AC current flow

[Monkeyblaine]

Im having a little trouble understanding AC current flow... My understanding is that the electrons jiggle backwards and forwards due to the waveform of AC, so I don't understand why there is a need for a neutral as the electrons don't need a return path as they don't actually move anywhere unlike in a DC circuit where they move round and round... If someone could correct my understanding of this I'd muchly appreciate it.

 

[needasparks]

You are correct in the way that AC flows both ways but remember we only use the movement of electricity.
If you think about it all electrons want to do is return to its source so if it flows backwards and forwards it does not go anywhere until we give it a return path (neutral). Abit like a boat with one ore, with a second ore you will travel in one direction instead of going round and round. bit more to it with three phase but best to master single phase first.

edit
Just read your post again, you should understand current flows in a different direction to electrons.
You need to separate the two as current is the product of flow.

 

[Knobhead]

It doesn’t matter which way the current it flowing, it’s got to have some where to go.

View attachment 11447

 

[Richard Burns]

Have a go at reading this thread http://www.electriciansforums.net/e...s-electrical-regulations/50378-ac-theory.html
it is not all correct but is asking the same questions.

 

[Darkwood]

This is so my field ... been a geek that i am but rather than write it all il link you to Wiki speed of electricity and take a good read on the electric drift... and going against everyones thinking there is little or no directional movement of the electrons but propogation of the electron field is what does the business, if you struggle to understand it just imagine a table full of ball bearings (each bearing been an electron) and you pushed a new ball bearing in at one end then because the table is full it will end up with a ball dropping off the other side, each ball has moved very little but the effect has jumped a great distance in a very small time.
Speed of electricity - Wikipedia, the free encyclopedia

 

[Monkeyblaine]

Ok i think I'm slowly getting it. But like I said before why is there a neutral? Do all the electrons shake in both conductors and without a neutral at a different potential they wouldn't be able to do this. The way I imagine a circuit working is once the switch on say a lighting circuit has been turned on the electrons travel up the line conductor through the load, down the neutral back to the star point of the supply transformer and then repeatedly throught that process. But if they just shake about I don't get it. Please explain in easy terms.

 

[Darkwood]

your struggling to get it because you assume the electrons do the business end.... your confusion comes from the fact that you've crossed over from the normal teaching methods about electrons whizzing around a circuit and used some references from quantum physics regarding the fact the electrons dont really move... this is a very deep field and takes alot of learning and understanding and if your to try understand it you have to let go and forget you teachings about electricity.
Its not something that can be explained in everyday terms as its a quantum world and nothing like the everyday things we see or understand, but just remember its the propogation of the electric field thats moving close to the speed of light in a active circuit not the electrons, you need a neutral for say a lightbulb because you need a repelling force and a attracting force for a circuit to exist.

If you want to understand this then abandon what school and college taught you about electricity and start reading into the world of quantum theory but dont mix the two together as its hard enough to get your head around as it is. The link i gave explains it in simple terms but because of its sheer complexity its still a hard subject to grasp.

 

[Monkeyblaine]

So in laymans terms there needs to be different conductors at different potentials for the electrons and electric field to do their business in AC and I should just accept it. As apposed to DC where the move from negative to positive like in the same way as AC they need different forces to attract them but they just move differently.

 

[Darkwood]

the propogation of the electro-magnetic fields flows in one direction in DC (negative to positive)..AC users the same principles but reverses direction 100times a second in our 50hz supply, forget all about electrons moving about it just makes it confusing but yes any circuit needs the basics - a attractive force and a repulsive force so removing the neutral in your example you remove either the repulsive or attractive end depending on which part of the cycle the AC waveform is in.

 

[Monkeyblaine]

Ok I actually understand that! Thank you very much for your help. Im happy to understand that and accept the rest. Thanks again.

 

[Monkeyblaine]

Just read your post again, you should understand current flows in a different direction to electrons.
You need to separate the two as current is the product of flow.

Just read that again. I was under the impression that current was the flow of elections per second in a conductor but here your saying it's two
Separate things. Can someone explain this please?

 

[Darkwood]

Again i think his post is misguiding and he might be refering to the standard teachings that current flow from positive to negetive but in reality its the other way around as ive already explained, and in standard teaching an amp is a measurement of charge passing a point in a circuit for 1 second, 6.241 x 10^18 electrons or 1 coulomb per second = 1 ampere. Again this is still taught at colleges and uni's even though its incorrect but its a simple practicle way to learn if you can associate electrons moving as carrying charge, if we were taught the reality of what happens you would have confusion as its quite hard to convey what is taking place in everyday terms.

So as i suggested before either stick with your basic electrons moving teachings and keep it simple or try to erase all you think you thought you knew and start again in the quantum theory of the speed of electricity. You wont lose out following your college teachings as the maths still works its just the fundamental working that are totally different.

 

[ezzzekiel]

Amazing all this time i thought it was all done by elfs 

 

[telectrix]

you mean it isn't???

 

[ezzzekiel]

Ooops better not mention santa 

 

[Marvo]

Firstly I would forget about the difference between AC and DC. If you look at a very small portion of an AC waveform it looks the same as DC.

Current doesn't flow in an AC circuit without a neutral for the same reason it doesn't flow in a DC circuit if you disconnect one side of the battery.

We only get into Elf/Santa theory in very advanced electrical study levels.

 

[Monkeyblaine]

Ok so now I understand that on a positive half cycle the current will be flowing through the line conductor, through the load and through the neutral and then in 1/100th of a second or the negative half cycle of the wave it will flow in the opposite direction from the neutral through the load and finally through the line... and visa versa every 1/100th of a second... am I getting it yet???

 

[Darkwood]

Ok so now I understand that on a positive half cycle the current will be flowing through the line conductor, through the load and through the neutral and then in 1/100th of a second or the negative half cycle of the wave it will flow in the opposite direction from the neutral through the load and finally through the line... and visa versa every 1/100th of a second... am I getting it yet???

If your supply is 50hz then in your example then yes.

 

[Darkwood]

Do remember though; without me trying to confuse you more, that neutral as we call it isn't always needed to have a circuit, you can have 2phase and 3phase circuits AC that use the potential difference between the phases and dont always need a neutral (contactor coils, transformers and 3ph motors), all you actually need is a constant potential difference between 2 wires and a load thats designed for the voltage present.

 

[Monkeyblaine]

Would I be also right in saying that the reason we don't have protective devices on our neutrals then is because it alternates so quickly that in the case of a short circuit it only needs to protect Half the cycle.

And if there is a fault to earth will the fault current alternate between travelling one way through the earth then back from the earth in the opposite direction 50 times a second until the protective device operates.

 

[Darkwood]

Would I be also right in saying that the reason we don't have protective devices on our neutrals then is because it alternates so quickly that in the case of a short circuit it only needs to protect Half the cycle.

And if there is a fault to earth will the fault current alternate between travelling one way through the earth then back from the earth in the opposite direction 50 times a second until the protective device operates.

No!! its for safety reasons, because the neutral is derived as is the earth from the centre tapping of the sub'-transformer it has the same potential and up until it is devided it effectively the same thing, so in normal conditions touching the neutral wont give you a shock, if you had a fuse break the neutral but leave live connected then that neutral would be 230v potential to earth, but show no volts to live as its floating at the same voltage. Also to note is if anything which had a broken neutral had a N/E fault then it would use earth as a return path leaving many hazards and can go undetected. If like we do you fuse the live then when operated it should 'if all wiring is ok' remove all potentials between conductors and to earth on isolated circuit making it safe to work on.

 

[Monkeyblaine]

Ok I get that. So the 230v would continue up the line through the load, down the neutral until the break thus making that particular neutral at 230v to earth and if that neutral was ever to come in contact with an earth it would use that as it's return and may make other things live and very dangerous.

 

[Guest111]

Would I be also right in saying that the reason we don't have protective devices on our neutrals then is because it alternates so quickly that in the case of a short circuit it only needs to protect Half the cycle.

And if there is a fault to earth will the fault current alternate between travelling one way through the earth then back from the earth in the opposite direction 50 times a second until the protective device operates.

no years ago we had fuses in both live and neutral but if the fuse in the neutral blew and the one in the live didnt the faulty equipment was still live and very dangerous

 

[Monkeyblaine]

On the negative half cycle when the current is flowing in the opposite direction I.e through the neutral. There is no voltage going through the neutral is there. It's just the current that alternates. Not the voltage??

 

[Marvo]

The 'voltage' is what pushes and pulls the current around the circuit. The AC voltage alternates between a positive value and a negative one which causes current to flow in alternate directions. You don't get current flow unless there's a voltage present.
Check out these books linked in this thread, there's one of them covers basic AC theory and I'm sure it will explain the principals better than I can.

 

[Darkwood]

yes as Marvo says you need to get a grasp of the basics first, remember the Neutral is derived from the centre tap of a 3ph transformer and so is the earth, this is your confusion you assume because it shows 0volts to earth it dosn't have a voltage but Voltage is relative so relative to live it is 230v.
If you have some standard probe testers their is usually 2 neons which detect whether its an AC supply or DC supply one neon lit will detect DC as it only see's the voltage in one half of the cycle if both light then its AC as its sees voltage in both positive and negative cycles the neons are just basic with each having a diode in reverse to the other so only been capable of lighting when the waveform matches its diode direction.

 

[Engineer54]

so is the earth

Not quite right, but i know what you mean...lol!!

 

[Monkeyblaine]

AC confusion

On the negative half cycle when the current is flowing in the opposite direction I.e through the neutral. There is no voltage going through the neutral is there. It's just the current that alternates. Not the voltage??

 

[telectrix]

Re: AC confusion

the voltage alternates between + and - relative to 0. 0 being the neutral, so there is no voltage in the neutral.

 

[Knobhead]

Re: AC confusion

The voltage and current alternate above and below 0V.
0V being referenced to earth AKA neutral.

 

[Knobhead]

Re: AC confusion

This may help
View attachment 11498

 

[Richard Burns]

Re: AC confusion

Voltage is a potential difference between two points; for ac, between live and neutral. It does not flow it is a value.
Current is the amount of charge transferred through the cable and is "pushed" by the voltage as a sort of pressure.
For the AC waveform that tony shows above the Line conductor (Live) has a voltage sine wave varying at 50Hz.
The neutral conductor is at a nominal zero volts with respect to the line conductor at all times.

The potential difference between the line conductor and the neutral conductor has a root mean squared value of 230V (nominally). This means that the peak voltage is +/-325V.

So you can say that the voltage of the live (with respect to the neutral) starts at zero rises to +325 volts drops to zero then drops down to -325V and back up to zero, then starts again.

Whilst the voltage on the live is positive wrt the neutral current flows one way and when the voltage on the live is negative with respect to the neutral it flows the other way.

 

[Marvo]

One thread deleted and two similar threads merged.

Please just stick with the one thread to discuss one topic Monkeyblaine. Thanks.

 

[Monkeyblaine]

Ac help please help!!!!!!

Im confused... I understand that on the positive half cycle the 230V goes through the line conductor, through the load and then through the neutral and on the negative half cycle it does the opposite but im confused what pushes the 230v through the neutral when its on then negative half cycle especially when the neutral stays at 0V the whole time... Can someone please help me understand AC!!!!!!!!!!

 

[Knobhead]

Re: Ac help please help!!!!!!

For goodness sake have a look at this link

Google

 

[Silly Sausage]

Re: Ac help please help!!!!!!

I'll give you an explanation about how electricity works but you'll have do some reading up first on Thermodynamics and Entropy!

 

[Knobhead]

Re: Ac help please help!!!!!!

I was thinking more along the lines of atrophy.

 

[Adam W]

Re: Ac help please help!!!!!!

Wasn't there another thread on this same subject earlier?

 

[Knobhead]

Re: Ac help please help!!!!!!

Echo.........
Echo......
Echo...

Worded diferent, but the same.

 

[Darkwood]

Re: Ac help please help!!!!!!

Monkey pls stop starting new threads, Marvo has spent his time and patience merging your last 2 and you start yet another on the same theme, we've all tried our best to simplify it for you and if you still struggle id just accept what we say and put you efforts into learning the trade and stop trying to confuse yourself, its great you feel the need to understand the underlying theory of AC but just give yourself more time and like most things the pieces do slot together and you have that personel VOILA moment.
Dont in anyway take this as an attack on your intellect its just sometimes takes longer to put the pieces together because of the complexity of the fundemental workings of electricity, i remember i struggled for years on certain aspects but it does just click one day .... just be patient!

 

[Knobhead]

Marvo, have a look in theories and electrical wiring.

Sorry I lost my rag with him.

 

[Darkwood]

One thread deleted and two similar threads merged.

Please just stick with the one thread to discuss one topic Monkeyblaine. Thanks.

Not sure he read this as i hate to inform you lol..... how your ability at merging 3 threads :dizzy2:

 

[Darkwood]

Re: Ac help please help!!!!!!

PS please refer to Richard Burns post in your previous thread hes done a good job at explaining it, i dont think you can really put it any more simpler.

 

[Knobhead]

This has got to be a wind up.

One of the better ones.

Think I’ll go and read Archie’s thread about new members again.

 

[Richard Burns]

Last help.

Voltage does not flow. Voltage is a pressure that causes current to flow.

Current flows. The current that flows is determined by the resistance (inductance) of the load and the applied voltage.

For a Current to flow there must be a circuit (for a.c.) from Line to Neutral and there must be a voltage between the two.
The voltage on the Line varies from +325V to -325V.
The voltage on the Neutral stays at zero.

When the voltage on the line is zero then no current flows as there is no voltage between line and Neutral
As the Line voltage rises up to +325V the current flow rises in proportion to the voltage (assuming a fixed load).
Conventional current flow is from Line to neutral.
The voltage on the line then falls to zero as does the current flow.
The voltage on the line then drops to -325V the current flow increases proportionally but going from Neutral to Line.

This diagram may help, showing a load of 230 ohms between Line to Neutral and demonstrating the current flow at points along one and a half cycles (0.03 seconds)
View attachment 11506


If you cannot get this then read the links you have been given.

 

[Darkwood]

your attatchment is invalid Rich and i dont mean that personally ... it asks you to contact the admin'

 

[Richard Burns]

your attatchment is invalid Rich and i dont mean that personally ... it asks you to contact the admin'

Sorry about that, I can view it, maybe it was too big (249kB)?
Oh well I tried!

 

[Monkeyblaine]

Neutral push!

Would I be right in saying that during the negative half cycle and the line conductor is at -230v RMS this is the attracting force which alternates the voltage and in a way sort of sucks the current the opposite direction (for argument sake) back through the neural and keeps alternating every 100th of a second, in a 50hz supply??

 

[Des 56]

Re: Neutral push!

Maybe,instead of thinking of a sucking force.try ignoring the actual neutral conductor

Try thinking about the star point of the transformer where the neutral originates,then imagine your single phase supply being connected directly onto the windings at that transformer,then picture the current and voltage as in the sine wave of the alternating cycles,at that transformer,it may help you this way

 

[Monkeyblaine]

Re: Neutral push!

Ok I understand that. But am still right in saying during the negative half cycle this is when the current changes direction and heads back up the neutral though the load then through the line. And it's because of this negative half cycle (-230v) that's the reason it is pushed back the other way.