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.