quetions

[snowplough]

Hi there,
Im new to this site but woould appreciate if any of ou could answer one of many quetions i have about electrics as i am currently doing m y level 2 electrics course.

 

[pushrod]

Hi pushrod, i see you made a common mistake when you visualise electrons whizzing round a circuit but may i correct you and hopefully enlighten you.
Electrons do move/migrate in a d.c. circuit due to a constant attraction but at a snails pace (in the region of mm per second) but in a.c. they tend to just sit there and vibrate with the changing direction what you are mixing it up with is the propagation of the electro-magnectic field which flys round at near light speed.
Check out this link....Speed of electricity - Wikipedia, the free encyclopedia
At 50 hertz it would change direction 100 times a second as it completes a positive and negative cycle in 1 hertz.

Thanks Darkwood but i was writing for the benefit of the OP and keeping it simple. Mind i don't think i said anything about the speed of electron drift and i did say full changes of direction , in case pedants took umbridge. If i had said oscillates with a frequency of 50 Hz as the pd goes from positive to negative and that actually involves 100 changes of direction per second i think the OP might have been a bit lost. Also not confused at all with electro-magnetic fields/radiation after all I am now retired having spent 30 years working in a science field and have post grad quals in physics and electronics (mind have forgotten most of it now ) No offence intended but i am not your average trainee spark.
What you need to help me on is which junction box to use under floorboards and interpreting some of the wiring regs
Cheers.

 

[Darkwood]

Thanks Darkwood but i was writing for the benefit of the OP and keeping it simple. Mind i don't think i said anything about the speed of electron drift and i did say full changes of direction , in case pedants took umbridge. If i had said oscillates with a frequency of 50 Hz as the pd goes from positive to negative and that actually involves 100 changes of direction per second i think the OP might have been a bit lost. Also not confused at all with electro-magnetic fields/radiation after all I am now retired having spent 30 years working in a science field and have post grad quals in physics and electronics (mind have forgotten most of it now ) No offence intended but i am not your average trainee spark.
What you need to help me on is which junction box to use under floorboards and interpreting some of the wiring regs
Cheers.

Hi pushrod , now ive seen your true colours as to say il will treat you with a little more grace , and respect, understand the simplification but it just mislead me and no offence taken as the content of my post probably undermined yourself but not intended will tread lightly around you from now on

 

[snowplough]

Hi PUSHROD AND DARKWOOD ,
iM SURE OU BOTH NO WHAT OUR ON ABOUT ,IM JUST THE ROOKIE, I HAVE ACTUALL LEARNT A LITTLE BIT NOW ABOUT THIS POTENTIAL DIFFERNCE AS IVE BEEN RACKING MY BRAIN ABOUT WHAT THE NEUTRAL WIRE IS FOR ETC, IM COMING TO THE CONCLUSION THAT A VOLT IS A DIFF BETWEEN TO POINTS ONE BEING HIGHER POTENTIAL THEN THE OTHER, A WORK AS BEEN DONE ON THE COULOMB BY MOVING IT FROM A LOW POTENTIAL TO A HIGH THEN OCE IT CAN GET BACK TO ITS SOURCE (THE POSIVE COULOMB ) IT CAN NEUTRALISE THE ATOMS SO THAT NO POTENTIAL EXITS, BUT BECAUSE THIS IS HAPPENING CONTINOUS ,LIKE IN A BATTERY THEN IT JUST KEEPS HAPPENING UNTILTHE PUMPING MECHANISM OF THE BATTERY STOPS. I ALO TAKE IT THAT ONCE THE ENEGISED COULOMBS GO THROUGH THE LOAD OR BULB THE LOSE THERE ENERG THROUGH FRICTION HENCE WHEN THE GET TO THE NEGATIVE CHARGE THEY HAVE LOST ALL THERE ENERGY AND SO THEY NEED TO START ALL OVER AGAIN , HENCE THE NEUTRA WIRE CONTAINS THE SAME FLOW OF ELECTRONS AS THE LINE WIRE BUT THEY HAVE NO ENERGY LEFT HAVIN GIVEN IT UP TO THE LOAD . IVE STILL LOADS OF OTHER QUESTIONS ONE BEING IN AC CIRCUITS DOES THE CURRENT FLOW THROUGH THE NEUTRAL WIRE FROM THE STAR POINT AND THROUGH THE LINE WIRE SO ITS BACK AND FORTH . OR DOES IT JUST ALTERNATE BETWEEN THE THREE PHASES , IE L1, L2, L3.

Sorry for shouting ,

Many thanks for all your help

Snowplough

 

[pushrod]

In an AC , 3 phase , system equipment can be connected in star or delta. Delta is usually for motor windings - load is balanced and will be no return current because supplies are 120degrees out of phase. A star system allows unbalanced loads in which case there will be a current in the neutral.

 

[snowplough]

Hi Pushrod,

Many thanks for your patience,

So i was so asking does the current alternate between the line and neutral ,ie through the neutral and up through the line, and then down the line and then through the neutral so it is alternatig , i hope im clear as i am still confused.

Many Thanks

Snow Plough

 

[pushrod]

From your reply i'm not sure if you have quite got the idea of alternating - you might have, i'm just not sure. With ac the current is there all the time, it's just that the electrons are oscillating rather than moving in a steady direction. You can still think of single phase AC largely as you would of DC.

 

[snowplough]

Cheers Pushrod,

Yes i probably assumed wrong then, i thought it alternates back and forth through the transformer which causes it to induce a voltage into the secondary transformer.

i need to do some googling i think to see if i can get my head around exactly whats happening with A.C .

Many Thanks

Snow plough

 

[Darkwood]

Hi Pushrod,

Many thanks for your patience,

So i was so asking does the current alternate between the line and neutral ,ie through the neutral and up through the line, and then down the line and then through the neutral so it is alternatig , i hope im clear as i am still confused.

Many Thanks

Snow Plough

Mains current a.c. goes backwards and forwards in a cycle (clockwise and anti-clockwise) at 50 hertz (50 times a second)
The cycling is achieved by varying the voltage on the live wire from about 325volts to -325volts and back again, the effective value of the voltage which is also known as RMS is about 230volts
So answer to your question is YES
When you stick you voltage meter on the mains then it is taking an average reading or if you splash out out on an expensive meter it may say 'True RMS' which is extremely accurate thats why you dont see 325volts and because it goes from 325 volts to -325 volts at 2 points in this cycle the voltage is effectively zero so the wire has no potential and no current flows but of cours this happens so quick its never save to touch.

 

[snowplough]

Many thanks Darkwood,

I understand that A.c Alternates 100 times per sec, (50 cycles per sec) i think.

Now in a battery say the current goes from the posive terminal through the live wire ,and then to the negative terminal via the neutrel wire with the load inbetween. so if you put a switch on the live side ie before the bulb then everything after the switch would remain dead even if you touched it , and everything before the switch would remain live, that is with current onl flowing one way.
So what i cant get my head around is if the current was to flow in both directions in a baterry say like in a domestic circuit even tho it was travelling at 50hz say , what difference would it make which wire you put the switch on as it is coming from both directions thus making the the current flow to either side of the switch if you no what i mean.

i know im missing something really simple as i know your gunna tell me, as they say its easy when you no, i have still loads more questions .

Thanks for your patience

Snowplough

 

[pushrod]

Can i ask have you been looking at the diagrams of simple alternators? If so this could be your problem because domestic electricity is not supplied like that. The electricity in your house is one phase from a 3 phase supply in the street that carries the electric field that causes the electrons to oscillate. The neutral ends up as an earth . Don't think of the neutral running back to the alternator to complete the circuit - it doesn't. It is much easier to think of AC behaving just like DC in terms of wiring things up.

In all the exams i did last year on the level 2 of the c&g 2330 there was not a single question requiring this sort of thought, so don't worry if you are having problems getting your head around it.

 

[Darkwood]

The supply arrives at you local substation after coming off the regional grid through a series of transformers, the final transformer will be a star wound transformer and up to this point their is no neutral, due to the nature of the design a star wound transformer has a center common point to all three phases after they have gone through the transformer coils, this point is used to create or tap off BOTH earth and neutral so effectively earth is neutral and vise versa, these can be in the same wire or seperate hence the different supply styles tns and tncs, at your house cutout they are seperated into earth and neutral if tncs or just connected as normal if tns.
So because neutral is effectively earth you are under normal conditions able to touch it without creating a circuit through you body, but unless by special design the earth and neutral must remain seperate in the installation as its very dangerous if you dont.

 

[snowplough]

Cheers Push rod,

Yes i have been looking at a simple alternater, that i why i am assuming the house line and neutral is the same.

I will just keep looking on net and asking questions as i am struggling to get my head round 3 phase ac from its creation at the alternator to how it ends up a singe phase in the home , like i said before i do night college and the lecturer tends to fly through stuff and i then end up back tracking trying to understand stuff wich leaves me back tracking.

Poor excuse i no but at ome point ,fingers crossed it all might make sense.

Many thank for you taking time to answer my quetions,

Snowplough

 

[pushrod]

i am struggling to get my head round 3 phase ac from its creation at the alternator to how it ends up a singe phase in the home ,

That's what Darkwood is explaining in the previous post. As he says the last transformer has 3 phases and a neutral. (star) and each house in a street will take off from one of the phases and a link to the neutral.

Have you found the Swindon massive site - halfway down the page are free to use revision questions with answers HERE - very useful

 

[snowplough]

Thanks Darkwood and Pushrod,

I have alwas taken it that because the earth rod at the star point is connected to the centre point at the star that,that is the return point back at the source so its at 0 potential, so any wire connected to it (neutral) will also be at 0 potential ,so by touching it like you said ,you cannot get a shock because the circuit is already complete ,i think?.

It was also the other bit i mentioned regarding A.C, and you did tell me that it does go down the neutral, which must be at the centre star point , and up through the line, back out of the house to which ever of the 3 phases its connected to, and then back to the star point, where there it must repeat the same route but this time through one the 3 phases to start with then back through the line in the house ,then out through the neutral back to the star point , I think?.

If i am right , i dont understand how having a switch on the line would make any difference, as if the current is going back and forth then to me you coud just as well have it on the neutral. As i have said before ,there is more to it then what i am thinking so at some point it will hopefull click, until then continous thanks to you both for your patience and stand by for tonnes more questions.

Regards

Snow plough

 

[pushrod]

Don't think of it as a push from the phase and then a push from the neutral.

It is a push from the phase and then a pull from the phase. Interrupting the phase with a switch will then turn everything off beyond that point.

Remember the simple alternator diagrams do not apply to domestic electricity.

(edit - I'm feeling confident that you'll be more happy this time)

Also read through this thread again looking at your questions and the answers you have been given to see if things make more sense. (post no 11 was to do with this).