Naked Science Forum

General Science => General Science => Topic started by: Mel769 on 15/01/2018 15:12:48

Title: Neutral has 0 voltage, so how does current flow?
Post by: Mel769 on 15/01/2018 15:12:48
Im having trouble getting my head round why it is that:-  the neutral in an AC system has 0 volts flowing (in respect to earth), then how does current travel through the neutral cable back to the transformer without any voltage pushing it.

I understand that a voltage is needed to propel the moving electrons (current) around a circuit, from its source, through a load, and back to the source via neutral.

Is it that there really is a voltage in the neutral, but as the neutral is at earth potential, it registers as 0 volts ??
Title: Re: Neutral has 0 voltage, so how does current flow?
Post by: Janus on 15/01/2018 16:51:05
Current flow just requires a voltage difference. As long as the "live" cable has a positive or negative voltage relative to to the neutral cable, current will flow.   Voltage is not an absolute value, but a relative one. There is no such thing as 0 volts in an absolute sense.  0 volts can be wherever we assign it.  We assign it to Earth because it is convenient to do so.
Title: Re: Neutral has 0 voltage, so how does current flow?
Post by: chris on 15/01/2018 17:13:17
@Janus is absolutely right.

The other term for voltage is "potential difference". When a battery or a power station pushes electrons into the positive line it gives them some potential; this may be either a positive or a negative potential; either way, their potential is greater than zero, so their natural tendency is to fall towards zero. So the electrons in the live / positive line try to move towards the neutral line to shed their potential energy. The neutral line is also connected to Earth and the power station, so by returning to the neutral line the electrons are effectively completing the circuit.
Title: Re: Neutral has 0 voltage, so how does current flow?
Post by: wolfekeeper on 15/01/2018 18:17:07
Basically, electricity is just a bunch of electrons crowding together, trying to get less crowded together, acting as an electrical fluid.

It's similar to water pressure/head. You can define sea level as a 'zero' water pressure, and if something has higher head then it will flow down to sea level, given the chance, or sea level will flow down into it, if it's below sea level.

Electricity is much the same, 'earth' is the level that electrical pressure (i.e. voltage) will flow down to (or will flow FROM earth into something if it has negative volts.)
Title: Re: Neutral has 0 voltage, so how does current flow?
Post by: Mel769 on 15/01/2018 19:48:22
Okay thanks guys, the crux of what im getting at is this. Say theres 120v between line and neutral, this potential difference causes current to flow, normally back to the supply transformer through the neutral (which is at earth potential 0V). Okay I get that. So in a live system there is current flowing through the neutral right, so why do i not get a shock if im standing on the ground and i touch the neutral, because there is still current going through it isnt there !!

I suspect it has something to do with the fact that there is no potential difference between the neutral and earth (that im connected to via the ground in this instance); so in that case there is no voltage between neutral and earth, but there is current flowing through the neutral from the live 120v. But i cant feel this 120v if i touch the neutral while im standing on the ground !!! WHY IS THIS PLEASE HELP
Title: Re: Neutral has 0 voltage, so how does current flow?
Post by: wolfekeeper on 15/01/2018 21:42:40
It's current that hurts you, not voltage. The current isn't flowing through you. If you unplugged the neutral and put yourself in the circuit, you'd die straight away.

But if you just touch neutral and you were earthed, basically, you have a resistance of (say) 1 k ohms, and the neutral wire to the earth has a resistance of a few ohms at most through the substation, so if you touch it, virtually all the current will go down the wire to earth and not through you. But you're not supposed to touch the neutral anyway.
Title: Re: Neutral has 0 voltage, so how does current flow?
Post by: hamdani yusuf on 15/01/2018 21:49:13
Okay thanks guys, the crux of what im getting at is this. Say theres 120v between line and neutral, this potential difference causes current to flow, normally back to the supply transformer through the neutral (which is at earth potential 0V). Okay I get that. So in a live system there is current flowing through the neutral right, so why do i not get a shock if im standing on the ground and i touch the neutral, because there is still current going through it isnt there !!

I suspect it has something to do with the fact that there is no potential difference between the neutral and earth (that im connected to via the ground in this instance); so in that case there is no voltage between neutral and earth, but there is current flowing through the neutral from the live 120v. But i cant feel this 120v if i touch the neutral while im standing on the ground !!! WHY IS THIS PLEASE HELP
the neutral is usually already connected to the earth by the utility providers.
Title: Re: Neutral has 0 voltage, so how does current flow?
Post by: syhprum on 15/01/2018 21:51:13
The current flowing back thru the neutral conductor might raise its voltage relative to the Earth by a few volts certainly not enough to give you a shock
Title: Re: Neutral has 0 voltage, so how does current flow?
Post by: Colin2B on 15/01/2018 23:13:39
I suspect it has something to do with the fact that there is no potential difference between the neutral and earth (that im connected to via the ground in this instance);
Yes, it has in fact everything to do with this.

so in that case there is no voltage between neutral and earth, but there is current flowing through the neutral from the live 120v. But i cant feel this 120v if i touch the neutral while im standing on the ground !!! WHY IS THIS PLEASE HELP
Current will only flow through something if there is a potential difference between 2 points on the object eg you. So your finger touches neutral and your feet touch earth (=neutral) so no current flows through you, just through the wire, so nothing to feel.
What you feel when you touch a live wire eg 120v is not the voltage, but the current flowing through you to earth. If you were suspended in the air with your feet touching the 120v wire you could touch the 120 with your finger and feel no current, feet and finger at same potential.
Title: Re: Neutral has 0 voltage, so how does current flow?
Post by: Mel769 on 16/01/2018 00:05:47
Thanks iit makes a bit more sense now