Question about AC current in a wire

This might be kind of weird, but how exactly does an AC signal flow in a wire? Suppose we have a copper wire with an AC signal (sine wave) flowing through it. Does it flow through it in the sense that if we suddenly freezed the signal and you took different chunks of the wire would they be at a different potential? Or does the potential of the entire wire vary such that when the particular potentials are plotted as a function of time they form the sine wave? (ie, at one point in time the potential of the entire wire is at 120 Volts and then a microsecond later it’s at 119.99 Volts, etc.)

I am having difficulty articulating this so if it doesn’t make sense then please let me know. Thanks guys.

There is a slight sort of “ripple” down through the wire, but this happens at about half the speed of light. This is pretty fast compared to the cycle time, so effectively the whole line is pretty much at the same voltage.

The wavelength of 60 Hz in the wire is about 2000 miles so any wire in a building, or the power lines in a town would be at the same potential. For high frequencies what you say is true. If you freeze the wave at any point in time the voltage and current along the line will be a sine wave if the input is a sine wave.

Your second guess is correct. To help you visualize it, remember that potential (voltage), does not “flow” through the wire. Current flows. That would be actual electrons travelling back and forth. The difference in potential is what causes them to flow.

When you measure the voltage potential, you are measuring the difference between the potential in the wire and some standard reference potential, usually the Earth’s potential, called ground or ground reference. That potential is taken to be the zero point for measurement. No matter where along the wire you measure that potential, it will be the same with respect to ground. On the other hand, if you measure the potential at two separate points along the wire, it will always be almost zero (it would only be exactly zero if you were measuring the potential difference along a perfect superconductor). There will be some lag between the change in potential at the source and the corresponding change downstream, but it would be at near the speed of light. For all practical purposes, the voltage changes at all points in a wire sinultaneously.

Thanks guys. I just always had this weird vision of a sine wave (or any signal) “squirming” through the wire moving up or down. It makes sense now because it’s a conductive material so it would all be at pretty much the same potential.

Well, that’s not right.

What David Simmmons said is correct; the potential DOES vary along the length of the wire. It DOES propagate over time at about 1/2c. And that seems to be the opposite of the message goBigRick got.

Without trying to start a flamewar, I respectfully suggest the only strictly accurate statement by Rhubarb is the last sentence, that “For all practical purposes …”

Given typical in-home wire lengths, practical measuring tools and 60Hz current, he’s 100% right, you can pretend the entire wire is at a single potential at all times. But that’s just as wrong as pretending the Earth is flat because for as far as I can walk in a day, it behaves as if it’s flat.
So I guess the bigger question is whether goBigRick is tryuing to understand what’s really happening in the wire, or is trying to understand how it appears to be happening.

A LOT of mis-knowledge about electrical phenomona is based on confusing these issues. Bad knowledge is enshrined in many textbooks and lots of practitioners have soft spots in their fundamental knowledge as well.

If you really want to understand what’s going on with electricity, and how much of what you thought you knew is wrong, or at least over-simplified to the point of falsity, check out this:

I understand what you’re saying, but keep in mind there would still be 120 VAC between the hot & neutral wires even if the neutral was not connected to earth ground.

When we talk about voltage in home wiring (or whatever), we should be clear about what we’re talking about, as there are many locations we could be referring to:

  1. The voltage between the hot and neutral wires is a 120 V RMS sine wave. This is true regardless of whether or not the neutral is ties to earth ground. And as others have pointed out, the phase velocity is so fast that for any structure the system can be electrically modeled as a lumped-parameter system (as opposed to a distributed-parameter system). This means that we can assume the instantaneous voltage is not a function of distance. However, because we’re dealing with AC, the instantaneous voltage is a function of time.

  2. Because the wiring in a building can be modeled as a lumped-parameter system, the voltage between any two points along a hot wire is zero. (Unless, of course, we take V = IR into account.)

  3. Because the wiring in a building can be modeled as a lumped-parameter system, the voltage between any two points along a neutral wire is zero. (Unless, of course, we take V = IR into account.)

LSL Guy:

Yes, the voltage does vary along a wire. This is due to two things:

  1. The wiring is a wave guide, the signal is time-varying, thus the voltage will be a function of distance as well as time. This will occur regardless of whether or not there’s current on the wires (i.e. this will occur regardless of whether or not there’s a load).

  2. The hot and neutral wires are resistors. Because V = IR, there will be a voltage between any two points along the hot wire when current is flowing. Same goes for the neutral wire… there will be a voltage between any two points along the neutral wire when current is flowing.

For the sake of these discussions, we are assuming #1 is negligible for the wiring in a given structure.

Let’s see – 60 hz should have a wavelength of approx. 5.7 meters according to the I’m-lazy-so-used-Google-to-do-it-calculator. So regardless of the speed of the current or voltage through the wiring, you’ll have peaks and troughs every 5.7 meters, or better yet, a single point every 5.7 meters will be at the same instantaneous potential.

Oh my goodness. Wavelength is equal to the speed of light in the medium divided by the frequency. For 60 Hz. in the ordinary insulated wire this is about 186000*.66/60 ~ 2000 miles.

Balthisar’s on-line calculator is correct.

For the wavelength of a 60 hz sound wave in air.

right idea, wrong calculator.

Doh! That’s why when doing real engineering at work, I don’t count on the first thing Google kicks back (and then I read it first).

Seriously, my apologies!

It is also not easily pictured, and not usually used in illustrations, but the flow of electrons model isn’t quite what happens.

Electrons do move and circulate, however far more energy is is passed one to another, like pass the parcel.

An energy charge is transfered atom by atom using electrons as the interface.

An analogy might be a tube of water, with a plunger at one end, and a piston at the other, force the plunger, the water does move, but only in direct relation to the plunger, not from one end to the other.

The energy transfer does go all the way along the tube to the far end.

When you get into higher levels of electronics, and into physics, its not the number of electrons being moved that is discussed, but rather the charge on the electron, the amount of energy imparted to it.

Well, what you are saying is not true in general. As I said earlier, for lines that are short compared to the wavelength the voltage between the wires can be treated as if it were the same throughout the line.

However for lines that are an appreciable fraction of a wavelength or more the picture changes. The voltage on the line consists of two waves the direct wave going from input end to output end and a reflected wave going from output end back to input end. If the line is terminated with the correct impedance at the output end the reflected wave is zero. Why the terminating impedance matters is beyond the scope of this explanation but take my word for it, it does matter.

For a long line, as LSL Guy said, the voltage is a function of both time and distance. For a properly terminated line if you “freeze” the voltage wave at any instant in time and travel down the line fron sending to receiving ends, the voltage will vary as a sine wave with decreasing amplitude for a sine wave in. The decreasing amplitude results from the line attenuation. If you stand in one place and measure the voltage variation with time it will also be a sine wave with the amplitude gradually decreasing as you measure in places farther down the line toward the receiving end.