Question on live and neutral wire

[mr166]

Sorry WW you weren't exactly wrong. I just poorly pointed out that
SWER is used in the US also.

 

[puf_the_majic_dragon]

So, forgive my ignorance, but I think I'm going to take this opportunity to educate myself just a little bit about AC grid electricity.
While I've done a lot of DIY electrical, and of course I read the instructions so I always wired up the hot and neutral correctly, I've never fully understood the difference between hot and neutral in AC from a physics perspective. Granted, most of my experience and knowledge is all in the home, after the mains - the description of the US grid using SWER helps a little bit...

So, basically, a generator has a magnet more or less "pushing" electrons around a coil of wire. In order for those electrons to respond to the "push", they need a place to go, hence a closed circuit with a source and return. Now I understood AC generators to more-or-less change the direction that they "push" the electrons in, which would mean that on the down phase the return wire became the source wire, and thus both wires could be "hot". But if the neutral is earthed, whether at the breaker panel or a grid transformer, then it could never possibly be hot - which leaves me wondering where the heck does the hot wire in an AC grid get its electrons? My hypothesis might be that on the down phase the generator is "pulling" electrons instead of pushing them, kind of creating an electron vacuum along the coil. Yes? No? I know I'm using layman's parlance, but I'm not afraid of a more technical explanation.

And another question - in an SWER scenario, what's to prevent the grounded neutral from acting as a short, especially with a capacitive load?

 

[sophiecentaur]

So, basically, a generator has a magnet more or less "pushing" electrons around a coil of wire. In order for those electrons to respond to the "push", they need a place to go, hence a closed circuit with a source and return. Now I understood AC generators to more-or-less change the direction that they "push" the electrons in, which would mean that on the down phase the return wire became the source wire, and thus both wires could be "hot". But if the neutral is earthed, whether at the breaker panel or a grid transformer, then it could never possibly be hot - which leaves me wondering where the heck does the hot wire in an AC grid get its electrons? My hypothesis might be that on the down phase the generator is "pulling" electrons instead of pushing them, kind of creating an electron vacuum along the coil. Yes? No? I know I'm using layman's parlance, but I'm not afraid of a more technical explanation.

It's really best if you realize that, as far as the generator and the load are concerned, there is no difference or order of importance between the two conductors. The Potential Difference between them just alternates at mains frequency. The fact that one of the wires happens to be connected to (or near) Earth has no effect whatsoever on the way the circuit works. Current flows alternately clockwise and anticlockwise. If you connect your 'scope probe to the live wire, its voltage will oscillate about zero volts and the neutral wire will have, nominally, no voltage swing relative to Earth. If you uncouple the Neutral back at the transformer, the two voltages will still waggle about and the difference between the two voltage waveforms will still the 'mains voltage' waverform but the absolute value of the mean between the voltages could be anything, depending on where one of the lines happens to be connected. The term Potential Difference says it all.

PS, many people feel that using the term 'electrons' somehow makes posts more learned or easier to understand. If you look at the wording used by 'people wot know', you will notice that they very seldom use electrons in explanations. They nearly always use the terms 'Current' and 'Charge'. because the nature of the charge carriers is not relevant in 99.9% of circuits. Electrons are far too hard to cope with when dealing with circuits. They do so many strange things.

 

[puf_the_majic_dragon]

It's really best if you realize that, as far as the generator and the load are concerned, there is no difference or order of importance between the two conductors.

Which is what confuses me, since that means the "neutral" is no different from the "hot".

PS, many people feel that using the term 'electrons' somehow makes posts more learned or easier to understand. If you look at the wording used by 'people wot know', you will notice that they very seldom use electrons in explanations. They nearly always use the terms 'Current' and 'Charge'. because the nature of the charge carriers is not relevant in 99.9% of circuits. Electrons are far too hard to cope with when dealing with circuits. They do so many strange things.

"Current" and "Charge", in this context, are rather abstract terms. I've known way too many electricians who use the words "current" and "charge" and don't have a clue what an electron is, much less what either word really means. Of course electrons do crazy things - this is a physics forum, after all, so why should we not talk about the crazy things they do?
So in physics, "current" describes the flow of electrons, and "charge" describes the number of electrons. "Potential difference" just describes a state where there are extra electrons on one end of a conductor and a shortage of electrons on the other end, and therefore the electrons will have a tendency to flow from higher to lower in order equalize the charge over the length of the conductor. If the local power plant operated strictly off of an electrical potential, electrons could move in either direction along the conductor in order to equalize charge - but that is not the case, as the direction those electrons move is strictly enforced (by the motion of the magnetic field of the generator). And those electrons can't flow if there is not a closed circuit between the point of negative net charge and the point of positive net charge.

Now, obviously there is a significant difference between the hot and neutral conductors, or this whole thread would make absolutely no sense, as a switch would have to switch *both* in order to completely disconnect the load. But the neutral is a dead wire when the hot is disconnected and, apparently, it just gets buried in the dirt at some point. So if we go back to the power plant, that generator has a giant coil around a spinning magnet and one end of that coil runs off to power your house - where does the other end of that coil go? Since it doesn't also run back to your house to close the circuit, it's got to close the circuit another way - is it just buried in the dirt as well? How does that actually work in order to form a closed circuit?

 

[sophiecentaur]

and don't have a clue what an electron is,

Exactly. And neither, I suggest, do you have a solid grasp of what they truly are (along with the vast majority of us). I would venture to say that you probably see them as tiny charge carriers. Does your understanding the way circuits work involve anything more than the charge on an electron? So why not avoid the term - which can only add complication - and use the accepted terms, using the Joule and the Amp instead of the number and rate of electrons? Like I said, it's common practice and there's a very good reason for it. It is not a cop-out to use the 'right' terms.

 

[sophiecentaur]

Now, obviously there is a significant difference between the hot and neutral conductors

Actually, as far as the circuit operation is concerned, there is no significant difference at all. The difference is in only terms of the way the distribution system works and relates to safety and protection of devices and cables - purely practical matters. The reason that the 'neutral' issue is raised so frequently on PF is that people don't start at the beginning of the story (basic circuits) and apply that rigorously when moving to AC and then to Mains supply. If only people used the term Potential Difference all the time (a long winded term which is shortened to Voltage) then the confusion would be less likely to arise. This would be because the word 'Difference' is so important. You could mount an AC generator and load on an insulated platform which is held at +5kV above Earth and the system would operate in exactly the same way, the 'electrons would all go in exactly the same direction and in the same numbers. Or you could connect one of the transmission cables to the +5kV source (relative to Earth) and the same thing would apply.

 

[Dale]

"Current" and "Charge", in this context, are rather abstract terms. I've known way too many electricians who use the words "current" and "charge" and don't have a clue what an electron is, much less what either word really means. Of course electrons do crazy things - this is a physics forum, after all, so why should we not talk about the crazy things they do?
So in physics, "current" describes the flow of electrons, and "charge" describes the number of electrons.

In many circuits, including circuits with batteries or other electrolytic conductors, the charge carriers include positive ions. So the terms "charge" and "current" are more corect and general than "number of electrons" and "flow of electrons". The electromagnetic effect of a current or charge typically does not depend on the sign of the charge carriers.

 

[Dale]

So if we go back to the power plant, that generator has a giant coil around a spinning magnet and one end of that coil runs off to power your house - where does the other end of that coil go?

Usually a power plant generator will have three coils, each producing a voltage 120 degrees out of phase with the other and all three referenced to a common neutral wire. The common neutral wire is usually grounded for safety and in normal conditions carries no current. If you look at power distribution lines you will usually see three large lines and a fourth much smaller line.

 

[sophiecentaur]

Whilst the above is all true, might it not be a lot to swallow for someone who thinks that, in a single phase system, there I s a fundamental difference between the two wires?

 

[Dale]

Probably. From a physics/electronics standpoint I agree that there is no fundamental difference whatsoever between the two wires. The only difference is with respect to safety, as described above.

The generator/fields/charge/current doesn't care if the load is an appliance or a human body. Power engineers do.

 

[William White]

Whilst the above is all true, might it not be a lot to swallow for someone who thinks that, in a single phase system, there I s a fundamental difference between the two wires?

in a real-world practical sense, there is a working difference: if you break the neutral, you've got a world of pain; if you touch the live you've got a world of pain!

in a three phase system breaking the neutral is not only very dangerous to people, it damages electrical equipment and can end up being a real nightmare
the conductors are clearly labelled differently for a reason, placed and protected differently for a reason, and defined in law as different for a reason.

 

[sophiecentaur]

If you read the contents of some of these posts, you can see that the 'difference' between the functions of the two conductors is actually perceived as fundamental. That was what I try to correct.
The people who 'know' do not seem to see what the problem is for the people who 'do not know'. To be helpful, PF needs to be aware of more than just the Engineering Facts. The Misconceptions are important.

 

[Nugatory]

Which is what confuses me, since that means the "neutral" is no different from the "hot".

You can interpret "neutral" as meaning "always at the same potential as the Earth nearby"; we make it so by connecting the neutral wire directly to the Earth (for example, by connecting it to a copper stake driven several meters into the ground).

The generator maintains a potential difference between the two wires. For a North American 240 V 60 Hz system, the potential is a sine wave with 60Hz frequency and peak-to-peak amplitude 310 V (Yes, 310! 240 is the RMS average potential across one full cycle. Google for "root mean square" if that doesn't make sense). In other words, the potential difference between the two wires is given by $V(t)=155\sin120\pi{t}$ and as far as the generator is concerned, the two wires are indeed not different in any way.

So if one wire is at the same potential as the earth, and the potential difference between the two wires is given by $V(t)$, then it must follow that the potential difference between the Earth and the other wire is also given by $V(t)$. That will be the hot wire. If you touch the hot wire while touching anything grounded, you'll be shocked; if you touch the neutral you won't. (Don't try this at home! The perfectly neutral neutral wire is an idealization even if some clown hasn't screwed everything up by putting a switch or fuse in the neutral, which is what this thread started out to be about).And to answer your original question: The current flow is through the hot and neutral, with the charge carriers moving first in one direction then the other. There's not a lot of current flow through the connection between the ground and the neutral, just whatever is needed to keep them at the same potential relative to one another while the potential of the hot wire oscillates relative to both.

 

[sophiecentaur]

And, on the subject of the charge carriers moving one way and then the other - how far will they actually move when they have only 1/100s, moving at a mean speed of a few mm/s??

 

[puf_the_majic_dragon]

You can interpret "neutral" as meaning "always at the same potential as the Earth nearby"; we make it so by connecting the neutral wire directly to the Earth (for example, by connecting it to a copper stake driven several meters into the ground).

The generator maintains a potential difference between the two wires. For a North American 240 V 60 Hz system, the potential is a sine wave with 60Hz frequency and peak-to-peak amplitude 310 V (Yes, 310! 240 is the RMS average potential across one full cycle. Google for "root mean square" if that doesn't make sense). In other words, the potential difference between the two wires is given by $V(t)=155\sin120\pi{t}$ and as far as the generator is concerned, the two wires are indeed not different in any way.

So if one wire is at the same potential as the earth, and the potential difference between the two wires is given by $V(t)$, then it must follow that the potential difference between the Earth and the other wire is also given by $V(t)$. That will be the hot wire. If you touch the hot wire while touching anything grounded, you'll be shocked; if you touch the neutral you won't. (Don't try this at home! The perfectly neutral neutral wire is an idealization even if some clown hasn't screwed everything up by putting a switch or fuse in the neutral, which is what this thread started out to be about).

And to answer your original question: The current flow is through the hot and neutral, with the charge carriers moving first in one direction then the other. There's not a lot of current flow through the connection between the ground and the neutral, just whatever is needed to keep them at the same potential relative to one another while the potential of the hot wire oscillates relative to both.

THIS answered my question :) and in such an elegantly simple way (way more simple than I was expecting).
And yeah, I did kind of hijack this thread, but I rationalize it because this understanding puts all of the other answers to the OP into a context. Your answer to my question answers the OP way better than any of the other responses did.

 

[NascentOxygen]

The generator maintains a potential difference between the two wires. For a North American 240 V 60 Hz system, the potential is a sine wave with 60Hz frequency and peak-to-peak amplitude 310 V (Yes, 310! 240 is the RMS average potential across one full cycle. Google for "root mean square" if that doesn't make sense). In other words, the potential difference between the two wires is given by $V(t)=155\sin120\pi{t}$ and as far as the generator is concerned, the two wires are indeed not different in any way.

Something is amiss here. Your figures fit a 110VAC supply with its peak value of 155V and peak-peak of 310V.

Any voltage difference of 240VAC 60Hz sine wave is $V(t)=339\sin120\pi t$, exhibiting a peak-peak amplitude of 678V.

 

[Nugatory]

Something is amiss here. Your figures fit a 110VAC supply with its peak value of 155V and peak-peak of 310V.

Any voltage difference of 240VAC 60Hz sine wave is $V(t)=339\sin120\pi t$, exhibiting a peak-peak amplitude of 678V.

Yeah, yeah, yeah... I slipped into 110V thinking there. Thanks, good catch.