# I Is AC energy really transferred as EM wave?

1. Oct 26, 2016

### 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?

2. Oct 26, 2016

### Staff: Mentor

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.

3. Oct 26, 2016

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.

4. Oct 26, 2016

### Jackson Lee

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?

5. Oct 26, 2016

### Jackson Lee

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

6. Oct 26, 2016

### davenn

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

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)

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

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

different from what ?

again, exactly what are you referring to / comparing ?

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

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

Dave

7. Oct 26, 2016

### Jackson Lee

Thanks for your reply. What I learnt 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.

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.

8. Oct 27, 2016

### Drakkith

Staff Emeritus
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).

9. Oct 27, 2016

### davenn

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 with

regards
Dave

10. Oct 27, 2016

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.

11. Oct 27, 2016

### Jackson Lee

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)

12. Oct 27, 2016

### Jackson Lee

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.

13. Oct 27, 2016

### Staff: Mentor

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 ]

14. Oct 27, 2016

### Drakkith

Staff Emeritus
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.

15. Oct 27, 2016

### 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.

16. Oct 27, 2016

### Drakkith

Staff Emeritus
Indeed. No dueling in the technical forums.

17. Oct 27, 2016

### Jackson Lee

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. )

Last edited: Oct 27, 2016
18. Oct 27, 2016

### Jackson Lee

Last edited: Oct 28, 2016
19. Oct 28, 2016

### jartsa

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.

20. Oct 28, 2016

### sophiecentaur

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.
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)