AC theory

[La Poste]

Greetings.

I have been thinking and reading a little about AC theory lately and will tell you what I think I know so far.
Please feel free to shoot me down in flames if I am wrong.

AC current does not actually move anywhere, well it does move but it moves backwards and forwards along the cable 50 times a second so in effect it doesn't actually travel along the cable it just moves back and forth a few centimetres.

The reason for this is because the RMS voltage goes 230V in a positive half cycle and 230 Volts in the negative half cycle with respect to 0 Volts thus the EMF (Volts) draws the current first one way and then the other.

The reason we only get a shock off the positive half cycle is because the neutral is grounded at various points and so the negative half cycle is kept as close to 0 volts as possible.

Now this might be wrong but it is how I visualise it at the moment.

If anyone has any comments to make about what is really going on inside a cable in terms of electron flow then please feel free to add, it would give me something to think about at work.

Thanks.

 

[i=p/u]

hysterisis, if you understand this youl get ac theory in a instant

 

[i=p/u]

What is alternating current (AC)? : BASIC AC THEORY

 

[i=p/u]

we use it because ? is your first question, it alternates and giving of a back emf which we use, the screw rule etc ..loads loads loads about it and i dont know where to start

 

[La Poste]

I understand about transformers, back EMF, etc but I was just trying to imagine electricity like the flow of water, what is actually happening in the cable in a domestic environment like a house.

If we have plus 230 Volts in the positive half cycle then if we were to touch a live cable and earth we would get a shock.

But in AC theory we also have negative -230 Volts RMS, if we were to touch this negative 230 Volts and earth would we also get a shock if the negative cable were not attached to the ground at various points along its length?
If the neutral were not earthed and it was at it's most negative point IE -230 Volts it would be just as dangerous as the live the only difference would be the electron flow would be from ground through us into the neutral cable instead of from live through us into the ground as is the case if we were to touch the positive live cable.

Does that make sense?

I'm just trying to get my head round the actual flow of electrons within an AC cable.
I mean the electrons just move back and forth at 50 times a second right, they don't actually go anywhere.

Maybe I should just go to bed. :wacko:

 

[micknew]

You are dead right regarding a shock between live and earth when the neutral is earthed at source.
Excuse the pun.

 

[Silly Sausage]

I understand about transformers, back EMF, etc but I was just trying to imagine electricity like the flow of water, what is actually happening in the cable in a domestic environment like a house.

If we have plus 230 Volts in the positive half cycle then if we were to touch a live cable and earth we would get a shock.

But in AC theory we also have negative -230 Volts RMS, if we were to touch this negative 230 Volts and earth would we also get a shock if the negative cable were not attached to the ground at various points along its length?
If the neutral were not earthed and it was at it's most negative point IE -230 Volts it would be just as dangerous as the live the only difference would be the electron flow would be from ground through us into the neutral cable instead of from live through us into the ground as is the case if we were to touch the positive live cable.

Does that make sense?

I'm just trying to get my head round the actual flow of electrons within an AC cable.

maybe I should just go to bed.

Hard to explain (for me anyway)

Forget the +/-, think potential difference.
For AC, there is a pd of 230rms to earth, that is what drives the current through whatever is connected between the source & earth/neutral (generally assume neutral is at the same potential as earth, that's why you don't get get a shock from touching neutral)

Remember that the neutral is on the low potential side (earth) of a load, so no shock hazard, see above.



The electrons themselves just oscillate within the conductors @ 50Hz, they go nowhere.


Hard thing to explain is electricity. Lecturer at uni asked us all once 'Does anyone know what electricity is?'

The answer is.............No one knows!

 

[La Poste]

The thing is if the neutral is held at 0 Volts by connecting it to the earth then there would be no potential difference or EMF between neutral and earth and so in the negative half cycle the electrons would not flow in an opposite direction along the neutral cable, there is simply no force to drive them.

So in effect with AC what we get is a current that moves slowly forward in one direction in the positive half cycle but does not move at all in the negative half cycle.

By tying the neutral to earth we are turning AC into a kind of DC that moves in one direction in the cable in little jumps at 50 times per second.

There is no reverse direction because there is no reverse PD, it has been eliminated by grounding the neutral.

Is that any good?

Actually edit that, if you were to touch a live cable single phase then the flow of electricity in your body would first be one way in the positive half cycle and then another way in the negative half cycle.

An AC electric shock would move electrons through your body in alternating directions at 50 times per second.

The thing I can't understand is if there is no PD in the neutral cable because it is grounded how can any electricity flow through it, the electrons would just remain stagnant and not move.

 

[micknew]

In a way you are right, you get a shock 50 times per second which equates to half of the sine wave.
There is still a movement of electrons in both directions.

 

[Andy 7671]

I like a deep thinker me.

I can remember asking my tutors the difference between electron drift and conventional current theory. Never did get a proper answer.

Sorry, but dont believe you cant get a belt off a neutral, check out what happens in a neutral of an unbalanced 3 phase load. Neutrals can be borrowed as well, Im sure Archy was just talking theoretical, just didnt want you to get a shock.

 

[Silly Sausage]

The thing is if the neutral is held at 0 Volts by connecting it to the earth then there would be no potential difference or EMF between neutral and earth and so in the negative half cycle the electrons would not flow in an opposite direction along the neutral cable, there is simply no force to drive them.

So in effect with AC what we get is a current that moves slowly forward in one direction in the positive half cycle but does not move at all in the negative half cycle.

By tying the neutral to earth we are turning AC into a kind of DC that moves in one direction in the cable in little jumps at 50 times per second.

There is no reverse direction because there is no reverse PD, it has been eliminated by grounding the neutral.

Is that any good?

Actually edit that, if you were to touch a live cable single phase then the flow of electricity in your body would first be one way in the positive half cycle and then another way in the negative half cycle.

An AC electric shock would move electrons through your body in alternating directions at 50 times per second.

The thing I can't understand is if there is no PD in the neutral cable because it is grounded how can any electricity flow through it, the electrons would just remain stagnant and not move.

I'm the world's worst teacher, so bare with me!

I think I see where you're struggling (in red)

As you know(!), current will only flow through a complete circuit (loop).
Think of the supply as the energy source which drives the current through the load.
If all the resistance is in that load, then all the energy in the system is dissapated in that load, the current still has to flow back to source, otherwise it would be open circuit. You won't experience a shock from the neutral (or earth or the potential you are at) because you are at the same potential, therefore no current will be driven through you.

You could have a 230V system where its earth/neutral potential is at, say , 100V with respect to you. If you touched that neutral/earth you'd see 100V and get a 'mild' shock!

 

[micknew]

You cannot not get a shock between N & E unless its connection at source is broken.
If its connection is broken the live will try to return to earth through you (if touched), giving you a shock.

 

[La Poste]

Thanks for your answers, really appreciated.

I guess what I am really stumped at is how can the current flow in the neutral if it is at zero volts.

There is simply no EMF to drive it along, this is where I am stuck.

If you touch the neutral at your light switch in your house it is at 0 Volts and if you touch the neutral at the transformer end it is also at 0 Volts so how does current move along it?

There must be some volt drop but just not enough to notice.

Time for a beer me thinks.

 

[micknew]

Best study how a generator generates electricity.
Don't worry you will understand it all but it takes time to sink in.
We have all been where you are in our learning at the moment.

 

[Chr!s]

Thanks for your answers, really appreciated.

I guess what I am really stumped at is how can the current flow in the neutral if it is at zero volts.

There is simply no EMF to drive it along, this is where I am stuck.

If you touch the neutral at your light switch in your house it is at 0 Volts and if you touch the neutral at the transformer end it is also at 0 Volts so how does current move along it?

There must be some volt drop but just not enough to notice.

Time for a beer me thinks.

The neutral will never be at 0 volts