Single wire AC

This is just a pondering that's probably very stupid but I hope to become a little less stupid by reading the responses from smart folks as yourself.

I'm curious about what might be required to have an AC open-ended circuit still perform work. For example, if just the AC's live wire was put into one side of a free standing capacitor (not limited to realistic specs or sizes), is there some level of 'compressability' of electrons (like they are said to gather up against a capacitor plate) to support an alternating current?

As it works, I think the AC's neutral and ground wires (connected at some point anyway) make the literal Earth work as the 'capacitor'. Is it a complete circuit from power station to your house and into the Earth and back to the power station? Or does the immensely large Earth, working as the ground, trick the circuit into thinking it's closed when it's really just a massive capacitor at the end to sink and source from as the current alternates?

So since the electrons don't really move in AC, and are more like the waves in an ocean (and ocean waves don't have a comparable need for a complete circuit, as such), I suppose one way to frame the question is- can a pulse of voltage potential travel down an open wire? Broadly, I think that's what antennas are, right?

Hope the random rambling makes enough sense to get some educating done.

good question.

i'd be interested to know the expert's views, too.

but, from a n00b's first understanding, wouldn't it be dangerous - if the voltage was low, i'm assuming nothing would happen - but if it were high enough - then a spark would arc to the nearest available conductor to ground.

i think that's what lightning is, no ?

EDIT:
oops - just re-read the question - and saw it's connected to a capacitor. (so obviously my scenario above would NOT happen... i think....)

I don’t think there’s any difference between a capacitor and just more wire. Wires have capacitance. Capacitors are just space for electrons to be. Capacitors place two such spaces next to each other so the gathering on one plate influences the electrons on the other plate. But half of a capacitor, is just electron real estate.

well, this will be a good test of my understanding of electrical theory.

i think there would be a small difference though, just at the beginning when the capacitor (presumably starting empty) had not reached it's full charge yet - once it is fully charged, then i guess it would be the same as an open ended wire.

when i read "AC", i still immediately think of 'Mains Voltage' - meaning DANGER, WATCH OUT ! !

but i suppose a smaller voltage would still pose the same question.

i'm guessing this would be relevant to the principle of wireless charging.

I'd say wireless charging is more like chopping a transformer in half, but that's another topic.

I'm curious about what might be required to have an AC open-ended circuit still perform work.

The earth can make a complete circuit... Birds can sit on high voltage power transmission lines and no current flows and nothing happens. You could hang safely from a high-voltage wire too. But, if you touch the tower that's holding-up the wire you've got a current-path to ground and you're dead!

So since the electrons don't really move in AC, and are more like the waves in an ocean

Current does flow, first one direction then the other. It's more like a tide than the waves. (And, the water does move up & down in a wave.)

Current also flows into/out-of a capacitor as it charges/discharges and AC current can "pass-through" a capacitor and "do work". The impedance (capacitive reactance) of a capacitor depends on the capacitor value and frequency. At DC (zero Hz) it has infinite impedance and at high frequencies a capacitor has low impedance.

Okay, let's not nit pick the wave analogy. Sound waves, then. Air molecules move back and forth and aren't actually moving with the wave. (But that was the point about the ocean waves that was relevant).

I use the phrase "don't really move" to mean they aren't traveling with the wave. There's a reason I didn't say "don't move". Maybe I just did too good a job of priming you to think I am really stupid, idk.

Hi,
I think you need to look here;

Electric drift[edit]
Main article: Drift velocity
The drift velocity deals with the average velocity of a particle, such as an electron, due to an electric field. In general, an electron will propagate randomly in a conductor at the Fermi velocity.[5] Free electrons in a conductor follow a random path. Without the presence of an electric field, the electrons have no net velocity. When a DC voltage is applied, the electron drift velocity will increase in speed proportionally to the strength of the electric field. The drift velocity is on the order of millimeters per hour. AC voltages cause no net movement; the electrons oscillate back and forth in response to the alternating electric field (over a distance of a few micrometers – see example calculation).

In copper at 60Hz speed of electron is approx 3.2m/s.
But the AC effect is almost instantaneous.
Consider a pipe full of bearings as the electrons in a conductor.
If you push one more bearing in one end, almost instantaneously a bearing pops out the other.
So even thought the bearing has only traveled a bearing diameter, its effect is a flow of bearings.
When it comes to electrons and capacitors, you have an Electric Field coming into operation.
When it comes to electrons and inductors , you have a Magnetic Field coming into operation.
Tom... :slight_smile:
Goooooooggllleeeeeee

So . . . . an AC live line going to one lead of a light bulb (let's assume incan) and the other lead of the light bulb goes to one lead of a capacitor, with the other lead of the capacitor left open. Is there some size of capacitor that will result in the light bulb lighting up?

INTP:
So . . . . an AC live line going to one lead of a light bulb (let's assume incan) and the other lead of the light bulb goes to one lead of a capacitor, with the other lead of the capacitor left open. Is there some size of capacitor that will result in the light bulb lighting up?

No, not at 50 or 60Hz, the return path through the air from the open lead has too high an impedance.
As you increase the AC frequency and circuit impedance changes , then interesting things can happen.
Tom... :slight_smile:

TomGeorge:
As you increase the AC frequency and circuit impedance changes , then interesting things can happen.
Tom... :slight_smile:

as INTP mentioned, is this what is happening with antennas ?

Just look up the details on 'SWER'. And, regarding antennas --- like RF electronics classes teaches things like ...... for a quarter-wavelength antenna.... along a transmission line....the impedance one-quarter wavelength away from an open circuit will appear as a 'short circuit'. So.... they teach standing waves, and AC voltage/current along the length of a resonant line. These kinds of effects happen when the dimensions of your components start to approach the wavelength of the AC signal(s) being used.

So with a 60Hz AC, the wavelength would be like 3100 miles. So, 775 miles of a single live wire is all it takes for AC to be happy to pretend it's a complete circuit and work can be pulled through whatever devices need power?

No, it just means that 775 miles of wire will act like a form of antenna and will radiate some of the 60 HZ depending on its efficiency.
To make an efficient antenna at 60 HZ you would need at a minimum a 1/4 wave vertical which means a tower 775 miles high, and a good ground plane.

I just remembered a vague tangent to this. I was helping out an AT&T installer, and he had this magical device that could determine the length of a cable, even if it was open-ended. Turns out capacitance can be used when there's at least a pair of wires running the same distance. But a tech that has my curiosity is the Time Domain Reflectometry. Send a pulse down a cable, see how long it takes to detect the reflection.

Maybe you see the tangent. What pulse is being sent down the open-ended wire? Is this related to the antenna stuff? Proper distance wire aligning with fractions of wavelengths result in standing waves, mismatches result in imperfectly overlapping reflections and therefore calculable distance with some sort of voodoo math that can tell the difference between a reflection coming from one period to the next period?

INTP:
This is just a pondering that's probably very stupid but I hope to become a little less stupid by reading the responses from smart folks as yourself.

I'm curious about what might be required to have an AC open-ended circuit still perform work. For example, if just the AC's live wire was put into one side of a free standing capacitor (not limited to realistic specs or sizes), is there some level of 'compressability' of electrons (like they are said to gather up against a capacitor plate) to support an alternating current?

You are inadvertently asking about antenna theory I think - the relevant constant is the permittivity of free-space, about 10^-11 couloubs/volt-meter. "One side of a free standing capacitor" could also
mean ground-return (and the capacitor is pointless for that, the waveform is balanced AC already).
You can also express the permittivity of free space in volts/coulomb, about 113,000,000,000 volts per (coulomb per metre) - so no, not very compressible at all.

As it works, I think the AC's neutral and ground wires (connected at some point anyway) make the literal Earth work as the 'capacitor'. Is it a complete circuit from power station to your house and into the Earth and back to the power station? Or does the immensely large Earth, working as the ground, trick the circuit into thinking it's closed when it's really just a massive capacitor at the end to sink and source from as the current alternates?

No, the earth is a resistor in this context.

So since the electrons don't really move in AC, and are more like the waves in an ocean (and ocean waves don't have a comparable need for a complete circuit, as such), I suppose one way to frame the question is- can a pulse of voltage potential travel down an open wire? Broadly, I think that's what antennas are, right?

Charge carriers do really move in AC, they (on average) move back and forth (a few microns would be
typical for mains conductors) - that's still movement. They also move randomly at enormous speeds which all cancel out and make no contribution to the large scale current (other than thermal or "Johnson" noise).

The pulse of voltage travels way faster than the electrons, basically the speed of light.

For mains frequencies you make the lumped-component approximation unless looking at very long transmission
lines (100's of km).

INTP:
I just remembered a vague tangent to this. I was helping out an AT&T installer, and he had this magical device that could determine the length of a cable, even if it was open-ended. Turns out capacitance can be used when there's at least a pair of wires running the same distance. But a tech that has my curiosity is the Time Domain Reflectometry. Send a pulse down a cable, see how long it takes to detect the reflection.

Maybe you see the tangent. What pulse is being sent down the open-ended wire? Is this related to the antenna stuff? Proper distance wire aligning with fractions of wavelengths result in standing waves, mismatches result in imperfectly overlapping reflections and therefore calculable distance with some sort of voodoo math that can tell the difference between a reflection coming from one period to the next period?

That's right. Energy is being sent down a wire. A voltage pulse of a finite width. This voltage pulse signal propagates (travels) along the wire. Depending on what's on the terminating end of the wire, and depending on the 'characteristic' impedance of the wire/line, the pulse can reach the terminating end and be partially reflected, or even fully reflected, or even not reflected at all. If there's any reflection ...... then that reflection will mean a signal will travel back toward the source. They teach this in transmission line theory. If the measurement unit at the source side is able to measure voltage as a function of time, then it may observe reflected signals. People might be able to make sense of the measured voltage-vs-time plot if the line connections are known, or if the line system is relatively simple.

This may be of some interest.

G line as its normally called is a single conductor low loss transmission line that works at
Microwave frequencies.
I made one once in my earlier years and it does work fairly well.
Its effectively a form of in side out waveguide where the launchers direct the wave down the wire, but it it remains confined to the wire.

So . . . . this voltage pulse, able to travel down an open circuit wire, could it also jump across an LED (with open circuit on the other side) to result in some light?

INTP:
So . . . . this voltage pulse, able to travel down an open circuit wire, could it also jump across an LED (with open circuit on the other side) to result in some light?

Maybe you wouldn’t get much of a light show with a voltage pulse. But if you put enough AC power into a resonating line (eg. open circuit line, one-quarter wavelength long … an antenna) and you wrap a coil around that line (with an LED placed across the terminals of that coil)… then you might get a light show.