Electricity: Is it a longitudinal wave?

[Skrphys]

I am a high school teacher and we were discussing waves and electricity in class today. One of my students asked me if electricity is a longitudinal wave or not and I had no idea how to answer.

So, I realize that electric fields are what drive electrons to move through conducting wires, but when electrons are moving through the wires are they moving in the same way a longitudinal wave moves? How exactly are signals transmitted through electrical wires? I know in college, we did experiments in which our output was a sinusoidal wave, but is that actually what is being transmitted through the wire, or is the output just a pictorial representation of the signal?

 

[ZapperZ]

I am a high school teacher and we were discussing waves and electricity in class today. One of my students asked me if electricity is a longitudinal wave or not and I had no idea how to answer.

So, I realize that electric fields are what drive electrons to move through conducting wires, but when electrons are moving through the wires are they moving in the same way a longitudinal wave moves? How exactly are signals transmitted through electrical wires? I know in college, we did experiments in which our output was a sinusoidal wave, but is that actually what is being transmitted through the wire, or is the output just a pictorial representation of the signal?

You need to be a bit more clear on the situation here. Charges will always have a force in the direction of the electric field.

If you have a DC current, then there is no "wave". In an AC circuit, the electric field oscillates back and forth. In both situations the charges respond due to the forces in the direction of the electric field (depending on the sign of the charge). So I guess you can call it a "longitudinal" wave, but this is misleading because that is not the direction of wave propagation.

Zz.

 

[sophiecentaur]

So I guess you can call it a "longitudinal" wave,

There can be a longitudinal component of any EM wave that's not out in Free space. This is yet another example where Classification without knowledge can lead to confusion. In the same way, although we say that Sound is a longitudinal wave, there can be transverse motion of the air around an obstacle.
Teacher have the worst job in this respect because they have to preach the standard, abbreviated ideas so that they can recognise the Exam questions and score the points.

 

[davenn]

So, I realize that electric fields are what drive electrons to move through conducting wires, but when electrons are moving through the wires are they moving in the same way a longitudinal wave moves?

have you heard of / read about electron drift ?

https://en.wikipedia.org/wiki/Drift_current
https://en.wikipedia.org/wiki/Drift_velocityFor DC the paths of the free charges ( electrons) in the wire is random and all over the width of the wire but because of the EMF ( potential difference between the 2 ends of the wire, there is a general movement of those randomly moving charges along the wire.

As @ZapperZ , commented for AC. The EM field on the outside of the wire ( which transports the energy) oscillates back and forward, 50/60 Hz for mains voltages ... much higher for audio and radio signals, along a wire.

The oscillating EM field also causes the charges (electrons) to oscillate back and forward about a point in the wire. So, for example, the charges in the filament of a light globe never leave the filament and continue into the rest of the circuit.

I know in college, we did experiments in which our output was a sinusoidal wave, but is that actually what is being transmitted through the wire, or is the output just a pictorial representation of the signal?

On an oscilloscope, you see that sine wave, or some variation of it depending of the oscillator type that is generating the signal. The sine wave is real in that it is showing you the cycling EM wave of the signal along the wire as the EM energy cycles from a positive peak down through zero to a negative peakhope that helped

Dave

 

[Skrphys]

There can be a longitudinal component of any EM wave that's not out in Free space. This is yet another example where Classification without knowledge can lead to confusion. In the same way, although we say that Sound is a longitudinal wave, there can be transverse motion of the air around an obstacle.
Teacher have the worst job in this respect because they have to preach the standard, abbreviated ideas so that they can recognise the Exam questions and score the points.

Indeed! I appreciate that you have recognized the disconnect between what is true and what needs to be taught. I guess what I’m trying to do here is to understand for myself so that I can simplify and explain to my students. Thanks for your reply!

 

[Skrphys]

have you heard of / read about electron drift ?

https://en.wikipedia.org/wiki/Drift_current
https://en.wikipedia.org/wiki/Drift_velocityFor DC the paths of the free charges ( electrons) in the wire is random and all over the width of the wire but because of the EMF ( potential difference between the 2 ends of the wire, there is a general movement of those randomly moving charges along the wire.

As @ZapperZ , commented for AC. The EM field on the outside of the wire ( which transports the energy) oscillates back and forward, 50/60 Hz for mains voltages ... much higher for audio and radio signals, along a wire.

The oscillating EM field also causes the charges (electrons) to oscillate back and forward about a point in the wire. So, for example, the charges in the filament of a light globe never leave the filament and continue into the rest of the circuit.
On an oscilloscope, you see that sine wave, or some variation of it depending of the oscillator type that is generating the signal. The sine wave is real in that it is showing you the cycling EM wave of the signal along the wire as the EM energy cycles from a positive peak down through zero to a negative peakhope that helped

Dave

Thanks, Dave! So what I’m gathering is that for AC the electrons in the wire are pushed forward and backward depending on the direction (i.e., sign) of the potential caused by the alternating EM field. So then, the transfer of energy would be from the electrons in the wire “hitting” off of each other. In other words, the electrons are the “medium” in which a longitudinal wave is traveling in order to transfer energy. Is this a correct way to conceptually explain what’s happening here?

Thanks again for your reply!

 

[davenn]

Thanks, Dave! So what I’m gathering is that for AC the electrons in the wire are pushed forward and backward depending on the direction (i.e., sign) of the potential caused by the alternating EM field.

Yes
Caveat ... pushed may not be the quite correct word, but close enuf

So then, the transfer of energy would be from the electrons in the wire “hitting” off of each other. In other words, the electrons are the “medium” in which a longitudinal wave is traveling in order to transfer energy

No, read again what I said in my second statement ...

...The EM field on the outside of the wire ( which transports the energy)...

.
Dave