Is AC energy really transferred as EM wave?

[Jackson Lee]

Hey, guys. There is an interesting question about EM waves. I know electricity is transferred on transmission line as electromagnetic wave. But electromagnetic wave is one type of transverse wave, then how is AC electricity like? I feel it hard to imagine it.
Besides, conductors could be used to eliminate EM wave, for which metal grid in the door of microwave oven is a good example. Then why is the AC current different?

What do you think about it?

 

[anorlunda]

You are making it overly hard by mixing up many models. I recommend three mental models to use thinking about electricity.

1. Quantum electrodynamics, QED. It deals with real and virtual,photons which are packets of EM waves.
2. Fields, using Maxwell's Equations. Fields propagate at near light speed in wires. At this level, forget about photons.
3. Circuits, using Ohm's law in real or complex form. At this level, we forget about fields. Power transmission lines are treated like that.

There are other models, such as for RF transmission including waveguides, and for simiconductors. But those three are the main ones.

If you try to reason at fractional levels between those three, things become much more difficult to get correct, complete and consistent.

 

[Windadct]

Personally, and I believe some here will disagree, I do not view the AC system as an EM wave propagation type phenomena. There are specific boundary cases where there is some EM Wave effect, but essentially all of the analysis is conducted without EM theory.

 

[Jackson Lee]

Personally, and I believe some here will disagree, I do not view the AC system as an EM wave propagation type phenomena. There are specific boundary cases where there is some EM Wave effect, but essentially all of the analysis is conducted without EM theory.

According to its propagation speed, it is reasonable to conclude it is certain kind of electromagnetic wave. However, what puzzled me is that EM wave is transverse wave, then what is it like when propagating through transmission lines?

 

[Jackson Lee]

You are making it overly hard by mixing up many models. I recommend three mental models to use thinking about electricity.

1. Quantum electrodynamics, QED. It deals with real and virtual,photons which are packets of EM waves.
2. Fields, using Maxwell's Equations. Fields propagate at near light speed in wires. At this level, forget about photons.
3. Circuits, using Ohm's law in real or complex form. At this level, we forget about fields. Power transmission lines are treated like that.

There are other models, such as for RF transmission including waveguides, and for simiconductors. But those three are the main ones.

If you try to reason at fractional levels between those three, things become much more difficult to get correct, complete and consistent.

Yeah, your response is helpful. However, what is the real thing like? Is it EM wave?

 

[davenn]

Personally, and I believe some here will disagree, I do not view the AC system as an EM wave propagation type phenomena. There are specific boundary cases where there is some EM Wave effect, but essentially all of the analysis is conducted without EM theory.

Then you really should change your views from an incorrect personal theory and learn the established theories

I know electricity is transferred on transmission line as electromagnetic wave.

This is not correct, as @anorlunda said, you are mixing up different things and coming out with the wrong idea

electricity is the flow of charge ( which is the flow of electrons), what we call current. The wire/transmission line acts as a waveguide
for the EM wave that propagates along the outside of the conductor(s) at close to the speed of light ( dependent on the makeup of the cable)

But electromagnetic wave is one type of transverse wave, then how is AC electricity like?

this doesn't even begin to make sense ... try again

Besides, conductors could be used to eliminate EM wave, for which metal grid in the door of microwave oven is a good example

you need to understand what is happening with the window grid on a microwave oven ... do some reading

Then why is the AC current different?

different from what ?

However, what is the real thing like? Is it EM wave?

again, exactly what are you referring to / comparing ?

According to its propagation speed, it is reasonable to conclude it is certain kind of electromagnetic wave.

EM waves are EM waves ... they just vary in frequency ... EM waves are generated by accelerating electrons. AC current is the common form of this

However, what puzzled me is that EM wave is transverse wave, then what is it like when propagating through transmission lines?

so why don't you think it cannot propagate along the outside of a transmission line conductor ?

Dave

 

[Jackson Lee]

The wire/transmission line acts as a waveguide for the EM wave that propagates along the outside of the conductor(s) at close to the speed of light ( dependent on the makeup of the cable)

Thanks for your reply. What I learned from textbooks is AC current is transferred as EM wave which is one kind of transverse wave. And in transverse wave, the displacement is perpendicular to the direction in which the wave travels. So I want to know what the AC current is like when propagating along transmission line.As for metal grid of microwave oven door, I suppose it is used to eliminate EM wave, technically to say is microwave, in order to protect users from danger. Metal conductors could be used for RF shielding. The following is material from Wikipedia.

How electromagnetic shielding works
Electromagnetic radiation consists of coupled electric and magnetic fields. The electric field produces forces on the charge carriers (i.e., electrons) within the conductor. As soon as an electric field is applied to the surface of an ideal conductor, it induces a current that causes displacement of charge inside the conductor that cancels the applied field inside, at which point the current stops.Similarly, varying magnetic fields generate eddy currents that act to cancel the applied magnetic field. (The conductor does not respond to static magnetic fields unless the conductor is moving relative to the magnetic field.) The result is that electromagnetic radiation is reflected from the surface of the conductor: internal fields stay inside, and external fields stay outside.Several factors serve to limit the shielding capability of real RF shields. One is that, due to the electrical resistance of the conductor, the excited field does not completely cancel the incident field. Also, most conductors exhibit a ferromagnetic response to low-frequency magnetic fields, so that such fields are not fully attenuated by the conductor. Any holes in the shield force current to flow around them, so that fields passing through the holes do not excite opposing electromagnetic fields. These effects reduce the field-reflecting capability of the shield.In the case of high-frequency electromagnetic radiation, the above-mentioned adjustments take a non-negligible amount of time, yet any such radiation energy, as far as it is not reflected, is absorbed by the skin (unless it is extremely thin), so in this case there is no electromagnetic field inside either. This is one aspect of a greater phenomenon called the skin effect. A measure of the depth to which radiation can penetrate the shield is the so-called skin depth.

So I feel very surprised why AC current, if it is EM wave, could propagate along transmission line which is made of metal conductor. Why is this kind of EM wave, I mean AC current, not eliminated while propagating?

I hope you can understand what I mean and help me resolve this problem. There must be something wrong, but I don't know where it is, please correct me. Thanks a lot again.

 

[Drakkith]

What I learned from textbooks is AC current is transferred as EM wave which is one kind of transverse wave. And in transverse wave, the displacement is perpendicular to the direction in which the wave travels. So I want to know what the AC current is like when propagating along transmission line.

Electric current, whether AC or DC, is not an EM wave. It is simply the flow of electric charges. The current does, however, respond to the voltage along the line, which itself is the result of the EM wave set up by the power source.

So I feel very surprised why AC current, if it is EM wave, could propagate along transmission line which is made of metal conductor. Why is this kind of EM wave, I mean AC current, not eliminated while propagating?

The current would quickly dissipate in a transmission line if it weren't for the fact that the power source is providing a constant input of power that more than compensates for the loss. Microwaves impinging on the metal screen of your microwave induce a current in the screen and as a result are heavily dissipated, only making it through with a very, very small amount of power (either that or they are reflected. I can't remember which).

 

[davenn]

Thanks for your reply. What I learned from textbooks is AC current is transferred as EM wave which is one kind of transverse wave. And in transverse wave, the displacement is perpendicular to the direction in which the wave travels. So I want to know what the AC current is like when propagating along transmission line.

if that is really what you are reading (and not a mis-interpretation) then you are reading the wrong books
you have made that statement several times

As I said earlier and Drakkith repeated,

Electric current, whether AC or DC, is not an EM wave. It is simply the flow of electric charges.

EM waves are EM waves ... they just vary in frequency ... EM waves are generated by accelerating electrons. AC current is the common form of this

In an AC circuit, say, the transmission of energy from your power generating station to your home, the electrons in the wires in the generator probably never make it to your home, they just oscillate back and forwards over a very short distance at 50 Hz/ 60Hz (depending where your are from) As they oscillate back and forwards, they undergo acceleration during each half cycle and its this cycling (oscillation) that generates the EM wave that I said above.
The energy is transmitted along the outside of the cable ( transmission line) via the EM wave

It is no different in a radio/tv/mobile phone etc radio signal, and the audio signal in your stereo system.
They are ALL AC signals, just different frequencies of oscillation

Things are a little different in a DC circuit and easier, but I don't want to get into that until you understand the basics of the AC system, which is what your OP was concerned withregards
Dave

 

[Windadct]

MY reference was assuming that the OP was referring top Power Transmission as AC - as in Anorlunda's 3rd reference (post 2)- which we posted about the same time. In power transmission and distribution the design, protection and theory exist and function with little to no consideration of EM propagation, even in line distance relay protection, one of the more complex protective strategies, the theory is all in complex impedance, and not in EM fields. There are some communication strategies used, line carrier relays, that use RF on the same distribution/transmission lines, but that is really a communication challenge, not a power one.

 

[Jackson Lee]

Electric current, whether AC or DC, is not an EM wave. It is simply the flow of electric charges. The current does, however, respond to the voltage along the line, which itself is the result of the EM wave set up by the power source.
The current would quickly dissipate in a transmission line if it weren't for the fact that the power source is providing a constant input of power that more than compensates for the loss. Microwaves impinging on the metal screen of your microwave induce a current in the screen and as a result are heavily dissipated, only making it through with a very, very small amount of power (either that or they are reflected. I can't remember which).

if that is really what you are reading (and not a mis-interpretation) then you are reading the wrong books
you have made that statement several times

As I said earlier and Drakkith repeated,

In an AC circuit, say, the transmission of energy from your power generating station to your home, the electrons in the wires in the generator probably never make it to your home, they just oscillate back and forwards over a very short distance at 50 Hz/ 60Hz (depending where your are from) As they oscillate back and forwards, they undergo acceleration during each half cycle and its this cycling (oscillation) that generates the EM wave that I said above.
The energy is transmitted along the outside of the cable ( transmission line) via the EM wave

It is no different in a radio/tv/mobile phone etc radio signal, and the audio signal in your stereo system.
They are ALL AC signals, just different frequencies of oscillation

Things are a little different in a DC circuit and easier, but I don't want to get into that until you understand the basics of the AC system, which is what your OP was concerned withregards
Dave

Thanks a lot to Drakkith and Dave. You two are really helpful. I think I got it. Beside, Mr. Dave, I notice you said energy is transmitted along the outside of cable via EM wave. What do you mean? Does that happen near the surface of cable or something else? Could you please give further explanation? You seems to be an real expert.(smile)

 

[Jackson Lee]

MY reference was assuming that the OP was referring top Power Transmission as AC - as in Anorlunda's 3rd reference (post 2)- which we posted about the same time. In power transmission and distribution the design, protection and theory exist and function with little to no consideration of EM propagation, even in line distance relay protection, one of the more complex protective strategies, the theory is all in complex impedance, and not in EM fields. There are some communication strategies used, line carrier relays, that use RF on the same distribution/transmission lines, but that is really a communication challenge, not a power one.

Yeah, actually, I am a student majoring in power. We seldom got involved with EM wave in the past and that is why I felt really surprised when reading AC current is certain type of EM wave. Maybe the author means AC electricity energy is transmitted via EM wave and I misinterpreted his words. All in all, thanks to all of you.

 

[anorlunda]

Beside, Mr. Dave, I notice you said energy is transmitted along the outside of cable via EM wave. What do you mean? Does that happen near the surface of cable or something else? Could you please give further explanation? You seems to be an real expert.(smile)

He meant skin effect. You can look that up on Wikipedia. But it is nothing you need to understand as a beginner on this subject.

[please, nobody mention poynting vector ]

 

[Drakkith]

Yeah, actually, I am a student majoring in power. We seldom got involved with EM wave in the past and that is why I felt really surprised when reading AC current is certain type of EM wave. Maybe the author means AC electricity energy is transmitted via EM wave and I misinterpreted his words.

The issue is that AC power transmission involves EM waves with low frequencies and enormous wavelengths (at 60 Hz the EM wave generated has a wavelength of 5 million meters) which are also traveling through a highly conductive medium. This is a very, very different scenario than light or radio waves traveling through free space and you don't even need to think of it as an EM wave in almost all cases. The equations and concepts specific to AC power transmission work just fine.

 

[sophiecentaur]

The word "really" can get us in trouble.
"Can be explained in terms of." Opens the possibility of perfectly valid alternatives and avoids deadly combat.

 

[Drakkith]

The word "really" can get us in trouble.
"Can be explained in terms of." Opens the possibility of perfectly valid alternatives and avoids deadly combat.

Indeed. No dueling in the technical forums.

 

[Jackson Lee]

He meant skin effect. You can look that up on Wikipedia. But it is nothing you need to understand as a beginner on this subject.

[please, nobody mention poynting vector ]

I have looked skin effect on Wikipedia and got the main idea, but there is something made me curious. The following is what I got from Wiki.

Conductors, typically in the form of wires, may be used to transmit electrical energy or signals using an alternating current flowing through that conductor. The charge carriers constituting that current, usually electrons, are driven by an electric field due to the source of electrical energy. An alternating current in a conductor produces an alternating magnetic field in and around the conductor. When the intensity of current in a conductor changes, the magnetic field also changes. The change in the magnetic field, in turn, creates an electric field which opposes the change in current intensity. This opposing electric field is called “counter-electromotive force” (back EMF). The back EMF is strongest at the center of the conductor, and forces the conducting electrons to the outside of the conductor, as shown in the diagram on the right.[1]

It is said that, according to theory of induction, induced current would oppose the change of magnetic field. Then when current increases in the upward direction, the outward induced current would be formed. However, when current decreases in the upward direction (you know this is AC current), then it seems induced current would change into inward direction so as to maintain magnetic field. If it is the case, the "skin effect" would change into "core effect".

In addition, besides skin effect, what puzzles me mostly is why energy is transmitted via EM wave, because according to Dave's description, it seems electrons' displacements are tangential to the direction wave travels, to some extent looks like sound wave, thus even though I know AC current propagates as fast as EM wave, don't you think it is more close to longitudinal wave? (EM wave is transverse wave.)

( PS: I know EM wave is existed, which I find while reading skin effect, but cannot understand why it is regarded as the carrier of energy. )
Could you please help me to deal with this question? Thx

 

[Jackson Lee]

The word "really" can get us in trouble.
"Can be explained in terms of." Opens the possibility of perfectly valid alternatives and avoids deadly combat.

That's ok. I don't care about this. They pointed out my problems and made me improved, so I am appreciative to all of them. Never mind about this, but thanks as well.

 

[jartsa]

I have looked skin effect on Wikipedia and got the main idea, but there is something made me curious. The following is what I got from Wiki.
It is said that, according to theory of induction, induced current would oppose the change of magnetic field. Then when current increases in the upward direction, the outward induced current would be formed. However, when current decreases in the upward direction (you know this is AC current), then it seems induced current would change into inward direction so as to maintain magnetic field. If it is the case, the "skin effect" would change into "core effect".

In addition, besides skin effect, what puzzles me mostly is why energy is transmitted via EM wave, because according to Dave's description, it seems electrons' displacements are tangential to the direction wave travels, to some extent looks like sound wave, thus even though I know AC current propagates as fast as EM wave, don't you think it is more close to longitudinal wave? (EM wave is transverse wave.)

( PS: I know EM wave is existed, which I find while reading skin effect, but cannot understand why it is regarded as the carrier of energy. )
Could you please help me to deal with this question? Thx

Skin effect is just parallel wires disturbing each other by inducing voltages on each other. And a thick wire is just many thin parallel wires - which disturb each other by inducing voltages on each other.

Now I'm so big-headed that I claim that the above explanation of skin effect is much better than the one in Wikipedia. Particularly bad In the Wikipedia article is the part which seems to be saying that conducting electrons move outwards on a AC-carrying thick wire.

 

[sophiecentaur]

Skin effect is just parallel wires disturbing each other by inducing voltages on each other. And a thick wire is just many thin parallel wires - which disturb each other by inducing voltages on each other.

You are giving no reason for this stratification. Are you assuming the wires are all insulated? Google "Litz Wire" and see how a wire, consisting of many thin insulated wires has much lower RF resistance than a single conductor of the same overall cross sectional area.

Particularly bad In the Wikipedia article is the part which seems to be saying that conducting electrons move outwards on a AC-carrying thick wire.

Why would you object to a model in which the electrons would move in elliptical paths? (Bearing in mind the thermal motion (massive) and the (very slow) drift velocity)

 

[davenn]

Skin effect is just parallel wires disturbing each other by inducing voltages on each other. And a thick wire is just many thin parallel wires - which disturb each other by inducing voltages on each other.

that is completely incorrect ... please do some reading of good quality papers

you don't need multiple conductors to get the skin effect

 

[Jackson Lee]

that is completely incorrect ... please do some reading of good quality papers

you don't need multiple conductors to get the skin effect

Yeah, that is what I want to say. I understand this part. Besides, Prof. Dave, could you please help me on the skin effect? Because I suppose there is another state, core effect, during certain stages.As for energy transmitted via EM wave, I have got it. Thx a lot.

 

[davenn]

Please ... I am no professor and definitely no expert ... just passionate Am just looking around for a good explanation for you

 

[jartsa]

You are giving no reason for this stratification. Are you assuming the wires are all insulated? Google "Litz Wire" and see how a wire, consisting of many thin insulated wires has much lower RF resistance than a single conductor of the same overall cross sectional area.

Let's say one 1 km long wire has inductance 100 ohms and another 1 km long wire, parallel to the first one, has inductance 200 ohms. Both wires are fed 100 Volts AC voltage. The current is smaller in the wire with the larger inductance. So the voltage drop per meter is the same in both wires, so if we connect for example the middle points of the two wires with a wire, there is no current in the wire.

Why would you object to a model in which the electrons would move in elliptical paths? (Bearing in mind the thermal motion (massive) and the (very slow) drift velocity)

See above.EDIT: It is possible that I got something wrong, so let me ask: Isn't it so that if we remove the "skin" where the AC current flows, what we we have then left is a wire where less AC-current flows at the same voltage, in other words what we have left is a wire with larger inductance? (The wire has no ohmic resistance worth mentioning)EDIT2:
https://en.wikipedia.org/wiki/Litz_wire
Let me design my own "Litz wire":

First one that does not work, but maybe proves some points:
Bunch of insulated thin wires, not wound or woven.

Then one that works:
Bunch of insulated thin wires, not wound or woven. Each thin wire is fed with a voltage that is adjusted so that the current is the same in every thin wire.

 

[Jackson Lee]

Let's say one 1 km long wire has inductance 100 ohms and another 1 km long wire, parallel to the first one, has inductance 200 ohms. Both wires are fed 100 Volts AC voltage. The current is smaller in the wire with the larger inductance. So the voltage drop per meter is the same in both wires, so if we connect for example the middle points of the two wires with a wire, there is no current in the wire.
See above.EDIT: It is possible that I got something wrong, so let me ask: Isn't it so that if we remove the "skin" where the AC current flows, what we we have then left is a wire where less AC-current flows at the same voltage, in other words what we have left is a wire with larger inductance? (The wire has no ohmic resistance worth mentioning)

Hi, Jartsa. Take it easy. We just come here to discuss questions and improve ourselves. If some words hurt you, I apologize for them. Actually, I suppose it is the statement that two parallel wires led to skin effect led to this minor debate, because the skin effect is caused by wire itself, but not its parallel one. Frankly speaking, I have made same mistake some time before. So, look this up on wikipedia and you will make a improvement. Some days later will be Thanksgiving Day, best wishes to you.

 

[davenn]

Let's say one 1 km long wire has inductance 100 ohms and another 1 km long wire, parallel to the first one, has inductance 200 ohms. Both wires are fed 100 Volts AC voltage. The current is smaller in the wire with the larger inductance. So the voltage drop per meter is the same in both wires, so if we connect for example the middle points of the two wires with a wire, there is no current in the wire.

inductance isn't measured in Ohms ... it's measured in Henrys, it kinda makes your post incorrect and pointless

 

[Charles Link]

At 60 Hz, most of the time the calculations can be done by assuming longitudinal electric fields, and by assuming the electrical current is the same at every point in the circuit. $\\$ For r-f frequencies and higher, the electrical signals in the circuits turn into transverse EM waves in the cables, and when the signal encounters a termination (such as a resistor) at the end of the circuit, the characteristic impedance of the cable along with the terminating impedance will determine the signal that is observed on the termination. A reflected wave can be observed/created there as well, and for a large terminating impedance, it is not uncommon to see a voltage pulse double in amplitude because of the reflected pulse, compared to that of a terminating impedance that matches the characteristic cable impedance for which there is no reflection. Meanwhile, the characteristic impedance of a cable gives the ratio of voltage to current for the transverse EM signal , but it is non-ohmic in nature and does not imply any resistive losses.

 

[jartsa]

inductance isn't measured in Ohms ... it's measured in Henrys, it kinda makes your post incorrect and pointless

Oh. What I meant was: Impedance was 100 ohms in one wire, and 200 ohms in the other wire, because inductances of wires were 10 mH and 20 mH.

 

[sophiecentaur]

At 60 Hz, most of the time the calculations can be done by assuming longitudinal electric fields

Can that be true? On a long domestic supply (say 5km) line, with, perhaps 50V drop, that would involve a field of 50/5,000 V/m (= 10^-3 V/m)). What calculations would involve that sort of field? When would the field be relevant or measurable?

 

[sophiecentaur]

Oh. What I meant was: Impedance was 100 ohms in one wire, and 200 ohms in the other wire, because inductances of wires were 10 mH and 20 mH.

And what about the interaction between the two wires?

Then one that works:
Bunch of insulated thin wires, not wound or woven. Each thin wire is fed with a voltage that is adjusted so that the current is the same in every thin wire.

Where could I buy some of that - and a source for feeding it?

Skin effect is just parallel wires disturbing each other by inducing voltages on each other.

"Is just"? There is a world of difference between two parallel wires and a single, thicker wire. The propagation of an EM wave over a conductor (E field, nominally transverse) is actually not transverse. There is a slight forward tilt to the wave front, due to losses in the conductor. A number of insulated wires or layers could not support or explain this phenomenon. Your model is flawed, I'm afraid.