Current producing electromagnetic force

[kgm2s-2]

When current is allowed to flow through an electric conductor, magnetic fields are produced. What actually causes the magnetic fields to appear?

 

[jtbell]

What sort of explanation are you looking for, that is, what do you mean by "actually causes"?

 

[kgm2s-2]

It means that when current is flowing through a straight wire, how is magnetic field created around that wire? That is, what happens in the wire which causes this phenomenon to occur?
And why does the magnetic field change direction (from clockwise to anticlockwise or vice versa) when the current is flowing in the opposite way,
according to the Maxwell's screw rule or right-hand grip rule?

 

[WMGoBuffs]

The superficial answer to your question is just "Maxwell's Equations," but I think what you're asking is much deeper, akin to "What actually <i>is</i> a magnetic field?" That's tougher to answer.

 

[cabraham]

When current is allowed to flow through an electric conductor, magnetic fields are produced. What actually causes the magnetic fields to appear?

That's a very good question. Unfortunately, neither I, nor anyone has ever found an answer. The "cause and effect" concept is intuitive at the macroscopic level. If you tee up a golf ball, then swing the club, hitting the ball and putting it into flight, it is intuitive that the kinetic energy of the club was partially transferred to the ball, "causing" it to move.

At the microscopic level, though, what causes what is usually unknown. It is impossible to define everything, since definitions would be circular. What causes the magnetic field has not yet been detected. If we ever do discover a smaller more basic particle, then the question becomes "what causes its behavior".

Some properties are basic and cannot be proven or derived from something else. When a wire carries a current, years of empirical observation have demonstrated the existence of a magnetic force, and no exceptions have ever been found. We postulate this, i.e. no formal proof is given. We don't really know what causes magnetism. Maxwell's equations quantify this property and are postulates. Maxwell's are basic and are not derived.

Likewise with electric fields, voltage, current, and Coulomb force. Why two like charges repel one another is unknown. We've observed that they do, and no one knows the "cause".

"Cause and effect" is difficult to distinguish when you're dealing with basic particles and their associated interactions. What "causes" gravity is along those same lines, i.e. it is postulated that two masses attract each other mutually from empirical observation. No one knows the cause. I hope this helps. Best regards.

Claude

 

[ice109]

snip

lots of philosophical mumbo jumbo but no real substance. there are theorized answers to this question, just because you don't know them ( and coicdentally i don't know them ) doesn't mean you need to resort to saying that it is absolutely unknown.

"The modern understanding of magnetism posits that all magnetic effects are actually due to relativistic effects[4] caused by relative motion between the observer and the charged particles."

 

[waht]

Magnetic field is just a relativistic effect of the electric field. When the charge is not moving, it has an electric field, and no magnetic field. However when it moves close to the speed or light, the electric field lines will be compressed, and a sort of tidal force will develop, which is the magnetic field. The strength of the magnetic field depends on how fast the charges move.

$$B = v * E/c^2$$

(cross product)

 

[cabraham]

lots of philosophical mumbo jumbo but no real substance. there are theorized answers to this question, just because you don't know them ( and coicdentally i don't know them ) doesn't mean you need to resort to saying that it is absolutely unknown.

"The modern understanding of magnetism posits that all magnetic effects are actually due to relativistic effects[4] caused by relative motion between the observer and the charged particles."

No offense ice109, but you are the one not making sense. If no one, neither you, nor I, nor anyone, knows the answer, then it is indeed *unknown*. When I say "unknown", I don't infer that it will never be known, nor do I infer that it doesn't exist. Unknown simply means just that, i.e. magnetic force can be sensed and measured but we don't have the answer as to what causes it. So it really is unknown. I'm not "resorting" to anything other than stating what centuries of science have and have not revealed.

As far as relative motion between the charged particle and observer goes, isn't that by definition what *current* is?! If charges are moving with respect to my reference frame, the charge per unit time is what is known as current, i.e. 1 amp = 1 coulomb / second. So what you're saying is that the magnetic field is due to *current*. But we already know that, don't we?! The OP asked us what causes the magnetic field around a current-carrying wire, and your answer is that the current causes it. Very well. This is what I mean by "circular definitions". Peace.

Claude

 

[jostpuur]

In my opinion this matter is highly mystical in main stream science. I can hear everybody explaining, that putting electricity and relativity together will give magnetism, but I have never seen actual proof for this claim. Some years ago, being not satisfied by these "rumours" about magnetism, I tried to deal with this problem myself.

Two facts before getting into business:

1: Somebody might say that you cannot use action at the distance in relativity. Nonsense! Just make sure that the action at the distance has proper time delay. Suppose partice A is in location $$(0,\boldsymbol{0})$$ in spacetime, and particle B in location $$(|r|/c, \boldsymbol{r})$$, then sure we can solve what force A is exerting on particle B.

2: You are not supposed to use forces in relativity. Nonsense! Just define $$\boldsymbol{F}:=d\boldsymbol{p}/dt$$. It is a good definition, and nobody has right to forbid this concept.

Now consider a following problem. A particle A is moving with velocity $$\boldsymbol{v}_A$$, and a particle B is moving with velocity $$\boldsymbol{v}_B$$, and we want to know what force particle A exerts on particle B. Let us assume that the locations in the spacetime are like in the earlier remark, so that the question makes sense.

I understood, that there is no need to postulate this answer. In particular there is no need to postulate magnetic field or Lorentz's force. It is sufficent to postulate Coulomb's force in the A particle's rest frame. Since we can calculate how four-position and the force itself transforms under boosts, we should be able to solve the force in any other frame. Unfortunately I cannot put in all the details, because the calculation is quite long, but I hope you understood the idea. So we postulate the force B feels is $$\boldsymbol{F}=\lambda \boldsymbol{r}/|r|^3$$, in the frame where A is rest, and solve what this force is in the original frame where particles have the given velocities. After lot of work the result is this

$$\boldsymbol{F}=\frac{\lambda(1-|v_A|^2/c^2)}{(|r|-\boldsymbol{r}\cdot\boldsymbol{v}_A/c)^3} \Big(\boldsymbol{r} - |r|\boldsymbol{v}_A\frac{1}{c} +\boldsymbol{v}_B\times(\boldsymbol{v}_A\times\boldsymbol{r}) \frac{1}{c^2}\Big)$$

Now it could be useful to divide this expression into particle A depending part, and particle B depending part, and it can be done with following definitions.

$$\boldsymbol{E}:= \frac{(1-|v_A|^2/c^2)}{(|r|-\boldsymbol{r}\cdot\boldsymbol{v}_A/c)^3} \Big(\boldsymbol{r} - |r|\boldsymbol{v}_A\frac{1}{c}\Big)$$

$$\boldsymbol{B}:= \frac{(1-|v_A|^2/c^2)}{(|r|-\boldsymbol{r}\cdot\boldsymbol{v}_A/c)^3} (\boldsymbol{v}_A\times\boldsymbol{r})\frac{1}{c^2}$$

With these definition the force is the given by the usual Lorentz force $$\boldsymbol{F}=\lambda (\boldsymbol{E}+\boldsymbol{v}_B\times\boldsymbol{B})$$. (Okey the constants aren't standard in these equations, but it's not important)

For some time I though I had all magnetism now figured out, but I later learned that these equations agree with the Maxwell's electromagnetism only in the special case where particle A is not accelerating, because the radiation terms are missing. And the original assumtion on the Coulomb's force is also wrong for accelerating source. Still, I would argue that these were not useless calculations, because they give good intuition on the nature of magnetism. In my opinion, the popular calculations that deal with lorentz contractions of current carrying wires are too special case like, to truly answer problems of magnetism and relativity.

If we now accept the fact that we don't need to worry about radiation yet, and are merely interested in the forces (as students in the early steps are), I can give a very specific answer to the question why and what magnetic field is. It is an artificial concept, invented by man, to make calculations easier and to divide the force expression into parts that depend on the source and the test particle separately. Ultimately, the force has its origin in Coulomb's force and Lorentz's transformations. So the question "what is magnetic field" is reduced to questions "what is Coulomb's force" and "what are Lorentz's transformations". That is a big step.

The more succesfull field point of view doesn't fully agree with this, and I sure don't want to fight against fields. But it is always good to know lot of different ways to deal with these phenomena, isn't it?

 

[jostpuur]

What I meant with those comments about the topic being mystical, is that even though I've got quite far on my own, I still don't know how to get nicely full electromagnetism with radiation, by somehow "putting electricity and relativity together".

For example I've read how Ohanian & Ruffini derive ED in their relativity book, and it's like "let us postulate nice differential equations". Well I guess it looks good if you have lived your whole life with field differential equations, and consider that approach trivially the only possibility, but I'm still sceptical about its rigourness.

The fact that many phycisists seem to think that the force is somehow old fashioned concept, that doesn't belong to relativity, also sets under question, that do these people know anything on this matter.

 

[jostpuur]

kgm2s-2, I was just taking part in the discussion, I don't know if my post helped you in any way. At what level are you? How much you know about physics?

 

[ice109]

snip

what i meant is that i haven't learned the physics yet. there is physics that explains b field. it is not unknown at all. I'm pretty sure quantum field theory explain the electromagnetic force but like i said i don't know it.

 

[stmartin]

In the atom, the protons and electrons are neutralized so that means that there is no charge=no electric force=no electric field. It is like moving neutral ball produces magnetic field.

 

[pervect]

In my opinion this matter is highly mystical in main stream science. I can hear everybody explaining, that putting electricity and relativity together will give magnetism, but I have never seen actual proof for this claim. Some years ago, being not satisfied by these "rumours" about magnetism, I tried to deal with this problem myself.

The situation is hardly "mystical" but there are some subtle and not-so-subtle points here.

Two facts before getting into business:

1: Somebody might say that you cannot use action at the distance in relativity. Nonsense!

Can you give an example of someone saying that you cannot use "action at a distance", for instance retarded potentials? I have a feeling of a strawman being scattered all over the floor here.

2: You are not supposed to use forces in relativity. Nonsense! Just define $$\boldsymbol{F}:=d\boldsymbol{p}/dt$$. It is a good definition, and nobody has right to forbid this concept.

Again, can you give an example of someone saying you can't use forces in special relativity?

On a separate note, there are other ways of handling forces in special relativity, i.e. 4-forces - using forces rather than 4-forces is not guaranteed to be the best choice.

Now consider a following problem. A particle A is moving with velocity $$\boldsymbol{v}_A$$, and a particle B is moving with velocity $$\boldsymbol{v}_B$$, and we want to know what force particle A exerts on particle B. Let us assume that the locations in the spacetime are like in the earlier remark, so that the question makes sense.

I understood, that there is no need to postulate this answer. In particular there is no need to postulate magnetic field or Lorentz's force. It is sufficent to postulate Coulomb's force in the A particle's rest frame.

Here is the first subtle point. You also have to postulate that in A's reference frame, the force on B is independent of B's velocity. This is true for electromagnetism, but is not true for some other "forces", i.e. gravity for instance.

Since we can calculate how four-position and the force itself transforms under boosts, we should be able to solve the force in any other frame. Unfortunately I cannot put in all the details, because the calculation is quite long, but I hope you understood the idea.

I haven't looked at this closely, but I believe that this is a perfectly standard calculation that one can find in textbooks, see for instance

http://en.wikipedia.org/w/index.php?title=Electromagnetic_four-potential&oldid=136870056

This has the general name of Lienard-Wiechart potentials. This is usually expressed in terms of a 4-potential, rather than a force. The force is just the gradient of the 4-potential, so it's fairly trivial to go from the force -> potential.

For some time I though I had all magnetism now figured out, but I later learned that these equations agree with the Maxwell's electromagnetism only in the special case where particle A is not accelerating, because the radiation terms are missing. And the original assumtion on the Coulomb's force is also wrong for accelerating source. Still, I would argue that these were not useless calculations, because they give good intuition on the nature of magnetism. In my opinion, the popular calculations that deal with lorentz contractions of current carrying wires are too special case like, to truly answer problems of magnetism and relativity.

I don't think the calculations were useless either, but it seems to me that a certain amount of re-inventing the wheel has occurred.

The way I would describe the situation would be this:

A coulomb force (more preciesely, the 4-force equivalent) alone, is not Lorentz invariant - it doesn't transform in the way that a force must according to relativity. The 4-force must transform just as any other 4-vector - and this defines how the 3-force must transform.

The combination of electric + magnetic forces together, is Loretnz invariant, therefore we call the force the "electromagnetic force" in relativity, just as we refer to "spacetime" in relativity rather than space+time. The electric and magnetic forces are two parts of the same entity.

However, in spite of the fact that the electric and magnetic forces are two parts of the same force, it is not quite true that one can deduce the precise form of the magnetic force from the electrostatic force (Coulomb's law). One has to make additional assumptions about the force on a moving charge.

 

[stmartin]

Magnetic field is just a relativistic effect of the electric field. When the charge is not moving, it has an electric field, and no magnetic field. However when it moves close to the speed or light, the electric field lines will be compressed, and a sort of tidal force will develop, which is the magnetic field. The strength of the magnetic field depends on how fast the charges move.

$$B = v * E/c^2$$

(cross product)

Why you thing that the electric field lines (electric force) will be compressed?

 

[kgm2s-2]

kgm2s-2, I was just taking part in the discussion, I don't know if my post helped you in any way. At what level are you? How much you know about physics?

Nevertheless, thanks for your contribution. I'm in O-level physics if you're wondering.

 

[stmartin]

So, let's conclude. Why does moving electron produces magnetic field?

 

[jtbell]

I gave an answer in the other thread where you asked this question:

https://www.physicsforums.com/showthread.php?t=175883

 

[stmartin]

I gave an answer in the other thread where you asked this question:

https://www.physicsforums.com/showthread.php?t=175883

There is nothing concretely said there. I just want to know, correctly how it is. I saw on some posts that said that the electric field is compressing and producing tidal force, which is the magnetic force. Can you explain better please. Thank you.

 

[stmartin]

Can somebody explain this please:
"Magnetic field is just a relativistic effect of the electric field. When the charge is not moving, it has an electric field, and no magnetic field. However when it moves close to the speed or light, the electric field lines will be compressed, and a sort of tidal force will develop, which is the magnetic field. The strength of the magnetic field depends on how fast the charges move"
Thank you.

 

[waht]

A tidal force is when there is an uneven distribution of forces. Most common tidal force is exerted by the moon on the oceans on earth.

You can think of a charge at high speeds similarly. Consider an electron at rest, the electric field lines are radially spread out evenly.

If you are familiar with special relativity, then at speeds close to speed of light things experience length contraction (in the direction of motion), time dilation, and mass increase, right?

As a result the electric field lines of an electron traveling close to the speed of light will be squeezed in the direction of motion (by length contraction). So much so, the electric field strength in front of and behind the electron (in the direction of motion call it x) will be much less than in y-z plane. The y-z plane will be a circular disc containing the strongest electric field lines that is perpendicular to the direction of motion.

The converse is also true. If an electron is stationary emanating radially electric field lines, and if you zip by this electron close to the speed of light, by length contraction effect will make you seem the front and back of the electric field are squeezed, and y-z plane disk will contain the strongest electric field densities.

It has been shown that this effect is responsible for the magnetic field. It's all relative.

 

[jtbell]

To put it another way, just like position and time are "mixed together" by a Lorentz transformation when you switch from one reference frame to another (e.g. from a frame in which a particle is stationary to a frame in which the particle is moving), so also are the electric and magnetic fields "mixed together" by a Lorentz transformation. A pure electrostatic field in the frame in which the source particle is stationary, transforms into a mixture of electric and magnetic fields in a frame in which the particle is moving.

However, I think pervect's post implies that you can't always go the other way: not all magnetic fields can be transformed into an electrostatic field by Lorentz-transforming them into a suitable reference frame.

I would not use the "tidal force" to describe the way magnetic forces are related to electric forces. The math seems completely different to me.

 

[stmartin]

A tidal force is when there is an uneven distribution of forces. Most common tidal force is exerted by the moon on the oceans on earth.

You can think of a charge at high speeds similarly. Consider an electron at rest, the electric field lines are radially spread out evenly.

If you are familiar with special relativity, then at speeds close to speed of light things experience length contraction (in the direction of motion), time dilation, and mass increase, right?

As a result the electric field lines of an electron traveling close to the speed of light will be squeezed in the direction of motion (by length contraction). So much so, the electric field strength in front of and behind the electron (in the direction of motion call it x) will be much less than in y-z plane. The y-z plane will be a circular disc containing the strongest electric field lines that is perpendicular to the direction of motion.

The converse is also true. If an electron is stationary emanating radially electric field lines, and if you zip by this electron close to the speed of light, by length contraction effect will make you seem the front and back of the electric field are squeezed, and y-z plane disk will contain the strongest electric field densities.
It has been shown that this effect is responsible for the magnetic field. It's all relative.

So you say that the electron's electric field is converted into magnetic field by the front and back side of the electrons, right? So that's why there is electromagnetic field and not all of the electric field is converted into magnetic, right? I didn't actually understand the last part (bold). Why y-z plane? I can't imagine what do you think.

 

[stmartin]

waht? ? c

 

[waht]

So you say that the electron's electric field is converted into magnetic field by the front and back side of the electrons, right? So that's why there is electromagnetic field and not all of the electric field is converted into magnetic, right? I didn't actually understand the last part (bold). Why y-z plane? I can't imagine what do you think.

Just forget about y-z or x, this is just a 3 dimensional coordinate system.

When you take the away the electron's electric field strength in the direction of motion where else is it going to go?

It's going to concentrate away from the direction of motion and into a form of a disk that is perpendicular do the direction of motion. So an electron traveling close to the speed of light will have the strongest electric field at 90 degrees to the direction of motion.

A magnetic field is just an invented construct designed to help us understand the electric field relativistically.

An electromagnetic field is when an changing electric field induces a magnetic field, and then the magnetic field will induce the electric and so on, it's just like a dog chasing his own tail.


I quote from wiki

" explained in 1905 that a magnetic field is the relativistic part of an electric field.[4] It arises as a mathematical by-product of Lorentz coordinate transformation of electric field from one reference frame to another (usually from co-moving with the moving charge reference frame to the reference frame of non-moving observer).

(However, the Lorentz transformation cannot be applied to electric fields unless it already presupposes the existence of magnetic fields and their inter relationship with electric fields under the terms of Maxwell's equations. As such, the magnetic field can hardly be considered as a by-product of the Lorentz transformation.)

When an electric charge is moving from the perspective of an observer, the electric field of this charge due to space contraction is no longer seen by the observer as spherically symmetric due to non-radial time dilation, and it must be computed using the Lorentz transformations. One of the products of these transformations is the part of the electric field which only acts on moving charges — and we call it the "magnetic field". It is a relativistic manifestation of the more fundamental electric field. A magnetic field can be caused either by another moving charge (i.e., by an electric current) or by a changing electric field. The magnetic field is a vector quantity, and has SI units of tesla, 1 T = 1 kg·s-2·A-1. An equivalent, but older, unit for 1 Tesla is Weber/m2."

Read this from top to bottom a couple of times

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

 

[stmartin]

Just forget about y-z or x, this is just a 3 dimensional coordinate system.

When you take the away the electron's electric field strength in the direction of motion where else is it going to go?

It's going to concentrate away from the direction of motion and into a form of a disk that is perpendicular do the direction of motion. So an electron traveling close to the speed of light will have the strongest electric field at 90 degrees to the direction of motion.

A magnetic field is just an invented construct designed to help us understand the electric field relativistically.

An electromagnetic field is when an changing electric field induces a magnetic field, and then the magnetic field will induce the electric and so on, it's just like a dog chasing his own tail.


I quote from wiki

" explained in 1905 that a magnetic field is the relativistic part of an electric field.[4] It arises as a mathematical by-product of Lorentz coordinate transformation of electric field from one reference frame to another (usually from co-moving with the moving charge reference frame to the reference frame of non-moving observer).

(However, the Lorentz transformation cannot be applied to electric fields unless it already presupposes the existence of magnetic fields and their inter relationship with electric fields under the terms of Maxwell's equations. As such, the magnetic field can hardly be considered as a by-product of the Lorentz transformation.)

When an electric charge is moving from the perspective of an observer, the electric field of this charge due to space contraction is no longer seen by the observer as spherically symmetric due to non-radial time dilation, and it must be computed using the Lorentz transformations. One of the products of these transformations is the part of the electric field which only acts on moving charges — and we call it the "magnetic field". It is a relativistic manifestation of the more fundamental electric field. A magnetic field can be caused either by another moving charge (i.e., by an electric current) or by a changing electric field. The magnetic field is a vector quantity, and has SI units of tesla, 1 T = 1 kg·s-2·A-1. An equivalent, but older, unit for 1 Tesla is Weber/m2."

Read this from top to bottom a couple of times

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

As far as I know the magnetic field produces electric field when there is presence of conductor, and magnetic field lines are cutted by conductor. I was interested about the story for compressing the lines of force. What happens with the gravitational field? Is it also compressed by the movement of the moon or what?

 

[waht]

As far as I know the magnetic field produces electric field when there is presence of conductor, and magnetic field lines are cutted by conductor. I was interested about the story for compressing the lines of force. What happens with the gravitational field? Is it also compressed by the movement of the moon or what?

The moon thing is not related to this at all. I've used the term "tidal" because it refers to a net effect caused by asymmetry. In the case of an electron moving at high speeds, there is an asymmetry in its electric field.

But there is a theoretical study of gravitomagnetism that in the case of a mass traveling close to speed of light will induce a fictional force that is analogous to when a magnetic field is induced by an electric charge traveling close to speed light. However this has not been observed experimentally.

Also there is no need for a conductor. By Maxwell's laws, a change in electric field will induce a magnetic field, and vice versa a change in magnetic field will induce an electric field. If you bring a conductor close by, a change in magnetic field will move the electrons in the conductor. The moving electrons will themselves induce their own magnetic field which might oppose the magnetic field you are changing (eddy currents).

But note the magnetic field is only an effect of the electric field relativistically. We call it a magnetic field to distinguish the behavior of the electric field.

hope that helps.

 

[bernhard.rothenstein]

Magnetic field is just a relativistic effect of the electric field. When the charge is not moving, it has an electric field, and no magnetic field. However when it moves close to the speed or light, the electric field lines will be compressed, and a sort of tidal force will develop, which is the magnetic field. The strength of the magnetic field depends on how fast the charges move.

$$B = v * E/c^2$$

(cross product)

As far as I know the equation you propose holds in a given inertial reference frame if we can start with an inertial reference frame in which only an electric field is detected.
There is another relationship between E and B
E=vxB
as well when there is a starting inertial reference frame in which only a magnetic field and a moving test charge are detected.
The transformation equations for E and B account for both cases.

 

[stmartin]

The moon thing is not related to this at all. I've used the term "tidal" because it refers to a net effect caused by asymmetry. In the case of an electron moving at high speeds, there is an asymmetry in its electric field.

But there is a theoretical study of gravitomagnetism that in the case of a mass traveling close to speed of light will induce a fictional force that is analogous to when a magnetic field is induced by an electric charge traveling close to speed light. However this has not been observed experimentally.

Also there is no need for a conductor. By Maxwell's laws, a change in electric field will induce a magnetic field, and vice versa a change in magnetic field will induce an electric field. If you bring a conductor close by, a change in magnetic field will move the electrons in the conductor. The moving electrons will themselves induce their own magnetic field which might oppose the magnetic field you are changing (eddy currents).

But note the magnetic field is only an effect of the electric field relativistically. We call it a magnetic field to distinguish the behavior of the electric field.

hope that helps.

Thank you very much. I understood it. Sorry for my english, but what is net effect? So moving the electron's electric field will have the strongest strength because of its speed of motion?

 

[cabraham]

Einstein's word on this topic

This is an interesting thread. Good points have been made, but there seems to be one issue that I have trouble with. Some have suggested that the electric field, herein called "E", is *more fundamental* than the magnetic field "H". In other words, some view E as the "root", and H as a relativistic consequence. I am not a physicist, and make no claims of being an expert, but I do believe that Albert Einstein's published findings still hold today. In his paper "On The Electrodynamics Of Moving Bodies", 30 June 1905, AE states the following:

quote

As to the interpretation of these equations we make the following remarks: Let a point charge of electricity have the magnitude “one” when measured in the stationary system K, i.e. let it when at rest in the stationary system exert a force of one dyne upon an equal quantity of electricity at a distance of one cm. By the principle of relativity this electric charge is also of the magnitude “one” when measured in the moving system. If this quantity of electricity is at rest relatively to the stationary system, then by definition the vector (X, Y, Z) is equal to the force acting upon it. If the quantity of electricity is at rest relatively to the moving system (at least at the relevant instant), then the force acting
upon it, measured in the moving system, is equal to the vector (X0, Y0, Z0).

Consequently the first three equations above allow themselves to be clothed in
words in the two following ways:—
1. If a unit electric point charge is in motion in an electromagnetic field, there acts upon it, in addition to the electric force, an “electromotive force” which, if we neglect the terms multiplied by the second and higher powers of v/c, is equal to the vector-product of the velocity of the charge and the magnetic force, divided by the velocity of light. (Old manner of expression.)

2. If a unit electric point charge is in motion in an electromagnetic field, the force acting upon it is equal to the electric force which is present at the locality of the charge, and which we ascertain by transformation of the field to a system of co-ordinates at rest relatively to the electrical charge. (New manner of expression.)

The analogy holds with “magnetomotive forces.” We see that electromotive force plays in the developed theory merely the part of an auxiliary concept, which owes its introduction to the circumstance that electric and magnetic forces do not exist independently of the state of motion of the system of co-ordinates. Furthermore it is clear that the asymmetry mentioned in the introduction as arising when we consider the currents produced by the relative motion of a magnet and a conductor, now disappears. Moreover, questions as to the “seat” of electrodynamic electromotive forces (unipolar machines) now have no point.

end of quote


Well, unless I'm misconstruing AE's use of the word "seat" which he places quotation marks around, it is clear to me that his position on the matter is beyond debate. "Seat" in this context implies "root" or "basis". The online M-W dictionary says the same. AE says that electric and magnetic forces can never exist independently under *dynamic conditions*. Also, he states that which one is the "seat" simply has no point.

This is still the case today. E and H are, well, mutual and co-existant. Neither is more "fundamental" than the other. That is all we can really say.

As far as the motion of charge is concerned, my understanding has been as follows. Again, I don't claim to be the final authority. A static charge has a static electric field. When the charge moves, we have current, I, and a magnetic field. However, if the *rate* of charge motion, the current I, is constant, i.e. direct current, we have constant energy. If a superconducting loop carries a 1 uA current indefinitely, the energy is 0.5*L*(I^2) in circuit terms, or 0.5*B*H in field terms. There is NO electric field in this case. Under static conditions, we can have a non-zero H with zero E.

However, if the current in the loop is time-varying, we have a time-varying energy. In other words, the power is non-zero. Since power is 0.5*E X H, where "X" is the vector cross product, both E and H must be non-zero. When a field, E or H, changes with time, the other one must be non-zero as well. In the first case, the charges were moving in the wire constituting a current, but the rate was steady. The energy was constant, and the time derivative of a constant is zero. Since power is zero, either E or H must vanish. For a lossless current carrying loop, E is zero. For a charged lossless capacitor, E is steady and non-zero, whereas H is zero.

I think the confusion involves "static" vs. "dynamic". Although charges are in motion with respect to my reference frame, we must determine if the *rate* of charge motion is steady or time changing. If charges move at a steady rate, we can have H w/o E. If charges are stationary, we have E w/o H. If charges move at a time-varying rate, we now have time-varying energy, which constitutes non-zero power. Non-zero power requires non-zero H as well as non-zero E. They cannot exist independently.

It's good to go over these concepts, as it keeps us on our toes. Best regards.

Claude

 

[stmartin]

Thank you very much. I understood it. Sorry for my english, but what is net effect? So moving the electron's electric field will have the strongest strength because of its speed of motion?
quote from phys.lsu.edu

Electrons, spinning as they orbit the nuclei of atoms, create magnetic fields. The direction of spin of each electron determines the direction of the magnetic field surrounding it.

I http://gickr.com/results2/anim_fa2cfba7-13bf-5784-7155-43e7058b1574.gif"

 

[stmartin]

waht? dfs

 

[stmartin]

waht? Can you tell me why the direction of spin of each electron determines the direction of the magnetic field surrounding it?

 

[jtbell]

In classical electrodynamics, a spinning object is like a loop of current, which produces a dipole magnetic field:

Magnetic field of current loop

In quantum electrodynamics... ehhh... I'll let someone else tackle that.

 

[stmartin]

In classical electrodynamics, a spinning object is like a loop of current, which produces a dipole magnetic field:

Magnetic field of current loop

In quantum electrodynamics... ehhh... I'll let someone else tackle that.

I am interested why the direction of spin of each electron determines the direction of the magnetic field surrounding it when the electric field lines are all same around the electron? So if it contracts itself when it moves from the front and back side, no matter which direction it moves the magnetic field will be same.