Five questions about electromagnetic radiation

[pleco]

This is from Wikipedia:

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

The electromagnetic waves that compose electromagnetic radiation can be imagined as a self-propagating transverse oscillating wave of electric and magnetic fields.

1. What is really meant by "self-propagating", how it works?

2. How many electric and magnetic fields are in one wave?

3. What is the strength of those electric and magnetic fields?

4. What does -q and +q represent in the above diagram?

5. Why would electric and magnetic fields oscillate?

 

[Einj]

1. By self propagating it means that the change of the electric field generates a magnetic field which is changing itself. The change of the magnetic field generate an electric field and so on.

2. The field is something that is extended all over space. There is one electromagnetic field, and it is oscillating.

3. What do you mean by "strenght"? If you are talking about the amplitudes of the electric and magnetic fields, they are not fixed. They depend on each particular case. However, the are related as B=E/c.

4. Two oscillating charge.

5. Because the two charges oscillate.

 

[Drakkith]

5. Because the two charges oscillate.

The two charges are oscillating in response to the EM wave, they are not causing the EM wave.

To elaborate on a few of the questions:

1. Self propagating means that once the wave has been created, it continues to move outwards from the source without requiring anything to act on it.

2. An EM wave is a wave within the EM field. As Einj mentioned already, there is only one field. The part of the field that is "waving" is the field vectors, which is what we use to determine how an electric charge will respond to an electric field. When the vector (the lines in the wave above) points up, a positively charged particle will be accelerated up, while a negatively charged particle will accelerate down. When the vector points down the charges will reverse their direction of acceleration. This is why antennas work. The EM waves cause the charges in the antenna to oscillate back and forth, which is detected with the electronics and amplified for processing.

4. The +q and -q represent a positive lyand negatively charged particle respectively. They are there to show you how an electric charge responds to the wave.

5. The vectors of each field oscillate in response to an accelerated charge(s) that creates the wave. The details of why this occurs would require study of electromagnetism and involve some math.

 

[pleco]

1. By self propagating it means that the change of the electric field generates a magnetic field which is changing itself. The change of the magnetic field generate an electric field and so on.

Generation doesn't sound like propagation to me.

2. The field is something that is extended all over space. There is one electromagnetic field, and it is oscillating.

So both the electric and magnetic field are originating around the same point?

3. What do you mean by "strenght"? If you are talking about the amplitudes of the electric and magnetic fields, they are not fixed. They depend on each particular case. However, the are related as B=E/c.

I mean what is usually meant by strength or magnitude. Electric field is measured in volts per meter and magnetic field is measured in teslas or amperes per meter. Is this magnitude known for EM wave? What does the amplitude represent anyway?

4. Two oscillating charge.

You mean electric charge, like electron and proton?

5. Because the two charges oscillate.

Wouldn't charges radiate equally oscillating EM wave even if they accelerated linearly in just one direction instead of going back and forth?

 

[pleco]

1. Self propagating means that once the wave has been created, it continues to move outwards from the source without requiring anything to act on it.

That doesn't sound like self-propagation, more like Newton's first law and ordinary propagation that applies to everything else. Self-propagating sounds more like self-propelling to me, as if it has its own propulsion or acceleration system built-in.

2. An EM wave is a wave within the EM field. As Einj mentioned already, there is only one field. The part of the field that is "waving" is the field vectors, which is what we use to determine how an electric charge will respond to an electric field. When the vector (the lines in the wave above) points up, a positively charged particle will be accelerated up, while a negatively charged particle will accelerate down. When the vector points down the charges will reverse their direction of acceleration. This is why antennas work. The EM waves cause the charges in the antenna to oscillate back and forth, which is detected with the electronics and amplified for processing.

So the electric field has its strongest negative polarity magnitude at the top amplitude points, strongest positive polarity magnitude at the bottom amplitude points, and zero magnitude where the blue line crosses k vector? Basically it goes ON and OFF while switching polarity from positive to negative?

4. The +q and -q represent a positive lyand negatively charged particle respectively. They are there to show you how an electric charge responds to the wave.

They represent receiver antenna rather than emitter, even though they are on the left side of the diagram away from the direction of propagation?

5. The vectors of each field oscillate in response to an accelerated charge(s) that creates the wave. The details of why this occurs would require study of electromagnetism and involve some math.

Alternating direction of an electric current in a piece of wire will emit EM waves proportional to amperes or voltage, rate of change in current direction, and maybe length of the wire? How does that compare with a direct current where charges are accelerated in only one direction?

 

[davenn]

They represent receiver antenna rather than emitter, even though they are on the left side of the diagram away from the direction of propagation?

No, I understand that to mean that that is how a positive and a negative point charge will respond to a passing EM wave
Drakkith will correct that if I got it wrong

Alternating direction of an electric current in a piece of wire will emit EM waves proportional to amperes or voltage, rate of change in current direction, and maybe length of the wire? How does that compare with a direct current where charges are accelerated in only one direction?

in a DC circuit, the electrons and resulting charge are not accelerated so there is no EM generated

 

[Dale]

1. What is really meant by "self-propagating", how it works?

Electric fields and magnetic fields follow Maxwell's equations and the Lorentz force law:
http://en.wikipedia.org/wiki/Maxwell's_equations#Conventional_formulation_in_SI_units
http://en.wikipedia.org/wiki/Lorentz_force#Continuous_charge_distribution

Those laws admit a wave solution:
http://en.wikipedia.org/wiki/Maxwel...s.2C_electromagnetic_waves_and_speed_of_light

One thing that characterizes waves is that they "self-propagate". I.e. a wave here now propagates into a wave there later.

2. How many electric and magnetic fields are in one wave?

You don't count fields. They are not discrete.

3. What is the strength of those electric and magnetic fields?

That is related to the intensity of the wave. In the visible range of the EM spectrum a stronger EM field corresponds to a brighter light.

4. What does -q and +q represent in the above diagram?

Charges, presumably from a dipole antenna.

5. Why would electric and magnetic fields oscillate?

Because they follow Maxwell's equations and Maxwell's equations admit a wave solution. See the response to 1.

 

[Dale]

in a DC circuit, the electrons and resulting charge are not accelerated so there is no EM generated

Interestingly, in a DC circuit you can get no EM generated even when individual electrons are accelerated, e.g. in a bend in a wire carrying DC. The key point is that the current density is static so the magnetic fields are static.

 

[davenn]

hi dalespam

The key point is that the current density is static so the magnetic fields are static.
That's what I really wanted to say ... a mental block stopped me putting into words

thanks :)

Dave

 

[Einj]

The two charges are oscillating in response to the EM wave, they are not causing the EM wave.

Actually at the beginning they are what causes the oscillation of the field. This is how antennas work. But your are probably right, after the field starts oscillating their motion is sustained by the field itself.

 

[vanhees71]

Of course, you are right. An electromagnetic wave (e.g., light or radio waves) must be created by acceleration of charges (in the classical picture, to which I'd like to stick for a while in this thread).

The sources (and the only sources) of the electromagnetic fields are charge and current distributions. If these are time dependent (and not just the Lorentz boost of a static charge distribution to another inertial frame, where they move with a constant velocity) electromagnetic radiation is produced.

These fields are dynamically propagating not only within matter containing charged particles but also in free space. That's what Wikipedia might mean by "self propelling". It's a bad wording, however, because there is nothing "self propelling" here. It's just like a ball you through, and it's moving further without being in contact with you anymore. In free space, i.e., without gravity and air friction, according to Newton's Laws it would just go ahead in uniform motion. It's not self-propelled. In a similar way also the free em. field travels on in free space and is not self-propelled.

It is also a wrong picture you get when it's said that the changing electric field causes a magnetic field and vice versa. You cannot interpret the corresponding homogeneous Maxwell equations (i.e., Faraday's Law and the Ampere-Maxwell Law) in a causal sense. There is only one electromagnetic field. It's even an observer dependent statement what you call "electric" or "magnetic" fields. The electromagnetic field as a whole is a frame-independent concept (described by an antisymmetric 2nd-rank tensor in Minkowski space, called the Faraday tensor).

The "strength of the field" in a certain sense can be associated with it's energy density, which is given (in Heaviside-Lorentz units) by
$$\epsilon=\frac{1}{2}(\vec{E}^2+\vec{B}^2).$$
It depends on how vigorously you wiggle the charges, labeled with $\pm q$ in the figure, which is, however, very misleading, if not entirely wrong. What seems to be depicted is the most simple example of a radiation source, the Hertzian dipole (which is realized approximately by, e.g., a linear antenna with a harmonic current; it becomes exact in the limit of vanishing length of such an antenna). The field looks entirely different from the plane wave plotted in the picture. A much better picture (even animated) can be found here:

http://en.wikipedia.org/wiki/Dipole_radiation#Dipole_radiation

The 5th question cannot be answered other than by "that's how nature is". The Maxwell equations which describe electromagnetic phenomena to high accuracy (as long as quantum effects can be neglected) are fundamental laws which have been found after some centuries of observations and just summarize in an elegant mathematical model these observations. They canno be derived from simpler principles and are fundamental in that sense. The existence of electromagnetic wave fields has been in fact predicted by Maxwell and have been found later in experiment by H. Hertz. This is a typical example for the way how science works: You start from a lot of observations, doing quantitative measurements and then find a fundamental law (which is a very rare event, by the way) which leads to further predictions about phenomena that may never have been observed before (or as in the case of electromagnetic radiation have not been realized as such; nowadays we know that visible light is nothing else the electromagnetic radiation in a specific range of wave lengths our eyes are sensitive to). Then you can try to do new experiments to check, whether electromagnetic waves really exist. If not, you have to modify your mathematical model or create a completely new one. If you find them, as is the case here, you have consolidated the validity of the model, and so on.

 

[Dale]

It is also a wrong picture you get when it's said that the changing electric field causes a magnetic field and vice versa. You cannot interpret the corresponding homogeneous Maxwell equations (i.e., Faraday's Law and the Ampere-Maxwell Law) in a causal sense.

I agree completely here. Causes always come before effects, and Maxwell's equations show the relationship of the E and B fields at the same time. So at most you can say that they are related or associated as described by Maxwell, but the "causes" language is incorrect.

Of course, you are right. An electromagnetic wave (e.g., light or radio waves) must be created by acceleration of charges (in the classical picture, to which I'd like to stick for a while in this thread).

The sources (and the only sources) of the electromagnetic fields are charge and current distributions.

However, I think that you are going a little too far here. Classical EM is perfectly consistent with EM waves that exist without ever having been created by a charge moving, (i.e. as a boundary condition of the universe or even as advanced potentials). I think that it is true that the EM waves that we deal with on a daily basis were all created by acceleration of charges, but classical EM is broader than that.

Just as you can say that an EM wave here now was caused by charge acceleration there yesterday, you can also say that an EM wave here now was caused by an EM wave there yesterday (ad infinitum).

I agree that we should stick with classical EM here, but I don't think that we need to make a broad sweeping statement about classical EM that is not completely required in classical EM and would have to be revised in QM and cosmology.

 

[sophiecentaur]

1. What is really meant by "self-propagating", how it works?

The reason or explanation for wave propagation is that it takes time (a delay) for an effect, in one place to reach another place. As you change the positions of the two charges (as they oscillate together and apart, for instance) and plot how the effect varies as you get further away, you will get a snapshot of the way oscillation varies in time, and that reveals itself as a 'graph' over the distance. The wavy graph will be moving away from the source at the speed (wave speed) that the effect can travel through the medium (space / air etc)
This is the same for EM and mechanical waves - it's just the mechanism that's different. The overall effect is that energy gets carried away from the source by the waves.

 

[vanhees71]

I agree completely here. Causes always come before effects, and Maxwell's equations show the relationship of the E and B fields at the same time. So at most you can say that they are related or associated as described by Maxwell, but the "causes" language is incorrect.

Indeed. If you solve Maxwell's equations, you'll find the retarded potentials in the Lorenz gauge and then by the appropriate derivatives the gauge-independent field equations which are also retarded quantities with respect to the sources $\rho$ and [/itex]\vec{j}[/itex]. No field components occur as sources of other field components (that's what's known as Jefimenko equations in the literature; see also the Wikipedia, which has a good article on it).

You can, of course, integrate also, e.g., the Ampere-Maxwell Law to express the magnetic field components in terms of the current and the magnetic field components. This, however is not a causal law, i.e., it's not expressing the solution as a retarded expression if you interpret $\vec{j}$ and $\vec{E}$ as sources.

However, I think that you are going a little too far here. Classical EM is perfectly consistent with EM waves that exist without ever having been created by a charge moving, (i.e. as a boundary condition of the universe or even as advanced potentials). I think that it is true that the EM waves that we deal with on a daily basis were all created by acceleration of charges, but classical EM is broader than that.

Just as you can say that an EM wave here now was caused by charge acceleration there yesterday, you can also say that an EM wave here now was caused by an EM wave there yesterday (ad infinitum).

I agree that we should stick with classical EM here, but I don't think that we need to make a broad sweeping statement about classical EM that is not completely required in classical EM and would have to be revised in QM and cosmology.

You are right, from just locally observing an electromagnetic radiation field in free space, I cannot conclude anything on the sources in a unique way. For this I'd need the field in the entire space in order to be able to use the Maxwell equations to get back the charge and current distributions.

Concerning the advanced solutions, you are also right. They are valid solutions of the Maxwell equations. The only trouble with them is to realize them in practice.

As an example take the usual thing of an antenna with some AC radiating off radio waves. It's to a good approximation dipole radiation. This is the retarded solution, which is easy to realize in the real world.

Now, of course you can just apply a "time reversal transformation", leading to the advanced solution. Since the em. interaction is time-reversal invariant you get a valid solution of Maxwell's equations, and there is nothing telling us that this is not possible to occur in nature. However, it's pretty difficult to realize in practice. What you'd need to do is to create a dipole field somehow in some distance from the antenna precisely such that it gets absorbed by the antenna causing the (time reversed) AC current we started from to create it. This is practically impossible to realize, and that's why we usually only consider the retarded solutions.

Another thing are attempts to get rid of the fields altogether to establish an interacting theory between point particles without the mediation of a field, i.e., a kind of action-at-a-distance model which does not violate special-relativistic causality. The most famous example is the absorber theory by Feynman and Wheeler, which they invented originally as a starting point for a relativistic quantum theory of a system of point particles without the fields in the hope to solve the problems with divergences in perturbative QED. As we all know, this idea has not survived, and Feynman finally solved the QED problem in terms of renormalization theory (on the way inventing path integrals and Feynman diagrams :-)).

 

[pleco]

in a DC circuit, the electrons and resulting charge are not accelerated so there is no EM generated

How about if the circuit was turned on and off, so electrons accelerate to their drift speed and then decelerate back to normal, repeatedly? How about if voltage supplied is increasing rapidly, or increasing and decreasing repeatedly? How about if the wire was a loop, would electrons following a curved path count as acceleration? Or how about if electron beam is bent with magnetic or electric fields, does that count as acceleration and do they emit EM waves then?

 

[pleco]

Electric fields and magnetic fields follow Maxwell's equations and the Lorentz force law:
http://en.wikipedia.org/wiki/Maxwell's_equations#Conventional_formulation_in_SI_units
http://en.wikipedia.org/wiki/Lorentz_force#Continuous_charge_distribution

Those laws admit a wave solution:
http://en.wikipedia.org/wiki/Maxwel...s.2C_electromagnetic_waves_and_speed_of_light

One thing that characterizes waves is that they "self-propagate". I.e. a wave here now propagates into a wave there later.

I read those equations say something like "mutually inductive", but nothing that says "self-propagating". I've also never heard of any other waves being regarded as self-propagating, but simply propagating and subject to Newton's first law as well. As far as I know we don't say photons are self-propagating, just propagating, also supposed to follow Newton's first law, aren't they?

You don't count fields. They are not discrete.

You count their sources and sinks, the points of attraction and repulsion. A single electric field at a single point in time can be either a source or a sink, that is either of positive or negative polarity. A single magnetic field must have both, a source and a sink, it has to be a dipole. Isn't that what Maxwell's equations say?

That is related to the intensity of the wave. In the visible range of the EM spectrum a stronger EM field corresponds to a brighter light.

Can you print down that equation or provide some reference about it?

 

[Drakkith]

How about if the circuit was turned on and off, so electrons accelerate to their drift speed and then decelerate back to normal, repeatedly? How about if voltage supplied is increasing rapidly, or increasing and decreasing repeatedly?

If the voltage or current is changing, then the circuit will generate EM waves. This can occur in DC circuits when you switch the circuit on or off or when you have variable resistors or other circuit components.

How about if the wire was a loop, would electrons following a curved path count as acceleration? Or how about if electron beam is bent with magnetic or electric fields, does that count as acceleration and do they emit EM waves then?

Steady current flowing through a loop of wire generates no EM waves since the magnetic field is static. Bending an electron beam will indeed generate EM waves. This is how the free electron laser works: http://en.wikipedia.org/wiki/Free_electron_laser

 

[Dale]

I've also never heard of any other waves being regarded as self-propagating, but simply propagating

That is fine. The English terminology is imprecise. The solution to a wave equation has a certain mathematical form. If you want to describe that form as "propagating" rather than "self propagating" that is fine as long as you are referring to the same equation.

You count their sources and sinks

Even that seems problematic to me, but even if you have a countable set of sources you still don't count the fields.

If you think this is a meaningful procedure then please provide a reference. I personally have never seen any professional reference count the number of fields.

Can you print down that equation or provide some reference about it?

Ceetainly. I will post it this evening when I get home.

 

[pleco]

These fields are dynamically propagating not only within matter containing charged particles but also in free space. That's what Wikipedia might mean by "self propelling". It's a bad wording, however, because there is nothing "self propelling" here. It's just like a ball you through, and it's moving further without being in contact with you anymore. In free space, i.e., without gravity and air friction, according to Newton's Laws it would just go ahead in uniform motion. It's not self-propelled. In a similar way also the free em. field travels on in free space and is not self-propelled.

Wikipedia says "self-propagating". It is only me who said that sounds as "self-propelling", because to me neither makes sense in the same way.

The "strength of the field" in a certain sense can be associated with it's energy density, which is given (in Heaviside-Lorentz units) by
$$\epsilon=\frac{1}{2}(\vec{E}^2+\vec{B}^2).$$
It depends on how vigorously you wiggle the charges, labeled with $\pm q$ in the figure, which is, however, very misleading, if not entirely wrong. What seems to be depicted is the most simple example of a radiation source, the Hertzian dipole (which is realized approximately by, e.g., a linear antenna with a harmonic current; it becomes exact in the limit of vanishing length of such an antenna). The field looks entirely different from the plane wave plotted in the picture. A much better picture (even animated) can be found here:

http://en.wikipedia.org/wiki/Dipole_radiation#Dipole_radiation

So is electric field in EM wave negative or positive, or both negative and positive in the same time, or little bit negative and then a little bit positive?

The 5th question cannot be answered other than by "that's how nature is". The Maxwell equations which describe electromagnetic phenomena to high accuracy (as long as quantum effects can be neglected) are fundamental laws which have been found after some centuries of observations and just summarize in an elegant mathematical model these observations. They canno be derived from simpler principles and are fundamental in that sense. The existence of electromagnetic wave fields has been in fact predicted by Maxwell and have been found later in experiment by H. Hertz. This is a typical example for the way how science works: You start from a lot of observations, doing quantitative measurements and then find a fundamental law (which is a very rare event, by the way) which leads to further predictions about phenomena that may never have been observed before (or as in the case of electromagnetic radiation have not been realized as such; nowadays we know that visible light is nothing else the electromagnetic radiation in a specific range of wave lengths our eyes are sensitive to). Then you can try to do new experiments to check, whether electromagnetic waves really exist. If not, you have to modify your mathematical model or create a completely new one. If you find them, as is the case here, you have consolidated the validity of the model, and so on.

Maxwell didn't just accidentally find a fundamental law or blindly attempt to formulate the wave equation. It is interesting then his theory was discarded while the equations were kept. It's like developing a theory to make pudding just to end up with the recipe for making a perfect sausage. Mmmm.

 

[Drakkith]

So is electric field in EM wave negative or positive, or both negative and positive in the same time, or little bit negative and then a little bit positive?

The electric field at anyone point alternates from + to - and back over time.

 

[pleco]

The reason or explanation for wave propagation is that it takes time (a delay) for an effect, in one place to reach another place. As you change the positions of the two charges (as they oscillate together and apart, for instance) and plot how the effect varies as you get further away, you will get a snapshot of the way oscillation varies in time, and that reveals itself as a 'graph' over the distance. The wavy graph will be moving away from the source at the speed (wave speed) that the effect can travel through the medium (space / air etc)
This is the same for EM and mechanical waves - it's just the mechanism that's different. The overall effect is that energy gets carried away from the source by the waves.

Wouldn't negative and positive charge create their own separate waves, or do we need both together to create a single wave?

Let a single electron accelerate vertically up and down 5 times per second over 1 meter distance, that is total of 10 meters per second. How many waves will be emitted in one second: 5, 10, or some larger number? In other words, is wave emitting continuous or happens only relative to discrete number of oscillations?

 

[pleco]

If the voltage or current is changing, then the circuit will generate EM waves. This can occur in DC circuits when you switch the circuit on or off or when you have variable resistors or other circuit components.

Steady current flowing through a loop of wire generates no EM waves since the magnetic field is static. Bending an electron beam will indeed generate EM waves. This is how the free electron laser works: http://en.wikipedia.org/wiki/Free_electron_laser

It makes sense. Thanks. Still plenty remains a mystery to me.

 

[pleco]

If you think this is a meaningful procedure then please provide a reference. I personally have never seen any professional reference count the number of fields.

Neither have I, that's why I'm asking. Although it looks like electric and magnetic fields in EM wave are referred to in singular rather than plural form when addressed separately.

 

[Drakkith]

Are you thinking that charges generate fields and that there should be as many fields as there are charges?

 

[pleco]

Are you thinking that charges generate fields and that there should be as many fields as there are charges?

I don't know. But from what I know it seems a single charge can create many EM waves, where each of those waves consists of a single electric and single magnetic field. It's even more unclear to me whether the electric field is a monopole and magnetic field a dipole, how their polarity and magnitude changes in respect to time and space, and whether is position of their sinks and sources defined in the same location, different locations, or not defined at all.

 

[Dale]

Can you print down that equation or provide some reference about it?

See page 3 here:
http://web.monroecc.edu/manila/webfiles/spiral/8.EMWaves.pdf

In natural units the magnitude of the intensity is EB. Later it calculates the average intensity for a pure sinusoidal (linearly polarized) wave which half of the maximum intensity.

 

[Dale]

I don't know. But from what I know it seems a single charge can create many EM waves, where each of those waves consists of a single electric and single magnetic field.

It doesn't matter. The count of fields doesn't show up in Maxwell's equations, so it doesn't influence any of the behavior of EM phenomena.

Furthermore, I do not know of any reference which identifies a wave as consisting of a single E and a single B field. So, this is not a topic for PF.

 

[Drakkith]

I don't know. But from what I know it seems a single charge can create many EM waves, where each of those waves consists of a single electric and single magnetic field. It's even more unclear to me whether the electric field is a monopole and magnetic field a dipole, how their polarity and magnitude changes in respect to time and space, and whether is position of their sinks and sources defined in the same location, different locations, or not defined at all.

It may help to think of the EM field as a single continuous field that extends throughout all of space and to think of electric charges as "altering" the field. That avoids the problem of having "sinks" and "sources". Besides, the field lines you see coming out of or going into charged particles are not real objects. They are there to help you visualize the field.

Some links to read:

http://en.wikipedia.org/wiki/Field_(physics )
http://en.wikipedia.org/wiki/Field_line

 

[pleco]

See page 3 here:
http://web.monroecc.edu/manila/webfiles/spiral/8.EMWaves.pdf

"Electric charges are surrounded by electric fields. When these charges move, the electric field changes. Additionally, these moving charges (or, more importantly, these changing electric fields) create magnetic fields. Since before the motion no magnetic field existed, this is a change in the value of the magnetic field (from zero to non-zero). By electromagnetic induction, this change in magnetic field creates emf, which is a change in the value of the electric field in a region of space. But this change in electric field must cause further change in the magnetic field, which must cause further change in the electric field, which must ..."


It sounds as if E and B fields would continue to increase each other in magnitude to infinity. It doesn't make sense to me, they don't really say what change and what value they are talking about. They say when charges move the electric field changes, whatever that means, but doesn't it electron keep its constant elementary electric field regardless of whether and how it moves or not?

In natural units the magnitude of the intensity is EB. Later it calculates the average intensity for a pure sinusoidal (linearly polarized) wave which half of the maximum intensity.

I see some energy flow vector is defined by E and B field vectors, but I don't see what any of those vectors practically mean or how could they be measured.

 

[Dale]

It sounds as if E and B fields would continue to increase each other in magnitude to infinity. It doesn't make sense to me, they don't really say what change and what value they are talking about. They say when charges move the electric field changes, whatever that means, but doesn't it electron keep its constant elementary electric field regardless of whether and how it moves or not?

This is why the language of physics is math, not English. The English description can never match the precision of the math. If you want it to make sense then, at some point, you will need to learn the math.

I can tell you that the fields do not increase in magnitude to infinity. That would violate the conservation of energy. I can also tell you that the field of a moving charge is different from that of a stationary one. A moving one has charge and current, while a stationary one has only charge.

The math makes all of this explicit.

 

[pleco]

It may help to think of the EM field as a single continuous field that extends throughout all of space and to think of electric charges as "altering" the field. That avoids the problem of having "sinks" and "sources". Besides, the field lines you see coming out of or going into charged particles are not real objects. They are there to help you visualize the field.

Some links to read:

http://en.wikipedia.org/wiki/Field_(physics )
http://en.wikipedia.org/wiki/Field_line

There are fields lines and then there are force lines. Electric field lines coincide with force lines, but magnetic field lines and force lines are perpendicular. The question is only what kind of lines those vectors in the diagram represent, where are they measured at and what are they relative to.

Here I isolated a single point at time t0 and five points of interest marked from p0 to p4. Is electric field stronger at point p0 or p1? Is electric field at point p1 positive or negative? Compared to p1, what magnitude and polarity is the electric filed at point p2, p3, and p4? Same questions for the magnetic field.

 

[vanhees71]

What are the initial conditions? At which time are you looking at the various points? How are the charges moving? Without this input, nobody can calculate what you want to know. Please look up the Wiki page on electric-dipole radiation which gives you a pretty good overview over the field created by such a dipole if the time dependence is harmonic and the wiggling of the charges started a very long time ago. As I already said the plane-wave picture is wrong (or only valid approximately very far away from the dipole).

The italics text in #29 is utter nonsense, as I've already explained earlier in this thread too. The correct equations to look at are Jefimenko's retarded solutions of the Maxwell equations for a given charge-current distribution. A time varying electric field cannot be straight-forwardly interpreted as the source of a magnetic (vortex) field and vice versa. This idea is simply a misinterpretation of Maxwell's equations that are local in space and time.

 

[pleco]

What are the initial conditions? At which time are you looking at the various points? How are the charges moving?

I'm not setting up conditions or deciding how charges are moving, just trying to interpret the diagram. I'm looking at any instant in time where blue vectors are depicted as vertical arrows pointing upwards.

Without this input, nobody can calculate what you want to know.

I'm not asking for any numerical values now, only about general properties and basic relations: greater, less or equal, positive or negative.

The italics text in #29 is utter nonsense, as I've already explained earlier in this thread too. The correct equations to look at are Jefimenko's retarded solutions of the Maxwell equations for a given charge-current distribution. A time varying electric field cannot be straight-forwardly interpreted as the source of a magnetic (vortex) field and vice versa. This idea is simply a misinterpretation of Maxwell's equations that are local in space and time.

It doesn't make sense to me either. As I remember Ampere's and Faraday's law it's moving E field that creates B field, not changing, just moving, and moving E fields are electric current. So basically it's electric currents that create B field depending on current density. In return changing magnetic field can create an electric current by making E fields move, but I never thought B field can change E field or create new ones.

 

[pleco]

The math makes all of this explicit.

Not enough if there can be misinterpretations like vanhees71 is talking about. Maxwell himself had a whole different idea about what those equations mean and represent. Apparently they can mean different things and be interpreted in different ways, unfortunately.

 

[Dale]

There are fields lines and then there are force lines. Electric field lines coincide with force lines, but magnetic field lines and force lines are perpendicular. The question is only what kind of lines those vectors in the diagram represent, where are they measured at and what are they relative to.

The vectors in the diagram are field lines.

Here I isolated a single point at time t0 and five points of interest marked from p0 to p4. Is electric field stronger at point p0 or p1? Is electric field at point p1 positive or negative? Compared to p1, what magnitude and polarity is the electric filed at point p2, p3, and p4? Same questions for the magnetic field.

The electric field is only shown at p0. The end of the vector is not a point in space, it is just a representation of the strength and direction of the E field at p0. This is a fairly standard method of representing vector fields.

http://mathinsight.org/vector_field_overview

Regarding the polarity, that depends on the direction of the coordinate axes, which are not explicitly shown. If right is x, up is y, and out of the screen is z, then the E and B at p0 are both positive.