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> Beginnner Q About The Flow Of Electrons
Mazzy
Posted: February 23, 2012 08:40 pm
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I have started learning about fundamental electrical theory and components this week, and basically I've come here with a question that I've not been able to find an answer to online, or in books.

If electrons are flowing from negative to positive, why do I feel a jolt from 120VAC line but not neutral?

Thanks!
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johansen
Posted: February 23, 2012 08:43 pm
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because you are connected to "neutral" via the ground bond at the breaker box.


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tekwiz
Posted: February 23, 2012 09:19 pm
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Yes, the neutral wires in your mains wiring are connected to ground in the breaker panel. Take the cover off your panel & you can see that none of the white wire connection blocks are insulated from the grounded panel.
There are a number of reasons for this apparent ground wiring duality, the biggest of which is safety.


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Jimthecopierwrench
Posted: February 23, 2012 09:33 pm
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The first part of the question has nothing to do with the actual answer.

A live 120V line will bite you under the 'right' conditions - such as when you're standing barefoot in the damp basement - and may not under others - such as touching the live conductor with one finger only while standing on a clean dry milk crate. But the reasons for these interactions have nothing to do with the sign - or direction of actual current flow.

Likewise, the neutral conductor won't (or shouldn't) because it's at the same local potential as the earth ground - that is it isn't live, and no other reason.

Now the other bit - about the current 'direction' - is a different discussion.

It matters as much as it's useful for description, identification, etc. Matters of course when you start talking about current manipulation through devices like diodes and transistors, if for no other reason than getting it 'backwards' means the device won't work. Even this in practice though is often reduced to arbitrary symbology though - like the arrows, and plus and minus symbols on schematic diagrams. Not also unlike boosting a dead car battery - a hundred million people simply connect 'red to red' with little or no thought as to which actual real or imagined net direction unssen 'things' might be migrating through conductors, or if's better yet to imagine the Newtyon's cradle model.

The signs are for frame of reference.

Take a 6 volt battery. The terminals have a 6V potential difference. One could be 0, the other +6, but it's a no less true to call the positive 0 and the negative -6. Even if you reference it to the earth, you could jam the negative side into the wet sandbox and say you have 'ground' and 6 Volts - but just as well turn it upside down and have ground and -6 Volts. See how that works?

Better still ... (or worse): The 120V line you were originally talking about, being AC has zero net flow in any one direction, but that's another story.


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Mazzy
Posted: February 23, 2012 09:44 pm
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Yes, I've been reading a bit about conventional vs electron current, and it makes sense to me with DC. With AC, it's a different matter possibly because I've not read as much about it (the basic articles and books I've read seem to be rooted in DC). So, if the line voltage was ground and the neutral wasn't, I could be shocked with the neutral line instead of line?

Thanks for the help.
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tekwiz
Posted: February 23, 2012 09:58 pm
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QUOTE (Mazzy @ February 23, 2012 12:44 pm)
Yes, I've been reading a bit about conventional vs electron current, and it makes sense to me with DC. With AC, it's a different matter possibly because I've not read as much about it (the basic articles and books I've read seem to be rooted in DC). So, if the line voltage was ground and the neutral wasn't, I could be shocked with the neutral line instead of line?

Thanks for the help.

Only if the neutral is no longer connected to ground, but you are. It's the DIFFERENCE in potentials that results in the shock.
AC is a bit difficult to comprehend on an intellectual level. It's often best just to think of it as 'electricity' until you must consider the AC properties. This isn't necessary for house wiring, but IS necessary for understanding electronics.
Things are often easier to consider if you don't consider them any deeper than is necessary for the task at hand.


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Trouble rather the tiger in his lair, than the sage among his books.
For to you, kings & armies are things mighty & enduring.
To him, mere toys of the moment, to be overturned at the flick of a finger.

Fortuna favet fortibus.
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Jimthecopierwrench
Posted: February 23, 2012 10:52 pm
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QUOTE
Only if the neutral is no longer connected to ground
One interesting example - and a quite realistic scenario - is removing a neutral line from a live panel.

You can safely touch the neutral bus, or any individual wire connected to it, but remove a neutral wire and it may suddenly become hot.

How? Because it may be connected to the hot wire through an appliance (plugged into the circuit which you've just 'lifted') or a (switched on) lighting circuit.

QUOTE
This isn't necessary for house wiring
Agreed. Best to look at it as simply 'supply' and 'return'.

Even the properties of AC that do need to be grasped for specialized electronics work do not require one to dwell upon the actual direction (magnetism excepted) of current in some physical sense. Magnitudes and vectors being abstract.


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themaster
Posted: February 23, 2012 10:57 pm
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Look at this image: power distribution

You'll see that one side of the power station's generator's coils are earthed, and that neutral also connects straight to earth. This image shows the 3 available phases (lives or "hots" as I think Americans call them). Most domestic dwellings have just one phase available.

You will only get a shock when you touch a phase conductor if your body completes the circuit by standing on a stone floor, or holding an earthed metal pipe, etc., because you are making a circuit by allowing the electricity to travel from live, through you, into the ground and back to the power station's generator since the generator's windings are earthed at one end. If you stand on an insulator like a wooden chair, then the electrons have nowhere to go, so do not flow through you.

Likewise you would get a shock if holding a phase conductor and a neutral conductor, because electrons can flow through you from phase to neutral and vice versa.

If you stand on a stone floor or hold an earthed metal pipe, and then grab the neutral, you will receive no shock because although electrons are flowing in the neutral, neutral and earth are connected at the power station and substations, and thus can be considered the same cable split into two - the electrons cannot flow through you because there is no potential difference. It'd be like holding both ends of a piece of wire and wondering why you aren't getting shocked from it.

The reason a separate earth is used (in TN-S and TN-CS systems - see wikipedia) instead of using neutral as both a neutral and an earth (TN-C), since they're connected together at the substation, is this:

Imagine a kettle with electricity flowing live to neutral (and vice versa) through the element, heating your water. Imagine the neutral is also used to 'earth' the metal outside of the kettle. Imagine the neutral in the cable to the kettle breaks. You now have the phase - live - flowing through the element, into the neutral that goes nowhere, and to the outside of the kettle! If you are standing on a stone or tile floor, or otherwise earthed, and touch that kettle, you get electrocuted. This is why TN-C electrical systems are rarely used.


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CWB
Posted: February 24, 2012 01:02 am
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ergo : "current path"

current flow (not to be confused with voltage/potential) cannot happen without supply and return paths . it may also be thought of as a "complete path" with AC .
in actuality it matters not whether the voltage source is AC or DC .

the amount of current flow is dependent on source current capabilities , voltage/potential and path resistance .
this is barring the effects of "loading down" of the current source and the capacitive and inductive reactance in/with AC circuits (etc) .
(but before getting into this , it is a good idea to get ones head wrapped around the "simple" stuff)

conventional versus electron flow in DC circuits ...
i have always thought in the correct or "electron flow" idea . electrons will always "go" towards a "less negative" potential .


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