
$$\vec{v} \cdot \vec{\nabla}) \vec{B} = \vec{e}_j v^k \partial_k B^j.$$

Sorry, not familiar with Ricci calculus.

@Orodruin @vanhees71 Is it correct to say that the x-component of $$(\vec{v} \cdot \vec{\nabla}) \vec{B}$$ reads $$v_x (\partial_x B_x)+v_y (\partial_y B_x)+v_z (\partial_z B_x)$$ ?
 
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  • #30
tade said:
Sorry, not familiar with Ricci calculus.

@Orodruin @vanhees71 Is it correct to say that the x-component of $$(\vec{v} \cdot \vec{\nabla}) \vec{B}$$ reads $$v_x (\partial_x B_x)+v_y (\partial_y B_x)+v_z (\partial_z B_x)$$ ?
yes I believe that is correct.
 
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  • #31
tade said:
As in, in this specific case of the moving magnet, ε represents t?
No. It is just a number. Again, the relation to ##t## comes from the chain rule.

tade said:
@Orodruin @vanhees71 Is it correct to say that the x-component of $$(\vec{v} \cdot \vec{\nabla}) \vec{B}$$ reads $$v_x (\partial_x B_x)+v_y (\partial_y B_x)+v_z (\partial_z B_x)$$ ?
Yes.
 
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  • #32
tade said:
Sorry, not familiar with Ricci calculus.

@Orodruin @vanhees71 Is it correct to say that the x-component of $$(\vec{v} \cdot \vec{\nabla}) \vec{B}$$ reads $$v_x (\partial_x B_x)+v_y (\partial_y B_x)+v_z (\partial_z B_x)$$ ?
Yes, that's the x-component of what I wrote from the very beginning. Sorry, I was not aware that you didn't know Ricci calculus. In the usual matrix-vector notation you have
$$(\vec{v} \cdot \vec{\nabla} \vec{B})=\begin{pmatrix}
v_x \partial_x B_x + v_y \partial_y B_x + v_z \partial_z B_x \\
v_x \partial_x B_y+ v_y \partial_y B_y + v_z \partial_z B_y \\
v_x \partial_x B_z + v_y \partial_y B_z + v_z \partial_z B_z
\end{pmatrix}.$$
 
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  • #33
Orodruin said:
$$\nabla \times \vec E' = \nabla \times (\vec v \times \vec B) = \vec v (\nabla \cdot \vec B) - (\vec v \cdot \nabla) \vec B = - (\vec v \cdot \nabla) \vec B,$$
because the magnetic field is divergence free. Thus ##\vec E' = \vec v \times \vec B## solves the Maxwell-Faraday equation.

by the way, the divergence of E should be zero right?
 
  • #34
tade said:
by the way, the divergence of E should be zero right?
Yes, this follows directly from ##\vec B## being divergence free.
 
  • #35
Orodruin said:
Yes, this follows directly from ##\vec B## being divergence free.
I was thinking about the divergence of v × B. It's a triple product, so it is equal to -v ⋅ (∇ × B).

If the divergence of E is zero, then so must the divergence of v × B.

But the curl of B is not always zero, as it is proportional to the current density.
 
  • #36
In vacuum the curl is zero. If there is not vacuum, the divergence of E will not be zero.
 
  • #37
Orodruin said:
In vacuum the curl is zero. If there is not vacuum, the divergence of E will not be zero.
If there's a current density but no net charge density, will the divergence of E be zero?
 
  • #38
tade said:
If there's a current density but no net charge density, will the divergence of E be zero?
No, but in the case described here there is no net charge density in the unprimed frame so since there is a current in the unprimed frame there will be a net charge density in the primed frame.
 
  • #39
Orodruin said:
No, but in the case described here there is no net charge density in the unprimed frame so since there is a current in the unprimed frame there will be a net charge density in the primed frame.
hmm, Einstein used this moving magnet and conductor problem as an introduction to his original paper on special relativity.

He argues that the electric and magnetic force explanations from different frames imply a preferred rest frame even though the effect is clearly one of relative motion.

So it seems like they managed to equate it mathematically without resorting to relativity.
 
  • #40
I must say, I lost track of what's the issue discussed here, and to say it friendly, the Wikipedia in this case is pretty incomplete (Pauli comes to my mind, who was much less friendly in such cases).

First of all, although it's not explicit in the usual notation of the local Maxwell's equations, they are relativistic equations, and they can be written covariantly, i.e., you can use the Maxwell equations to calculate what happens in this famous case using any frame of reference (it's what Einstein discusses in the introduction in his famous "electrodynamics of moving bodies" paper of 1905 as the motivation for his reanalysis of the space-time description necessary to make these Maxwell equations obeying the special principle of relativity).

That said, I think it's not a very simple problem though, because you need the full Maxwell equations, and it's not easy to find a formulation, where you can really do the calculation analytically to the end, but you can do it at least formally, i.e., just keeping the fields, currents etc. in general terms. It's also good to start by thinking about what can be concretely measured.

I'd say, we take the conductor as a conducting ring and having put a volt meter in series (comoving with the ring). Supposed the motion is slow enough such that the voltmeter can follow the changes of the fields, what's measured is the electromotive force along the ring. I'm using Heaviside-Lorentz units (which are much more convenient for relativistic arguments than the SI).

(a) Moving-ring frame

In this case we have a given magnetostatic field ##\vec{B}(\vec{x})## obeying the corresponding static Maxwell equations, ##\vec{\nabla} \cdot \vec{B}=0##, ##\vec{\nabla} \times \vec{H}=\vec{j}=0##, ##\vec{H}=\mu \vec{B}##, but that's not relevant for our question here.

Now we have the moving ring, described by ##C:\vec{x}=\vec{r}(t,\lambda)=\vec{r}_0(\lambda)+\vec{v} t##, where ##\lambda## is the parameter for the curve. Also let ##\vec{v}(t,\lambda)=\partial_t \vec{r}(t,\lambda)=\vec{v}=\text{const}##. Further let ##A## be an arbitrary surface with ##C=\partial A##. Now we use Faraday's Law in integral form to get the electromotive force. We need the magnetic flux through the moving surface with the ring as its boundary:
$$\Phi(t)=\int_{A} \mathrm{d} \vec{x} \cdot \vec{B}.$$
The time derivative is not 0 although the magnetic field is static, because ##A## is moving, i.e., we get for the electromotive force
$$\mathcal{E}=-\frac{1}{c} \dot{\Phi}(t) = \frac{1}{c} \int_{\partial A} \mathrm{d} \vec{x} (\vec{E}+\vec{v} \times \vec{B}) =\frac{1}{c} \int_{C} \mathrm{d} \vec{x} \vec{v} \times \vec{B}.$$
That's the reading of the volt meter when calculated in this frame of reference.

(b) Moving-magnet frame

That's slightly more complicated ;-)). We have to use the Lorentz transformation of the fields first. In this frame the em. field becomes time dependent and has both electric and magnetic components. As is derived in any complete textbook of electromagnetics (most elegantly using the four-tensor formalism and writing the em. field components in terms of the antisymmetric Faraday four-tensor ##F_{\mu \nu}##),
$$\vec{E}_{\parallel}'(t',\vec{x}')=\vec{E}_{\parallel}(t,\vec{x})=0, \quad \vec{E}_{\perp}'(t',\vec{x}') = \gamma [\vec{E}_{\perp}(t,\vec{x}) +\vec{\beta} \times \vec{B}(t,\vec{x}),$$
$$\vec{B}_{\parallel}'(t',\vec{x}')=\vec{B}_{\parallel}(t,\vec{x})=\vec{B}_{\parallel}(\vec{x}), \quad \vec{B}_{\perp}'(t',\vec{x}')=\gamma[\vec{B}_{\perp}(t,\vec{x})-\vec{\beta} \times \vec{E}(t,\vec{x})]=\gamma \vec{B}_{\perp}(\vec{x}).$$
Now
$$\vec{x}=\gamma(\vec{x}'+\vec{v} t') \qquad (*),$$
and since the area and its boundary don't move, we simply have
$$\mathcal{E}'=\int_{\partial A} \mathrm{d} \vec{x}' \cdot \vec{E}' = \int_{\partial A} \mathrm{d} \vec{x}' \cdot \gamma [\vec{\beta} \times \vec{B}(\vec{x})] = \frac{1}{c} \int_{\partial A} \frac{\mathrm{d} \vec{x}}{\gamma} \cdot (\gamma \vec{v} \times \vec{B})=\mathcal{E},$$
as it should be. In the prelast step we have used (*) to transform the line element ##\mathrm{d} \vec{x}'=\mathrm{d} \vec{x}/\gamma##. Note that this is the correct transformation since the integral is to be taken at constant coordinate time, ##t'##!
 
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  • #41
vanhees71 said:
I must say, I lost track of what's the issue discussed here, and to say it friendly, the Wikipedia in this case is pretty incomplete

I was asking Oroduin how physicists had established the mathematics of the moving magnet and conductor problem before relativity.
 
  • #42
Orodruin said:
$$
\nabla \times \vec E' = \nabla \times (\vec v \times \vec B) = \vec v (\nabla \cdot \vec B) - (\vec v \cdot \nabla) \vec B = - (\vec v \cdot \nabla) \vec B,
$$
because the magnetic field is divergence free. Thus ##\vec E' = \vec v \times \vec B## solves the Maxwell-Faraday equation.

I was thinking of the magnetic field at the center of a Helmholtz coil.

When we increase the size of the coil, but keep the magnetic flux density at the center the same, ##\nabla \vec B## gets smaller.

This should make ##\vec E'## smaller. But it has no effect on ##\vec v \times \vec B##. Is this correct?
 
  • #43
tade said:
I was asking Oroduin how physicists had established the mathematics of the moving magnet and conductor problem before relativity.
Before relativity, Heinrich Hertz, among others, tackled the problem and came pretty close to the modern relativistic view, but it's pretty difficult to understand compared to the very clear solution of all the problems concerning "electrodynamics of moving bodies" within the completed (special) theory of relativity, which is due to Minkowski.
 
  • #44
vanhees71 said:
Before relativity, Heinrich Hertz, among others, tackled the problem and came pretty close to the modern relativistic view

thanks, do you know what their solution to the scenario mentioned in #42 was?
 
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