Let's say we have a 120 Volt AC source

  • Context: Undergrad 
  • Thread starter Thread starter Evil Bunny
  • Start date Start date
  • Tags Tags
    Ac Source Volt
Click For Summary

Discussion Overview

The discussion revolves around the behavior of a 120 Volt AC generator that is not grounded, specifically focusing on whether current would flow if one pole is connected to the earth while the other remains unconnected. Participants explore concepts of electric potential, reference points, and the implications of grounding in electrical circuits.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants argue that current will flow if there is a nonzero potential difference between the ground and the "hot" line, suggesting that an electric field would cause electrons to flow.
  • Others contend that the voltage at a single point is relative and that without a reference point, the potential difference is meaningless. They emphasize the importance of measuring voltage differences between two points.
  • A participant states that if one pole of the generator is at 120 Volts and the other is unconnected, measuring between either pole and the earth would yield 0 Volts, questioning the validity of measuring potential relative to the earth.
  • Another participant challenges this by stating that if one pole is at a higher potential than the other, it is impossible for both to have the same potential relative to the earth, leading to the conclusion that connecting one pole to earth would allow current to flow.
  • Some participants suggest that the reference point for measuring voltage can be arbitrary, and it does not necessarily have to be the earth, but could be any defined point in space.
  • A later reply introduces an experimental approach using a battery to test the claims, suggesting that measurements between battery terminals and the earth would yield 0 Volts, reinforcing the argument that potential differences are relative.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between potential differences and grounding. There is no consensus on whether current would flow in the described scenario, and multiple competing interpretations of electric potential and reference points remain unresolved.

Contextual Notes

Participants highlight the importance of defining reference points when discussing electric potential. The discussion also touches on the limitations of common measuring instruments and the potential for small differences in voltage to be difficult to measure accurately.

  • #31


Where did anyone say it would jump to both simultaneously?

The arc will jump through the path of least resistance - this will either be to the other pole or to the ground. There's no more to it than that.
 
Physics news on Phys.org
  • #32


Evil Bunny said:
Let's take our suspended generator and crank it up to a million volts instead of just a wimpy 120 volts. Would that arc jump from one pole of the generator to the Earth or would it jump from one pole of that generator to the other?

Naty1 said:
In general, it will arc along the shortest air path...

Let's put them the exact same distance apart. Pole B and the Earth are now the exact same distance from Pole A. We start increasing the voltage until an arc occurs. The arc originates from Pole A... where does it go?

I say it goes to Pole B even if it was farther away than earth. The "desire" to go to the Earth is not there. Correct?

Lightning... on the other hand... :eek:
 
  • #33


Evil Bunny said:
Let's put them the exact same distance apart. Pole B and the Earth are now the exact same distance from Pole A. We start increasing the voltage until an arc occurs. The arc originates from Pole A... where does it go?

I say it goes to Pole B even if it was farther away than earth. The "desire" to go to the Earth is not there. Correct?

No. There is absolutely no desire.

As per my previous post, it follows the path of least resistance. (This of course, assumes the ground and pole b are equal in charge.)
 
  • #34


jarednjames said:
Where did anyone say it would jump to both simultaneously?

Post #25 except for the "simultaneously" part. Would one happen first? Again... little explanation. Hard to tell.

jarednjames said:
The arc will jump through the path of least resistance - this will either be to the other pole or to the ground. There's no more to it than that.

But there is. Didn't we say that there wasn't even a voltage present between the ground and the pole? Why would it ever want to go to the ground in the first place?
 
  • #35


Evil Bunny said:
Post #25 except for the "simultaneously" part. Would one happen first? Again... little explanation. Hard to tell.

He said both were likely to happen, not that both would. There is a difference.
But there is. Didn't we say that there wasn't even a voltage present between the ground and the pole? Why would it ever want to go to the ground in the first place?

There is always a potential difference between two points. It's just a matter of how much of a difference.

If the pole is at 1 million volts and the ground is zero (as per your previous post) then there is a potential difference between the ground and the pole.

You may want to go back and learn the basics of electricity.
 
  • #36


jarednjames said:
If the pole is at 1 million volts and the ground is zero (as per your previous post) then there is a potential difference between the ground and the pole.

The million volts was between the two poles. 0 volts between pole and ground, remember...
 
  • #37


Evil Bunny said:
The million volts was between the two poles. 0 volts between pole and ground, remember...

If your pole is at 1 million volts and the ground is at zero, there is a potential difference between them.

Think of an electric line power line. The line is at 400,000V and the ground is your zero reference. The only reason it doesn't arc is because the path of least resistance is to follow the wire.

Now, if I get close enough to the wire, suddenly the path of least resistance is to arc through the air, through me and into the ground. And so it changes direction and follows that route instead - until I move away.
 
Last edited:
  • #38


The utility company ties one of their two poles to the ground. That is the only reason there is a potential there. We did not do that. That is a very critical point to this thread. Go back and read carefully.
 
  • #39


Evil Bunny said:
The utility company ties one of their two poles to the ground. That is the only reason there is a potential there. We did not do that. That is a very critical point to this thread. Go back and read carefully.

What you have posted is the inversion of one of the standard procedures for servicing of HV lines.

They use a helicopter to allow someone to work on the lines whilst live. As the helicopter is not grounded, there is no potential (or very little) potential between it and the lines - so no arc.

In this case, it won't arc to the ground but to the other pole. EDIT: Assuming path of least resistance is to the other pole.

However, with a significant charge imbalance you will get an arc between the pole and the ground - as with static (lightning).
 
Last edited:
  • #40


jarednjames said:
In this case, it won't arc to the ground but to the other pole.

However, with a significant charge imbalance you will get an arc between the pole and the ground - as with static (lightning).

Perfect... Ok so my main reason for posting this is because I am having a hard time understanding why we can develop a large potential (between the two poles of my imaginary hovering generator) and not have the electrons make their way from the Earth to one of these poles. I would think that these electrons in the Earth would be attracted to one of these poles because of a huge difference in charge, but this is apparently not the case.

Why?

I think the explanation was given by Naty1 in post # 12 of this thread but I'm still having a hard time grasping the concept.

Lightning was also brought into this conversation and rightly so... Obviously there are scenarios where the potential between cloud and ground becomes so great that we ionize the air and lightning occurs. What is the difference? Is it sheer magnitude of voltage? Are we simply not generating enough voltage to "encourage" interaction between the generator pole and ground? Or does voltage (between the two generator poles) have absolutely nothing to do with this "charge" that causes lightning?
 
Last edited:
  • #41


Evil Bunny said:
Lightning was also brought into this conversation and rightly so... Obviously there are scenarios where the potential between cloud and ground becomes so great that we ionize the air and lightning occurs. What is the difference? Is it sheer magnitude of voltage? Are we simply not generating enough voltage to "encourage" interaction between the generator pole and ground? Or does voltage (between the two generator poles) have absolutely nothing to do with this "charge" that causes lightning?

It's worth pointing out, if you moved the generator so that the path to Earth had less resistance than to the other pole, it would arc to the earth. This is like static electricity from a car.
The car becomes charged due to sunlight on a hot day. It is not grounded to the earth, but when you touch it you ground it and so receive the shock.
This is true for electrical lines as well, regardless of whether or not they are in grounded at some point. The reason it arcs is not because the company ground the cables at some point.

In the case of lightning, it is only when the charge builds up enough to give you a large enough driving voltage to arc to the ground. So yes, it's all about voltage.
 
  • #42
Evil Bunny said:
The utility company ties one of their two poles to the ground.
In the USA, at most (but not all) transformers, the center tap of the transformer is is grounded, so that at the final step down transformer, and at a household, the neutral ends up +/- 10 volts of Earth ground. At my house, it's close, about .3 to .5 volts, but it's not zero. The final step down transformer only has 2 lines going into it, 6900 volts ac, and steps the voltage down to 220 volts, with a non-grounded center tap to provide a neutral. At my house hold, there is a grounded rod used for the third round pin socket on 3 pin plug sockets. The wide pin socket hole is neutral, and the narrow pin socket plug hole is one of the two end taps from the transformer, to provide 110 volts. A 220 volt socket would use both ends of the transformer, plus the grounded line.

jarednjames said:
They use a helicopter to allow someone to work on the lines whilst live. As the helicopter is not grounded, there is no potential (or very little) potential between it and the lines - so no arc.
There's a huge potential beteen helicopter and the line. The first part of the procedure is to extend a probe connected to the helicopter and the line being worked on, so that both helicopter and the line end up at the same potential, You often see significant arcing during the connection process. The line is then connected to the helicopter while the worker moves onto the line, then the line is disconnected. This processis repeated again when the worker leaves the line. The worker also uses a faraday cage like suit so any current flow or transformer effects of working on a single wire go thorugh the suit and not the worker. Example video:



There are also line inspector robots a bit less than 1 meter long that rely on a transformer effect from the two points on a single line to power them so they can move on high power, high tension lines. (I couldn't find a video of these in action). Just the field from these lines is strong enough to light up flourescent bulbs:

http://www.stopgeek.com/richard-boxs-light-field.html
 
Last edited by a moderator:
  • #43


rcgldr said:
There's a huge potential beteen helicopter and the line.

Initially yes, I meant later with the guy jumping about. The fact the helicopter never gets to the point of creating the path of least resistance means they can get to a zero potential level. So no arc when he's messing about.

It's what I was trying to clarify with my last post regarding moving closer to the earth.
 
  • #44


jarednjames said:
It's worth pointing out, if you moved the generator so that the path to Earth had less resistance than to the other pole, it would arc to the earth.

No it wouldn't. It's all about where these electrons want to go. They don't want to go to the earth, they want to go to the other pole. We talked about it in this very thread. If you put a straight wire from pole A to earth, virtually no current would flow (the same is true between pole B and the earth). If you put a straight wire from pole A to pole B the wire would quickly melt from all the current.

jarednjames said:
This is like static electricity from a car.

No, it's not actually... And this is what I'm trying to find out. What is different about it? This "static" electricity that shocks you when you touch your door knob or your car or allows you to stick a balloon to the wall is the same as lightning.

But it's not the same as having a voltage potential between these two wires of the "source" (generator) and their relationship to the earth.

This is the whole point of the thread. I'm looking for an explanation of what is different about it.

I guess I'm just not explaining it very well. I'm probably not asking it the right way or something.
 
  • #45


rcgldr said:
In the USA, at most (but not all) transformers, the center tap of the transformer is is grounded, so that at the final step down transformer, and at a household, the neutral ends up +/- 10 volts of Earth ground. At my house, it's close, about .3 to .5 volts, but it's not zero. The final step down transformer only has 2 lines going into it, 6900 volts ac, and steps the voltage down to 220 volts, with a non-grounded center tap to provide a neutral. At my house hold, there is a grounded rod used for the third round pin socket on 3 pin plug sockets. The wide pin socket hole is neutral, and the narrow pin socket plug hole is one of the two end taps from the transformer, to provide 110 volts. A 220 volt socket would use both ends of the transformer, plus the grounded line.

The primary of the transformer has one side attached to HV (6900VAC in your example) and the other side attached to the "return" which is the wire located underneath the HV line parallel with it on your utility pole). The "return" side of the primary is also attached to ground by a large conductor and a rod driven into the earth... The secondary has two "hots" and one center tapped neutral coming out of it. The center tapped neutral absolutely is grounded. It is grounded with the same conductor that is grounding the primary of the transformer. The two "hots" give you 240 Volts between them. Either of the "hots" give you 120V when referenced to the center-tapped neutral.

The bare copper wire (third round pin) is used for "bonding" metal parts in your structure together and they are also tied into the "neutral" as well as the "ground rod" at your service entrance.
 
  • #46


Evil Bunny said:
The primary of the transformer has one side attached to HV (6900VAC in your example) and the other side attached to the "return" which is the wire located underneath the HV line parallel with it on your utility pole). The "return" side of the primary is also attached to ground by a large conductor and a rod driven into the earth... The secondary has two "hots" and one center tapped neutral coming out of it. The center tapped neutral absolutely is grounded. It is grounded with the same conductor that is grounding the primary of the transformer. The two "hots" give you 240 Volts between them. Either of the "hots" give you 120V when referenced to the center-tapped neutral.

The bare copper wire (third round pin) is used for "bonding" metal parts in your structure together and they are also tied into the "neutral" as well as the "ground rod" at your service entrance.

I'm curious how you can know all this without knowing the reasons (physics) behind it - or at least some knowledge.
 
  • #47


Evil Bunny said:
No it wouldn't.
No, it's not actually...

Right, you need to check a few things here. I suggest you read up what charge is and what voltage is before we continue.

As rcgdlr said, you get an arc between the helicopter and the power lines - note that the helicopter isn't grounded.

Initially, the PD between the two is so high, you get the arc - much like static shocks.
 
  • #48


jarednjames said:
I'm curious how you can know all this without knowing the reasons (physics) behind it - or at least some knowledge.

The reason for grounding the distribution system in the United States is put forth in the NEC. They mention things like stabilizing voltages and protection against line surges and lightning strikes and unintended contact with high voltage sources... the article is 250.4 (A)(1) I think but I don't have it in front of me.

I definitely have some knowledge, but I'm looking for more. I don't understand how we can have a potential voltage on a pair of wires (like our generator scenario) and not have a potential between either one of those wires and the earth. It just seems like there would be some "charge" on the wire that would want to interact with the electrons in the earth.

My brain hurts... I need to take a break from this and regroup.
 
  • #49


Evil Bunny said:
The bare copper wire (third round pin) is used for "bonding" metal parts in your structure together and they are also tied into the "neutral" as well as the "ground rod" at your service entrance.
Then why do I see .3 to .5 volts between ground and neutral at my house? I've asked a few of my friends to do measure the difference at their houses, and the voltage is a bit under 1 volt, but it's never zero. I've seen the final transformer (it's less than 100 feet from my house) when it was being serviced. Two well insulated lines going in, 3 well insulated lines going out (the two hots and neutral), none of them are grounded.
 
  • #50


Since my meaning was still in dispute:

Friction causes some clouds to become charged with static electricity. Some become positive, some become negative so act like battery poles.

If you watch the heavens you can see some lightning flashes between one cloud and another.

You can also see lightning flashes between some clouds and ground.

Both types of flash act to discharge the clouds and we all know just how energetic and destructive a lightning flash can be.

I am only trying to get a better understanding of how electricity (whatever that means) works...

Then I (respectfully) suggest you prick up those long bunny ears and do more listening and less expounding.
 
  • #51


rcgldr said:
Then why do I see .3 to .5 volts between ground and neutral at my house? I've asked a few of my friends to do measure the difference at their houses, and the voltage is a bit under 1 volt, but it's never zero. I've seen the final transformer (it's less than 100 feet from my house) when it was being serviced. Two well insulated lines going in, 3 well insulated lines going out (the two hots and neutral), none of them are grounded.

The reason for voltage between neutral and ground is probably voltage drop. They are tied together in your panel and then attached to ground just outside of where your panel is located. They are tied together at no other point in the system, so as you get further away from the box the voltage might tend to "drift".

As to why they aren't grounded at your utility pole, I don't have an answer. Every one that I've seen, including the one at my house, has a large conductor connected to a ground rod at the base of the utility pole. At any rate, the neutral is most certainly grounded at your panel.
 
  • #52


How does this sound?

If you only have 1 wire connected to a generator, you still get a potential along the wire that's going to earth. However, since the other side of the circuit for the generator isn't connected, you don't have a complete circuit. The generator applies force to try to move the electrons in the circuit, but can't because there are no "replacement" electrons to replace the ones moving away from the generator. As the generator tries to move the electrons, a potential that is opposite develops and keeps the electrons from moving. If you get the voltage high enough, there will be enough potential for the disonnected side to pull electrons from the molecules in the air to replace the ones being lost on the other end. This results in arcing. Like the analogy to water that is commonplace, you must have water to replace the water that you pump in order to keep pumping the water. (Say that 3 times fast) That's why you need a complete circuit for the generator.

Also, just because you have a potential difference between 2 points doesn't mean that you will measure a voltage between them. You will only measure a voltage if there is current capable of flowing between those points. Holding multimeter leads in the air reads 0 voltage because the resistance of the air is so high at the distance between the leads that current CANNOT flow, meaning that you have 0 voltage. (Not to mention the fact that the multimeter doesn't have anything to measure because you aren't measuring a circuit anyways)

Likewise, the reason that power lines don't arc to ground isn't because the wire is the path of least resistance, but because the resistance of the air is so great that there isn't anywhere for the electricity to flow to. The path of least resistance is simply the path that the majority of the current will flow through.
 
  • #53


Evil Bunny said:
Transformer ... as to why they aren't grounded at your utility pole.
It's not on a pole, it's much larger than the transformers you see on poles, and it's underground, within a large metal cylinder, (don't remember what the floor was made of, perhaps cement), and a vented cap. There are 2 large wires going in, 3 going out, through the sides of the cylinder and into the transformer. The transformer itself may be grounded via bolts in the floor. The cylinder appears it can get filled with water, but somehow it doens't become a hazard at the ground above.

On a side note, some transmission lines in the USA have just 3 hot lines at different phases. There's a grounded line above them, but it's only there for lightning protection, and not part of the circuit. Where is the "return" path for the current from these 3 lines? For home usage, only one of the hot lines is used, and I assume a grounded wire, to feed the intial step down transformer, which I also assume has a grounded center tap on the output side.
 
  • #54


Drakkith said:
Likewise, the reason that power lines don't arc to ground isn't because the wire is the path of least resistance, but because the resistance of the air is so great that there isn't anywhere for the electricity to flow to. The path of least resistance is simply the path that the majority of the current will flow through.

That has to be the best contradiction in terms I've read in a while.

"It doesn't arc to the ground because the wire is the path of least resistance, but because the air is the path of greatest resistance." = The wire is the easiest route for the current to flow in.

I understand what you're trying to say though.

Now, evil bunny. You claim you want to learn and yet you keep arguing with us. You're not going to learn anything like this. I think Drakkiths explanation covers it nicely.
 
  • #55


rcgldr said:
It's not on a pole, it's much larger than the transformers you see on poles, and it's underground, within a large metal cylinder, (don't remember what the floor was made of, perhaps cement), and a vented cap. There are 2 large wires going in, 3 going out, through the sides of the cylinder and into the transformer. The transformer itself may be grounded via bolts in the floor. The cylinder appears it can get filled with water, but somehow it doens't become a hazard at the ground above.

I would bet that one of the two wires going in is the return for the primary and it is connected to one of the three going out (the center tapped neutral of the secondary), and they are both connected to ground either inside the concrete floor there or not far away... Lots of transformers are submerged in oil for cooling, insulation, surge suppression, etc. Yours is a bit different... Most people have those "bucket" type transformers hung on the utility pole.

rcgldr said:
On a side note, some transmission lines in the USA have just 3 hot lines at different phases. There's a grounded line above them, but it's only there for lightning protection, and not part of the circuit. Where is the "return" path for the current from these 3 lines? For home usage, only one of the hot lines is used, and I assume a grounded wire, to feed the intial step down transformer, which I also assume has a grounded center tap on the output side.

3 Phase Wye transformers are common in US electrical distribution systems. There is usually a single return for all three phases. Only one of the phases will be used to power a residential dwelling. One block of the neighborhood might be fed off phase A, the next block fed off of Phase B, and the third block might be fed off of phase C for example. But there will be a return. Look closely and see where the two primary wires are connected. One of them will most likely be grounded and also connected to the "return" wire of the primary... it will also likely be connected to the center tapped neutral on the secondary. You can usually look up and see them all physically connected together.
 
Last edited:
  • #56


Drakkith said:
How does this sound?

Sounds good. I think I agree with most of it.


Drakkith said:
If you only have 1 wire connected to a generator, you still get a potential along the wire that's going to earth. However, since the other side of the circuit for the generator isn't connected, you don't have a complete circuit. The generator applies force to try to move the electrons in the circuit, but can't because there are no "replacement" electrons to replace the ones moving away from the generator. As the generator tries to move the electrons, a potential that is opposite develops and keeps the electrons from moving.

This is a very concise and clear explanation. It is also confirmation of what I've been saying (admittedly a lot less clearly than you have here). These electrons "want to get back to the source" is what I've been saying and it hasn't been received well... Needing replacement electrons is another good way of putting it. Maybe it's easier to understand that way. Either way, that other pole of the generator needs to be involved before anything exciting happens. Agreed?

Drakkith said:
If you get the voltage high enough, there will be enough potential for the disonnected side to pull electrons from the molecules in the air to replace the ones being lost on the other end. This results in arcing.

Okay, now it's getting a little fuzzy again. I contend that the arc comes into existence because of a potential rising to the point where the air between the two poles of the generator turns into a conductor. The air becomes ionized and a path from one pole to the other pole is created, and we see an arc. We are now "pumping the water" out one pole and "squirting" it back into the other. Completing the circuit.

Are you saying something different than this? Not sure what you meant by pulling electrons from air molecules...

jarednjames said:
Now, evil bunny. You claim you want to learn and yet you keep arguing with us. You're not going to learn anything like this. I think Drakkiths explanation covers it nicely.

Yes, he covers it very nicely. I'm not trying to argue with anybody here. I'm sorry if I am giving that impression. His "replacement" analogy is a very good one. The reason these electrons don't ever want to go into the ground is because the ground offers no path back to the other pole of the generator. And if there is nothing "replacing" the electrons that would venture into the earth, then there is no way to "pump them out" of the other pole and into the ground.

I think we are all in agreement that electrons will never ever want to go into the ground unless the ground ends up in the circuit as a return path back to the source.

Now... Lightning is different because? The two poles would be cloud and ground (or cloud and cloud)? I guess...
 
  • #57


Evil Bunny said:
I think we are all in agreement that electrons will never ever want to go into the ground unless the ground ends up in the circuit as a return path back to the source.

We don't agree with that.

You only have to look at lightning, or more prominently, the sparks to the helicopter in rcgdlr's video to realize that isn't true.
 
  • #58


Okay... I'll rephrase that then...

Electrons acting on forces derived from an AC or DC voltage source with two poles will never ever want to go into the ground unless the ground ends up in the circuit as a return path back to the source.

Do you agree with that?

Now... Lightning is a different animal.
 
  • #59


Evil Bunny said:
Okay... I'll rephrase that then...

Electrons derived from an AC or DC voltage source with two poles will never ever want to go into the ground unless the ground ends up in the circuit as a return path back to the source.

Do you agree with that?

Now... Lightning is a different animal.

Is lightning not DC?
 
  • #60


A battery is a voltage source. Is lightning similar to a battery?

Honest question...
 

Similar threads

Undergrad Magnetism: Same Pole Repulsion & Heteropolar Attraction
  • · Replies 32 ·
2
Replies
32
Views
3K
Undergrad LRC Series Circuit with an AC Source
  • · Replies 10 ·
Replies
10
Views
1K
Undergrad How far do electrons travel in a 120 volt circuit?
  • · Replies 27 ·
Replies
27
Views
17K
Undergrad Motional EMF in Faraday disc, co-rotating magnet axial mean flux
  • · Replies 5 ·
Replies
5
Views
975
Undergrad Does Ohm's Law work for Light Bulbs?
  • · Replies 57 ·
2
Replies
57
Views
14K
Undergrad Two questions. 180 degrees out of phase.
  • · Replies 6 ·
Replies
6
Views
10K
High School Switch-to-lightbulb wire delay
  • · Replies 5 ·
Replies
5
Views
2K
Undergrad How a Battery creates a Potential difference in an electrical current
  • · Replies 14 ·
Replies
14
Views
6K
AC generator, current and ground
  • · Replies 26 ·
Replies
26
Views
3K
Tripping Behavior of Circuit Breakers
  • · Replies 42 ·
2
Replies
42
Views
5K