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Alternating Current, live and neutral wires

  1. Nov 5, 2016 #1
    Hi there,

    I am a teacher and will be teaching the topic of alternating current soon. I have not taught this before and have certainly not studied this in many years so I am trying to get a full understanding and am looking for a bit of clarification.

    So I know that the current supplied by the mains is alternating current and that this means the current is flipping direction many times a second and that consequently this has an associated frequency.

    Presumably this would mean that in a circuit the current would be flowing in one dirrection through the wires and components one moment and then flowing the opposite direction another moment.

    The main problem I am having is understanding how this works with the neutral and live wires. The live wire supplies 230V and carries the current into the appliance. The neutral wire returns the electricity to the
    generator after it has passed through the appliance. The neutral wire completes the circuit and the neutral wire is at approximately zero volts.

    If the current is alternating then how is this consistant with the fact that the current only enters an appliance from one direction (from the live wire) and exits it through the neutral wire? Wouldn't this be a direct current?

    I have probably made a lot of mistakes in the question and my apologies for this, just trying to to get a better understanding.

    Thanks for any help you can provide.
     
  2. jcsd
  3. Nov 5, 2016 #2
    You need to revise the concept of voltage. Saying that a single wire supplies 230V is incorrect. Ditto with the neutral wire at 0V: 0V with respect to what? (Thinking of earth?) How does this apply to two-wire systems?

    Saying that a point in a circuit is at such and such a voltage is only acceptable when the reference is understood. I am not a teacher, but I think that in a teaching context every opportunity should be taken early on to establish that voltages are differences. So, a voltage at a point or through a component is nonsense, but a voltage across a component is meaningful.

    I would not speak of the neutral wire "returning the electricity to the generator", that is wrong in several ways. Avoid using the word "electricity" to refer to anything but the general phenomenon. When referring to any measurable phenomenon like voltage, current, or power, use those words.

    The descriptions are indeed inconsistent. This is because the fact you assert is not true. Speaking of a return wire in a two-wire ac system is incorrect. Are you aware that the voltage in these systems is also time-varying?
     
    Last edited: Nov 5, 2016
  4. Nov 5, 2016 #3

    jim hardy

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    Here's the script i use

    The humble flashlight has a lot of lessons in it.
    Get yourself an old fashioned two cell metal one to use for a prop while you're speaking.

    Get a cheap meter, too i use the $9.95 Walmart analog one.


    Draw the flashlight circuit on the board, two batteries and a switch and a bulb. Bulb on right, batteries left.

    Remove one battery from your flashlight and hold it up . Tell the kids
    "Every quantum of positive charge that comes out the positive end (tap the top) of this battery must get back to its negative end.(tap the bottom)
    Or, if you prefer to think of charge as negative and carried by electrons, every bit of negative charge that comes out the bottom must get back to the top."

    Then move to the drawing on the board and trace out the loop, saying
    "If that charge must flow through the switch and the bulb and another battery to get there, that's what it will do."
    Then reassemble the flashlight and turn it on, saying "That's Kirchoff's Current Law for DC. DC on this drawing always flow clockwise(gesture around your loop) for positive charge carriers which we call conventional current or counterclockwise for negative charge carriers which we call electron current."
    Take a breath and see if they got it.
    Continue:
    "In more highly educated circles they always use conventional positive current but many trade schools still teach electron current. You should learn to think in positive current , that's how most textbooks are written, and you know , 'When in Rome' .
    You will however need to be conversant in both and i assure you they both work out to give same answers."

    Take a breather
    continue
    "Notice we have not even mentioned the terms Earth, Ground, Hot or Neutral.
    What do you think would happen if i measured the voltage between this flashlight and earth ground? Would any current flow to deflect the meter needle?
    (draw a meter on the board from battery negative to ground, maybe the frame of your chalkboard.)
    Ask class. In my class half the kids always say 'Yeah current wants to flow to ground'

    Ask class "How does going to ground help that charge find its way back to other end of the battery from which it came ?"
    Connect your real meter from flashlight metal to something in the room that's grounded (I had already verified that my chalkboard frame was grounded) and have a kid run through all the ranges to show no current flows.

    "Now we have demonstrated Kirchoff's Current Law which simply says "Current only wants to flow back to whereever it came from." That's why it flows in closed loops and if ground isn't in that loop current won't go there."
    That'll generate some consternation.
    Explain that lightning is obeying KCL because charge was carried aloft with water vapor that evaporated from the ground ahead of the thunderstorm, lightning is just that charge getting back to the earth where it came from a little while ago
    Kirchoff will hold an IOU temporarily. Or maybe i should say he gives out rain checks.
    Anyhow lightning imprints us early with the misconception that 'electricity for some reason likes ground' when it's really just cashing in a Kirchoff's Rain Check.

    Next i say
    "So, what would happen if i connected my flashlight's frame to earth ground? (i set mine on the metal chalk rail which i knew from prior test was grounded) Would current flow to ground? No, we already checked for that with the meter."
    Then i move to the drawing on the board and draw a line from negative down to a ground symbol.
    "If i measure from ground to flashight body i'll measure zero volts
    If i measure from ground to positive i'll measure positive three volts
    if i measure from ground to junction of the two batteries i'll measure positive one and a half volts."

    Then i erase my ground wire and ask "What if instead of grounding the flashlight body i had instead reached inside and grounded the junction of the two batteries?" and redraw my ground wire there. Gesturing as i speak, continue
    "Now from ground to flashlight body i'll measure minus one and a half volts.(write it on board)
    From ground to positive of top battery i'll read plus one and a half volts.(write it on board)
    From ground to battery junction i read of course zero.
    From flashlight body to top of batteries i'll read positive three volts.(write it on board too)
    You see, voltage is a potential difference between two points and nobody said either one of those two points has to be ground.
    And that's Kirchoff's voltage law, voltages add up."

    "Okay boys, now you have the basic nomenclature and concept.
    The only difference between DC and AC is this -
    DC always flows the same direction around the loop either clockwise or counterclockwise .
    AC reverses its direction 50 or 60 times a second depending what country you're in , so it shuttles back and forth around the loop. (Gesture wildly, you want to be the next Doc Brown)
    But when you train your brain to freeze-frame your thinking, at any instant AC is DC .
    Kirchoff still applies.
    AC current is usually delivered to us by a transformer winding not a battery.
    But it's still as simple as our two cell flashlight. (draw another flashlight circuit with two transformer windings instead of batteries, no ground)
    AC current wants only to get back to the far end of the transformer winding from whence it sprang
    and if but only if ground will take it there is it happy to flow that way .

    If the transformer winding isn't grounded it cannot send any current to ground. It's no different from that flashlight sitting over there.

    Most of the transformer windings you'll encounter outside of industry have two 120 volt windings in series. That makes for 240 volts but for some reason it's called 230. That's a leftover from the old days when voltages were a little bit lower."

    Continue
    "
    In the UK they ground one end of that winding (draw it on the board) so the 'hot' end will read 230 or 240 volts to 'ground'.
    The wire on transformer's hot end has a circuit breaker in it and is said to carry current to the load,
    The end that's grounded they call 'neutral' , and the wire on that end is said to return current to the transformer
    but that's strictly true only while current is flowing clockwise (assuming your drawing has load on right side of transformer) be aware current is cycling at power line frequency." (gesture back & forth)

    erase ground

    "In the US they ground the junction of the two windings and call that point neutral (draw it in)
    so the two transformer ends will both read 120 volts AC to ground and both are called 'hot' (write 120 and 120 beside them)
    but freeze framing, at the instant when one is +120 the other is -120 so there's 240 between them (add signs to drawing)
    your 120 volt appliances like lights and TV sets are connected between one hot and neutral, and the outlets in your house are divvied up among the two hots in hopes of somewhat balancing the load on the two windings,
    your 230 volt appliances like water heater and furnace are connected between the two hots and might not even have a neutral wire....

    and that's house wiring at its simplest."

    whew!

    Sorry for the long long essay
    but in an hour you can plant basics that'll last the boys a lifetime
    and will prevent much wayward thinking.

    Then go on to "that's why letting a hot wire accidentally touch ground gives such impressive sparks. Current can get back to its transformer by going through ground. That's because we ground transformers to keep things orderly, but it makes people think electricity magically 'likes ground' which isn't so.

    When messing with house wiring in the US always remember 'The Black wire is the one with smoke in it.
    It always goes to the short slot in an outlet and to the center pin of a lamp socket."


    Read IEEE Green Book for an excellent introduction to grounding. Google finds it.

    whew - sorry , but i think this is important.

    Good luck !

    old jim

    (edited for typos and dumb mistakes )
     
    Last edited: Feb 28, 2017
  5. Nov 6, 2016 #4

    dlgoff

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    Some animations; just for fun. Compliments of http://www.pbs.org/wgbh/americanexperience/features/general-article/light-acdc/ [Broken].

    acdc_anim_dc.gif acdc_anim_ac.gif
     
    Last edited by a moderator: May 8, 2017
  6. Nov 6, 2016 #5

    tech99

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    This often comes up with teachers and pupils.
    First imagine a DC mains supply provided by a battery. The two wires just form a circuit. Current flows out on one wire and back on the other. There is 230 volts DC between the wires. Now imagine that at the battery end I make an additional connection from one wire to ground. This does not affect the operation in any way. But now suppose that at the house I touch the wire which is not grounded and stand on wet ground. Now a secret additional circuit exists via my body and the ground. The wire I touch is called the live. If I touch the other wire, there is no such sneaky circuit, and this wire is called the Neutral. It is not grounded at the sending end. Notice that there is now a voltage of 230 volts between both wires, but also between the Live wire and Ground.
    Now you might think that grounding one wire has created a safety problem. For some purposes a supply which is floating and isolated from ground is a good safety feature. Power tools are sometimes fed from an isolating transformer to achieve isolation. But for a large network, the problem is that one or other wire might be accidentally connected to ground, and then we have a safety issue for all other users. So it is best to solidly connect one wire to ground at the sending end and call this the Neutral wire.
    Now to understand AC. Suppose I place a reversing switch at the battery. If I reverse it instantly, the lights and heating continue to work without knowing what has happened. Now imagine I reverse it 50 times each second. Still the same. This is AC.
    Hope this helps.
     
  7. Nov 6, 2016 #6

    jim hardy

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    @maximus123

    i hope you're not one of those who never checks back on his question.

    Let us know how the class goes ?
     
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