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Neutral in electrics?

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Droid
530199.  Tue Mar 31, 2009 3:42 am Reply with quote

Quote:
and the live wire varies between +240v and -240v?


From my memories of A level physics, the live wire actually varies between +(240V x square root) of 2 and -(240V x square root of 2) which is around 339V.

How do you type a square root symbol anyway? I've forgotten.

 
suze
530301.  Tue Mar 31, 2009 9:20 am Reply with quote

Droid wrote:
How do you type a square root symbol anyway? I've forgotten.


With some difficulty, actually. So here's one: .

It's character U+221A, and hence Alt-8730 will work on some computers. But you can't enter it directly from the keyboard.

 
Posital
530394.  Tue Mar 31, 2009 12:28 pm Reply with quote

bobwilson wrote:
Quote:
there are two types of electromagnetism to match the two ones for gravity (mentioned elsewhere (long and short)).

there are? I didn't know that?

No - some idiot mentioned that a while ago... and it's nearly April the 1st...

 
Davini994
530398.  Tue Mar 31, 2009 12:53 pm Reply with quote

Family wrote:
What purpose does the neutral serve in an AC electric circuit?

Aren't we overcomplicating this question a bit chaps?

i.e. isn't it enough to say that we need a circuit for alternating current, so we need both an 'in' and 'out' (live and neutral) for current to flow?

http://en.wikipedia.org/wiki/Alternating_current

 
bobwilson
530488.  Tue Mar 31, 2009 5:41 pm Reply with quote

But that's the problem Dave

Quote:
In alternating current (AC, also ac) the movement (or flow) of electric charge periodically reverses direction. An electric charge would for instance move forward, then backward, then forward, then backward, over and over again. In direct current (DC), the movement (or flow) of electric charge is only in one direction.


If the current is switching direction (periodically, here, I believe means 60 times a second in the UK from memory) then what's the difference between the Live wire and the Neutral wire? When the current is in one direction it goes from Live to Neutral; in the other direction it goes from Neutral to Live.

I know that can't be right but I'm buggered if I can figure out why it isn't.

Is there an electrician in the house?

 
grabagrannie
530548.  Tue Mar 31, 2009 9:34 pm Reply with quote

bobwilson wrote:
Quote:
ooh nitwit - so they are catching up then. In the UK ALL domestic applicances - from 25W lights to televisions to stereos - light use or heavy use - have three pins.

But in many cases, especially the smaller appliances, the earth pin is a dummy, not connected to any wire going to the appliance. The earth pin is there to open the sockets for the live and neutral pins (that's why it's a bit longer than the other two). This is only allowed if the appliance is double-insulated (shown by a symbol of one square inside another).
And your memory is getting to be unreliable, Bob. In the UK (and Europe so far as I know) the mains is at 50 Hz (cycles per second). It is 60 Hz in the USA.
I believe that 3-phase electricity is for industrial use. Domestic electricity is suppled as single-phase.

 
Posital
530554.  Wed Apr 01, 2009 12:06 am Reply with quote

bobwilson wrote:
If the current is switching direction (periodically, here, I believe means 60 times a second in the UK from memory) then what's the difference between the Live wire and the Neutral wire? When the current is in one direction it goes from Live to Neutral; in the other direction it goes from Neutral to Live.

I know that can't be right but I'm buggered if I can figure out why it isn't.

Is there an electrician in the house?

Yes that's right.

Think of ac being a bit like the tides. At high tide, water gets pushed up the thames. At low tide it comes out again.

If you disconnect the two (by raising the thames barrier) then the thames remains calm (ignoring the river flow) but the north sea still goes up and down.

(Bude canal is probably a better example, but less well known)

Is that any help or am I missing the point?

PS: The UK operates at a deadly 50Hz.

 
mckeonj
530577.  Wed Apr 01, 2009 3:37 am Reply with quote

The supplied alternating current has a sine wave form, because the generators are rotating (at 50Hz in UK and Ireland, 60Hz in USA & Canada). This makes for some fearsome maths (math). Would it be any better if the AC had a square waveform? This could be achieved by inserting an integrator in the transformer.

 
exnihilo
530583.  Wed Apr 01, 2009 3:59 am Reply with quote

suze wrote:
Droid wrote:
How do you type a square root symbol anyway? I've forgotten.


With some difficulty, actually. So here's one: .

It's character U+221A, and hence Alt-8730 will work on some computers. But you can't enter it directly from the keyboard.


Another win for the Mac then √ is generated by the simple Option+V keystroke and is entirely available directly from the keyboard.

(I know nothing about electricity, but felt I should contribute.)

 
Southpaw
530592.  Wed Apr 01, 2009 4:35 am Reply with quote

http://en.wikipedia.org/wiki/Domestic_AC_power_plugs_and_sockets

 
Davini994
530777.  Wed Apr 01, 2009 1:20 pm Reply with quote

bobwilson wrote:
But that's the problem Dave

Quote:
In alternating current (AC, also ac) the movement (or flow) of electric charge periodically reverses direction. An electric charge would for instance move forward, then backward, then forward, then backward, over and over again. In direct current (DC), the movement (or flow) of electric charge is only in one direction.


If the current is switching direction (periodically, here, I believe means 60 times a second in the UK from memory) then what's the difference between the Live wire and the Neutral wire? When the current is in one direction it goes from Live to Neutral; in the other direction it goes from Neutral to Live.

I know that can't be right but I'm buggered if I can figure out why it isn't.

Is there an electrician in the house?


The p.d. between the live and the negative is sometimes positive and sometimes negative to cause the flow in two directions.

So the negative can be nowt without it being a contradiction.

mckeonj said this several pages ago and is probably shaking his head in wonder right now.

 
mckeonj
530811.  Wed Apr 01, 2009 3:11 pm Reply with quote

Still baffled by current alternating with respect to neutral?
Then it is time to introduce "Mr Chad", who will explain it.

This image was popular during WW2 in Britain, appearing on walls everywhere and asking pertinent questions, like "WOT, NO BEER?!".
He was adopted by the Americans and renamed Kilroy, after the infamous "KILROY WAS HERE".
The image originated from an illustration of the sine wave, used to teach military personnel working with RADAR the elements of electrical theory.
The addition of + and - signs in the segments of the curve served to indicate the polarity, and also provided the 'cross eyes' of Mr Chad.

It can be seen that the line voltage varies above and below the 'neutral', and is zero briefly at two points in the cycle.
The effective voltage, that delivers the power, is a sort of average voltage over time, ignoring the +ve and -ve signs. This is the so-called Root Mean Square voltage, a neat bit of maths that gives a sensible answer.
Poor Mr Chad rarely got sensible answers to his questions.
(This account is based on things my father told me; during WW2 he was too old for military service, but was directed into war related civilian employment as an electrician with the Fleet Air Arm.)

 
soup
780169.  Wed Jan 26, 2011 6:09 am Reply with quote

Posital thats for three phase. I know of no domestic set up which uses three phase .

I am no expert on AC current (I can 'do' the numbers but I have virtually no understanding).

As I think of it the Live wire carries the push and pull of AC current + to -240 (well 230 10%) volts whilst the neutral wire completes the circuit and allows electron flow.

Switches and fuses should always be on the Live wire.

For extra points explain why it doesn't really matter if the live and neutral wires are reversed in lights/lamps (uncoloured wire).

 
aTao
780352.  Wed Jan 26, 2011 2:05 pm Reply with quote

EEK!

Well, I guess its not that serious, given that the penultimate post is from April 2009 and all the posters sill post. But owch, you guys play with this stuff and write that???

In a domestic UK situation:
Live: 240 Volts RMS (Root Mean Square) 50Hz. Or 240 * √2 * sine (50 * time in seconds).
Neutral: Up to 4 or 5 Volts RMS, depends on load and cabling.
Earth: 0 Volts.

All measured with respect to "ground" which is a copper spike driven into the ground with the earth cable attached (which is why Earth is 0V)

So, Whats the neutral for?

Electricity needs a circuit to flow round, no matter if its going live to neutral, or neutral to live, or swapping around all the time, it has to have some way to flow back to whatever made the voltage in the first place. If there was no circuit then what ever was connected to the voltage source would match that voltage exactly and then there would be no voltage difference along the connecting wires and so no current flowing in them.*

So, whats the difference between live and neutral in an AC circuit? from the power transmission point of view, nothing what so ever. The voltage that is important is the voltage difference between live and neutral and in AC the current alternately flows from one, then the other. You could raise both by any amount and it would have no effect on the power since its only the difference thats important.
However, power transmission is only part of the story, electricity is dangerous and protection is necessary. If there is a fault and a conductor connects to something you might touch, how can this be detected and acted upon? One way is to tie one half of the supply to ground (or very nearly), then, if the other half also connects to ground a low resistance circuit is formed which can draw enough current to blow a fuse, disconnecting the supply to the broken equipment. If both wires were floating then a single fault would merely mean that the whole circuit was then referenced to the fault point and could be tricky to detect.

Notes: Newfangled stuff means that wiring fault detection is now done by measuring and comparing the current flowing in the 2 conductors. If the difference is above (30mA in UK) then theres something adrift and the circuit is disconnected. Fuses or other newfangled over current measuring stuff is only used to detect circuits that are drawing more power than expected or that the supply cabling can safely deliver.
UK building sites use 110V AC with no earth reference (ie floating), the idea being that a single cable fault is not immediately dangerous, but that equipment is maintained such that any damaged gear is not used. If you chop through a live wire, you dont get a jolt, but you DO stop using it.

*In high voltage AC or high frequency AC then the charging of the circuit has a noticeable effect, but 240V 50Hz its almost negligible.

 
Posital
780396.  Wed Jan 26, 2011 5:05 pm Reply with quote

soup wrote:
Posital thats for three phase. I know of no domestic set up which uses three phase.
Electricity is delivered to the substation in three phases - then each of the three phases gets farmed out to different areas - so each area gets a single (albeit different) phase.

I mentioned the three phases above, because it's integral to how neutral is created.

 

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