Question on live and neutral wire

[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.