Undergrad Do Electric field lines propagate by themselves away from a charge?

[gionole]

In the limit of an infinite number of infinitesimal kinks, the result is no longer a kink. It's a smooth curve that interpolates between the E-field lines before and after the smooth (non-impulsive) acceleration.

If kinks are not produced in realistic examples(real world),

Question 1: why does every animation actually show it ?

Question 2: Then whats EM wave if not kinks ? Something going out from charge which is what if not kink ?

 

[Dale]

Question 1: why does every animation actually show it ?

Not every animation does.

Question 2: Then whats EM wave if not kinks ? Something going out from charge which is what if not kink ?

An EM wave can form from any shape. It does not need to have any kinks. A more useful and more common representation is a sinusoidal wave. Any well behaved wave can be described as a sum of sinusoidal waves. You don’t need infinite acceleration, just non-zero acceleration.

 

[renormalize]

If kinks are not produced in realistic examples(real world),

Question 1: why does every animation actually show it ?

Question 2: Then whats EM wave if not kinks ? Something going out from charge which is what if not kink ?

Answer 1: The simplest pictures/animations are for an infinite acceleration of a charge, which leads to a kink.
Answer 2: As I mentioned in my post, for a smooth acceleration, the kink is replaced by a smooth curve. For example, here is a depiction of the E-field lines of a charge undergoing a finite oscillating acceleration due to simple harmonic motion in the vertical direction (https://physics.weber.edu/schroeder/mrr/MRRtalk.html):

 

[Drakkith]

if not from 10 to 10.11, it will be from 10 to 10.01. Acceleration is this. At some point changes velocity from some value to another. For sure, changing velocity means that speed has to change. So changing value from 10 to 10.01 is realistic.

Unfortunately this isn't possible. Velocity change is a continuous process, not a discrete one. That is, given some change in velocity $\Delta v$ during some time period $\Delta t$, you can always divide both of these by a real number (2 or 3 or whatever) to get a smaller change in velocity over a smaller period of time.

So moving from 10 m/s to 10.01 m/s we get $\frac{0.01}{\Delta t}$ where $\Delta t$ is a non-zero number. Assuming the acceleration is constant (which it doesn't have to be, but it doesn't change my argument much) we can break this into two time periods of $\frac{\Delta t}{2}$ each, where the velocity change is now 0.005 m/s. This process can be continued forever, yielding ever smaller velocity changes over smaller time periods.

Another way: change in velocity is given by: $v_f=v_0 + at$ where $v_f$ is the final velocity, $v_0$ is the initial velocity, and $at$ is the acceleration multiplied by some time period. As we make $\Delta t$ smaller, the term $at$ also gets smaller, approaching zero in the limit of $t->0$ and causing $v_f$ to approach the initial velocity $v_0$. Simply plugging in zero for $t$ obviously gives us no change in velocity since $at$ is zero, but the relevant bit here is that the limit takes us through all real numbers and is a continuous 'process', not discrete. There is always a smaller number for $t$ and so we always get a smaller velocity change. There is simply no way for velocity to change from 10 to 10.01, or any two real numbers, over 0 time, which is what is required for an instantaneous jump.

One final thing to note is that $a$ must always be a finite number. $a$ can be any number you want, no matter how large, but it MUST be finite. It must be a number (infinity isn't a number). Trying to plug in infinity into the velocity equation leads to nonsensical results.

 

[gionole]

Answer 1: The simplest pictures/animations are for an infinite acceleration of a charge, which leads to a kink.
Answer 2: As I mentioned in my post, for a smooth acceleration, the kink is replaced by a smooth curve. For example, here is a depiction of the E-field lines of a charge undergoing a finite oscillating acceleration due to simple harmonic motion in the vertical direction (https://physics.weber.edu/schroeder/mrr/MRRtalk.html):
View attachment 327463

Are these still kinks ? If not, what you call them ?

 

[gionole]

Unfortunately this isn't possible. Velocity change is a continuous process, not a discrete one. That is, given some change in velocity $\Delta v$ during some time period $\Delta t$, you can always divide both of these by a real number (2 or 3 or whatever) to get a smaller change in velocity over a smaller period of time.

So moving from 10 m/s to 10.01 m/s we get $\frac{0.01}{\Delta t}$ where $\Delta t$ is a non-zero number. Assuming the acceleration is constant (which it doesn't have to be, but it doesn't change my argument much) we can break this into two time periods of $\frac{\Delta t}{2}$ each, where the velocity change is now 0.005 m/s. This process can be continued forever, yielding ever smaller velocity changes over smaller time periods.

Another way: change in velocity is given by: $v_f=v_0 + at$ where $v_f$ is the final velocity, $v_0$ is the initial velocity, and $at$ is the acceleration multiplied by some time period. As we make $\Delta t$ smaller, the term $at$ also gets smaller, approaching zero in the limit of $t->0$ and causing $v_f$ to approach the initial velocity $v_0$. Simply plugging in zero for $t$ obviously gives us no change in velocity since $at$ is zero, but the relevant bit here is that the limit takes us through all real numbers and is a continuous 'process', not discrete. There is always a smaller number for $t$ and so we always get a smaller velocity change. There is simply no way for velocity to change from 10 to 10.01, or any two real numbers, over 0 time, which is what is required for an instantaneous jump.

One final thing to note is that $a$ must always be a finite number. $a$ can be any number you want, no matter how large, but it MUST be finite. It must be a number (infinity isn't a number). Trying to plug in infinity into the velocity equation leads to nonsensical results.

To be honest, I understood your well described case. But the tricky part is we say kinks are only produced while charge changes velocity in zero time. I understand velocity cant change in zero time but why is this required for kink to be produced ? If you imagine charge which was moving constantly till x1 position and then accelerates(lets make this 1m/s), this means charge changes velocity continously from 10m/s to 10.00001 then to 10.000002 and so on(I did not include more decimals for simplicity
But I get it they will be continous) but you all are saying that when velocity changes continously, kink wont be produced. And this is what is really hard to grasp. Look at this this way. Charge when accelerated definitely moved from x1 to some very very close position(the closest neighbour of x1),
Then when charge did this, its field lines didn’t follow along(since field lines have the old speed - its true that new speed and old speed difference will soooo small but still different) so we got a case where old field lines are a little bit retarded and this should cause the kink formation so the field line from charges new position gets joined with old field line which then moves outward with the speed of light. Where would the exact mistake be in my case and why dont you call this kink ?

 

[Drakkith]

I understand velocity cant change in zero time but why is this required for kink to be produced ?

Ideally, a kink is an instantaneous change in the field line over zero distance and time. This is only possible if the particle has an instantaneous change in its velocity, which as has been explained isn't possible. In practice, if you zoom into the region of a kink in a field line you will find a smooth transition. It just looks like a kink if the acceleration happens over a sufficiently short period of time relative to how closely you're looking at the field line.

So a field line that looks like it makes a sudden angle change of 20 degrees will actually have that change occur over a small time period. Zooming in to the sharp 'elbow' will show that it is curved over some distance, not an instantaneous bend. It will actually look like your elbow instead of two straight lines coming together.

 

[Vanadium 50]

I'm not looking for math(maxwell equations) discussion, but some logical sense

Unfortunately, crankpots say this a lot. One problem with this is that when people talk like crackpots, people assume they are crackpots. The second problem is that physics is a quantitative science - it doesn't say "what goes up must come down" but where and when it comes down. The next problem is you have numbers in your OP, but want them explained without math. Not easy. Finally, the whole sentence can be rephrased as "I know there is an explanation, but I don;t want to learn it. I want a different explanation that is easier to learn." As was said a long time ago, there is no royal road to mathematics.

Electric fields are mathematical models. I will not get into a discussion of how "real" they are, bui electric field lin-es are products of our imagination. One can say "the field there is twice as strong as there" but not "thjere are 100 lines there but only 50 in that other place". As products

Now, take a look at the "kinks. A field line is the direction a test charge will move. Will a test charge move along a kink? If it does it's for zero time - these pictures are points in time - so the net motion is zero. Hard to get worked up about something that happens for zero time.

 

[gionole]

Ideally, a kink is an instantaneous change in the field line over zero distance and time. This is only possible if the particle has an instantaneous change in its velocity, which as has been explained isn't possible. In practice, if you zoom into the region of a kink in a field line you will find a smooth transition. It just looks like a kink if the acceleration happens over a sufficiently short period of time relative to how closely you're looking at the field line.

So a field line that looks like it makes a sudden angle change of 20 degrees will actually have that change occur over a small time period. Zooming in to the sharp 'elbow' will show that it is curved over some distance, not an instantaneous bend. It will actually look like your elbow instead of two straight lines coming together.

So you call “kink” a line which is not bent even at least a little bit. So it is the straight joined vector between a new line and old field lines. And as said, if acceleration is not super fast in short period of time, you say that we will still have old and field lines joining together but instead of a straight line, now they will be connected with curved line which also moves outwards with speed of light ?

 

[Dale]

Are these still kinks ? If not, what you call them ?

Waves.

I have never heard the term “kinks” in this context before you. Not in 30 or so years of doing this stuff

 

[gionole]

Unfortunately, crankpots say this a lot. One problem with this is that when people talk like crackpots, people assume they are crackpots. The second problem is that physics is a quantitative science - it doesn't say "what goes up must come down" but where and when it comes down. The next problem is you have numbers in your OP, but want them explained without math. Not easy. Finally, the whole sentence can be rephrased as "I know there is an explanation, but I don;t want to learn it. I want a different explanation that is easier to learn." As was said a long time ago, there is no royal road to mathematics.

Electric fields are mathematical models. I will not get into a discussion of how "real" they are, bui electric field lin-es are products of our imagination. One can say "the field there is twice as strong as there" but not "thjere are 100 lines there but only 50 in that other place". As products

Now, take a look at the "kinks. A field line is the direction a test charge will move. Will a test charge move along a kink? If it does it's for zero time - these pictures are points in time - so the net motion is zero. Hard to get worked up about something that happens for zero time.

Well, to be sure, one can know math but still did not know what EM wave is. I have seen lots of people like this, they explain it to me in terms of math but have no idea what it is logically. I dont agree there and definitely Einshtein would disagree with you as well.

Maxwells equations should not be needed to understand the waves at all. I dont want to go into discussing this - as dont want to deviate from the current discussion

 

[gionole]

Waves.

I have never heard the term “kinks” in this context before you. Not in 30 or so years of doing this stuff

Maybe this is where we got some misunderstanding what we call kink. So just copying here my last reply:

So you call “kink” a line which is not bent even at least a little bit. So it is the straight joined vector between a new line and old field lines. And as said, if acceleration is not super fast in short period of time, you say that we will still have old and field lines joining together but instead of a straight line, now they will be connected with curved line which also moves outwards with speed of light ?

 

[renormalize]

Are these still kinks ? If not, what you call them ?

They are continuous curves with continuous derivatives (tangents). Curves with kinks have sharp corners with discontinuous tangents.

 

[renormalize]

And as said, if acceleration is not super fast in short period of time, you say that we will still have old and field lines joining together but instead of a straight line, now they will be connected with curved line which also moves outwards with speed of light ?

Yes.

 

[Dale]

Maybe this is where we got some misunderstanding what we call kink.

Why are you focused on these kinks at all? It is a non-realistic wave. You will be much better off learning about realistic waves like sinusoids.

Your focus on kinks is leading you into all sorts of misunderstandings.

-The static E field does not have a speed

-E field lines do not have an age

-There is no identification of points along a field line

-There is no identification of field lines at different points in time

I don’t know if you have been exposed to some horrible reference that it putting all this into your mind, but you really need to try to mentally clear what you think you know and start fresh. What you are describing here is unrecognizable as electromagnetism to me.

 

[gionole]

Why are you focused on these kinks at all? It is a non-realistic wave. You will be much better off learning about realistic waves like sinusoids.

Your focus on kinks is leading you into all sorts of misunderstandings.

-The static E field does not have a speed

-E field lines do not have an age

-There is no identification of points along a field line

-There is no identification of field lines at different points in time

I don’t know if you have been exposed to some horrible reference that it putting all this into your mind, but you really need to try to mentally clear what you think you know and start fresh. What you are describing here is unrecognizable as electromagnetism to me.

Agreed. One time, I watched the khan academy's youtube video and it stuck with me. Khan Academy does pretty horrible jobs "sometimes" at many things such as this.

But If I forget about kinks, I'm not sure I know why wave is produced then. I understand what wave is in medium for sure. Energy travels in a rope as each molecule goes up and down. This seems easy. Sound waves are understood as well, as each molecule hits next one "horizontally". So in the end, energy travels from one point to another and each molecule does something(moves up/down or right and left - just talking only about rope and sound for this).

But I realized I couldn't understand the EM wave the same way as I did the rope/sound example, since it travels into vacuum, so when I got exposed to kink, I thought it was easier to move to this way and here I'm.

But if we even forget the "kink", we can still see in the realistic case that some shape is formed which moves outwards with speed of light. As I understand the shape is formed with the following reason. The charge moved from `x1` to `x2` with increasing speed, however now you wanna put it, it's undisputable that the lines that constant moving speed charge had till x1 still would move with old speed, so once charge appeared at x2, the old field lines are not at x2. they're a little bit distance away. Take a look. https://ibb.co/SsQVpFt (you can see here old field lines didn't catch up to x2 due to speed change - even though you say speed change is not discrete, there still should be produced the following scenario as on my link). Now, as far as I understand, from my image, we get curved line instead of straight line. Is this correct ? if so, I have to imagine why curved line and not straight line would be produced and why that curved line wouldn't be produced with instantenous change speed. Thoughts ?

 

[Dale]

Are you familiar with the concept of a potential?

They are less complicated and easier to understand than fields. Waves in the potentials are almost trivial, and if you need the fields then it is just some math to go from the potentials to the fields.

 

[gionole]

Are you familiar with the concept of a potential?

They are less complicated and easier to understand than fields. Waves in the potentials are almost trivial, and if you need the fields then it is just some math to go from the potentials to the fields.

Do you mean this ? https://en.wikipedia.org/wiki/Electric_potential

 

[gionole]

Agreed. One time, I watched the khan academy's youtube video and it stuck with me. Khan Academy does pretty horrible jobs "sometimes" at many things such as this.

But If I forget about kinks, I'm not sure I know why wave is produced then. I understand what wave is in medium for sure. Energy travels in a rope as each molecule goes up and down. This seems easy. Sound waves are understood as well, as each molecule hits next one "horizontally". So in the end, energy travels from one point to another and each molecule does something(moves up/down or right and left - just talking only about rope and sound for this).

But I realized I couldn't understand the EM wave the same way as I did the rope/sound example, since it travels into vacuum, so when I got exposed to kink, I thought it was easier to move to this way and here I'm.

But if we even forget the "kink", we can still see in the realistic case that some shape is formed which moves outwards with speed of light. As I understand the shape is formed with the following reason. The charge moved from `x1` to `x2` with increasing speed, however now you wanna put it, it's undisputable that the lines that constant moving speed charge had till x1 still would move with old speed, so once charge appeared at x2, the old field lines are not at x2. they're a little bit distance away. Take a look. https://ibb.co/SsQVpFt (you can see here old field lines didn't catch up to x2 due to speed change - even though you say speed change is not discrete, there still should be produced the following scenario as on my link). Now, as far as I understand, from my image, we get curved line instead of straight line. Is this correct ? if so, I have to imagine why curved line and not straight line would be produced and why that curved line wouldn't be produced with instantenous change speed. Thoughts ?

@Dale could you answer this ? I think I'm close if my logic in this is correct. The only thing to understand now with this is why it is curved while velocity change is not instantenous and why it is straight(kink) when it's instantenous.

 

[DaveC426913]

why kink is curved while velocity change is not instantenous and why it is straight when it's instantenous.

The answer to your question is in your question.What is the shape of the path if the change in direction of a moving point is not instantaneous?

What is the shape of the path if the change in direction of a moving point is instantaneous?

 

[Dale]

Do you mean this ? https://en.wikipedia.org/wiki/Electric_potential

Yes, and also this

https://en.m.wikipedia.org/wiki/Magnetic_vector_potential

 

[gionole]

The answer to your question is in your question.What is the shape of the path if the change in direction of a moving point is not instantaneous?

What is the shape of the path if the change in direction of a moving point is instantaneous?

Yes, but couldn't imagine this. From mobile, it shows me you uploaded an attachment, but there's no attachment.

 

[Dale]

@Dale could you answer this ? I think I'm close if my logic in this is correct. The only thing to understand now with this is why it is curved while velocity change is not instantenous and why it is straight(kink) when it's instantenous.

You are making many of the same mistakes that I have already pointed out. Do you really need me to go through line by line and repeat once again my same criticisms I have already stated?

I would rather move forward with the potentials than rehash the kinks.

 

[Dale]

The nice thing about the potentials is that (in the Lorenz gauge) they are very cleanly related to the charge and current. You don’t need to trace any field lines or make any connections.

For any charge you can think at each moment in time it sends out a spherical scalar ”bubble” that expands outward at $c$. This wave is proportional to the charge density, and it decays as $1/r$.

Similarly, for any current you can think at each moment in time it sends out a spherical vector bubble” that expands outward at $c$. This vector wave is proportional to the current density, and it decays as $1/r$.

This is very straightforward to visualize. If there are multiple charges or currents then the potentials just add together.

 

[DaveC426913]

Yes, but couldn't imagine this. From mobile, it shows me you uploaded an attachment, but there's no attachment.

The attachment was not germane.

What does the distance per time graph look like ( broadly) for an object that starts at v1 and accelerates smoothly to v2? Is it straight, angled, or curved?

 

[Drakkith]

But I realized I couldn't understand the EM wave the same way as I did the rope/sound example, since it travels into vacuum, so when I got exposed to kink, I thought it was easier to move to this way and here I'm.

I don't know where field lines come into this. EM waves are propagating disturbances in the EM field and their measurable effects are an oscillation in the electric and magnetic field vectors of the EM field. That is, if we place a bunch of detectors in a row and cause an EM wave to pass by them the detectors will show the field vectors oscillating back and forth around some mean value. This is analogous to placing a bunch of buoys in water and watching as their height changes upon the passage of a water wave (aka a gravity wave). Or the change in air pressure as a sound wave passes.

A field line is a line made by connecting different points in the field such that the line is tangent to the vector at every point. That's it. This in itself has nothing to do with EM waves.

 

[weirdoguy]

I have seen lots of people like this, they explain it to me in terms of math but have no idea what it is logically.

How can you tell if you yourself don't know what it is "logically" (whatever that means)? Or you just want everyone to adjust to your undefined "logic"? And since it is undefined I guess you will never be satisfied.

definitely Einshtein would disagree with you as well.

Oh you would be surprised. Also bringing up Einstein does not enhance your arguments. The one and only language we use doing physics is math, and Einstein knew that very well.

 

[gionole]

You are making many of the same mistakes that I have already pointed out. Do you really need me to go through line by line and repeat once again my same criticisms I have already stated?

I would rather move forward with the potentials than rehash the kinks.

Why mistakes ? I think I know the confusion between us.

Well, I think I was even calling a "curved joinining lines" between old and new lines as a kink. and you guys have only been calling `kink` the straight joining line` and thats where our misconception arised.

Whats the mistake here ? i dont get it at all. There is still something that joins old and new lines. If you dont call it kink, then you can call whatever you want, but it is still a joining curved line which propagates and which changes E gradually.

 

[vanhees71]

The em. waves in a vacuum propagate according to the retarded Green's function of the D'Alembert operator,
$$\Box=\frac{1}{c^2} \partial_t^2-\Delta,$$
which is
$$G(t,\vec{x},t',\vec{x}')=\frac{\delta(t-t'-|\vec{x}-\vec{x}'|/c)}{4 \pi |\vec{x}-\vec{x}'|}.$$
In the Lorenz gauge the corresponding solutions to the wave equations for the potentials,
$$\Box \Phi=\frac{1}{\epsilon_0} \rho, \quad \Box \vec{A}=\mu_0 \vec{j},$$
read
$$\Phi(t,\vec{x})=\int_{\mathbb{R}^3} \mathrm{d}^3 x' \frac{\rho(t-|\vec{x}-\vec{x}'|/c,\vec{x}')}{4 \pi \epsilon_0|\vec{x}-\vec{x'}|},$$
$$\vec{A}(t,\vec{x})=\int_{\mathbb{R}^3} \mathrm{d}^3 x' \frac{\mu_0 \vec{j}(t-|\vec{x}-\vec{x}'|/c,\vec{x}')}{4 \pi |\vec{x}-\vec{x'}|}.$$
I.e., the changes of the fields due to changes in the sources (charge and current distributions) propagate according to Huygens's principle, i.e., each local perturbation causes a spherical wave moving outward with the speed of light from the source. This holds for the physical fields, which are gauge invariant,
$$\vec{E}=-\partial_t \vec{A} -\vec{\nabla} \Phi, \quad \vec{B}=\vec{\nabla} \times \vec{A}.$$

 

[Dale]

I understand what wave is in medium for sure. Energy travels in a rope as each molecule goes up and down. This seems easy.

Energy travels in the EM field wherever $\vec E \times \vec B$ is nonzero. So if you understand a rope wave by focusing on energy traveling, then you should do the same for EM.

Sound waves are understood as well, as each molecule hits next one "horizontally". So in the end, energy travels from one point to another and each molecule does something(moves up/down or right and left - just talking only about rope and sound for this).

Again, you understand by looking at energy.

But I realized I couldn't understand the EM wave the same way as I did the rope/sound example, since it travels into vacuum,

What has that got to do with anything? The EM fields still transfer energy just like the rope and the sound. The problem is that you abandoned a sensible approach for understanding waves, looking at the energy, and instead focused on the E field lines. That is a useless approach here.

so when I got exposed to kink, I thought it was easier to move to this way and here I'm.

And now that you have found it is not easier, why do you persist in going down this unfruitful path?

But if we even forget the "kink", we can still see in the realistic case that some shape is formed which moves outwards with speed of light.

Yes. This is an EM wave.

As I understand the shape is formed with the following reason. The charge moved from `x1` to `x2` with increasing speed, however now you wanna put it, it's undisputable that the lines that constant moving speed charge had till x1 still would move with old speed,

Field lines do not have a speed.

so once charge appeared at x2, the old field lines are not at x2.

Field lines don’t have an age. There are no old or new field lines.

they're a little bit distance away. Take a look. https://ibb.co/SsQVpFt (you can see here old field lines didn't catch up to x2 due to speed change - even though you say speed change is not discrete, there still should be produced the following scenario as on my link).

I don’t understand that drawing. But I don’t need you to fix it. I prefer to abandon this whole approach.

Now, as far as I understand, from my image, we get curved line instead of straight line. Is this correct ?

The E field lines for an EM wave are indeed not straight.

if so, I have to imagine why curved line and not straight line would be produced and why that curved line wouldn't be produced with instantenous change speed.

And once you succeed in that, what have you gained? Do you think this produces an understanding of EM waves? Maybe it does. I haven’t seen anyone try this approach, so I cannot say that it will definitely fail. It just seems like there has been no progress so far using this approach.

Why not take a break from it for a while. Either pursue the energy approach or the potentials approach for a bit. See if a different approach helps. If it does, great, if not then you can always come back to this approach.