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Does current flow in both wires or in one in our homes?

  1. Nov 9, 2015 #1
    If we have a source of AC current with two terminals ( A and B ). We connect a device with this source. Now we say that one wire is live and other is neutral. But my question is that, how one wire is neutral. As we say that in AC current the terminals change the polarity. Its mean that some time A will be positive and some time B terminal will be positive. So when A is positive current will flow through one wire and when B is positive then current will flow through the other wire. both wires will carry current.
    May be my concept is not correct. Please help me...
     
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  3. Nov 9, 2015 #2

    Geofleur

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    Imagine what happens in each of these cases, without connecting the terminals to each other through any appliance:

    (1) You touch the live terminal and not the neutral one. This is dangerous, because then a current could form between the terminal and the earth, through you.

    (2) You touch the neutral terminal and not the live one. Then you are just touching a wire that effectively has its other end connected to the earth. Nothing happens.

    The terminology comes from what happens in those two cases. If you complete the circuit and then touch the "neutral" terminal then, yes, you are in danger again.
     
  4. Nov 9, 2015 #3

    jbriggs444

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    Please do not touch the neutral wire either.

    While it is probably true that nothing will happen when you touch a neutral wire, in practice, the neutral wire is not guaranteed to be at zero potential compared to the ground you are standing on. In the U.S., the neutral wire and the ground wire are separate. The presence of other devices on the neutral wire could result in a non-zero voltage there.

    An outlet equipped with a "Ground Fault Interrupter" can serve as some protection against the result of touching either the live or the neutral wire. https://en.wikipedia.org/wiki/Residual-current_device
     
  5. Nov 9, 2015 #4
    Your concept is correct.

    The neutral is called such because of the voltage to earth, not because of current. The same is true of "balanced" or "unbalanced" transmission lines. The balance or unbalance is determined by potential to earth or surrounding space, although it becomes more complex than "neutral" because balanced also can mean currents are equal and out-of-phase.

    There are three wires from the line transformer in the residential drop. Two "hots" of approximately equal voltage (120 volts nominal) with the same phase relationship but 180 degrees apart, and one neutral that is grounded at the pole and at the house to earth. The neutral in your house could carry anywhere from all of the load current to none of the load current, depending on how the house loads are balanced between the two 120V to neutral circuits.

    You may not know it, but you are (in effect) indirectly touching the neutral wire hundreds of times a week.

    If the system is just a two wire with a hot and neutral, the neutral would have all of the current of the hot (opposite direction flow) but the neutral should be near zero volts to earth or ground. If it is a three wire system with neutral like a house, the neutral still is very near zero voltage to ground but could have anywhere from zero to full current depending on load balance between the hots (the hots are out of phase). If the neutral is not near zero volts to ground, it is a safety issue. :)

    The American home ties the neutral and a small "safety ground" to earth at one point, the mains box. The small safety ground is the little bare wire in the standard cable, and is not designed to handle much current. It connects to the wall plate screws, metal wall plates, light fixture metal, and cabinets of electronics like a desk computer. So all that metal in the room eventually ties to the neutral at the fuse box. Even your telephone and cable TV cable tie to that same neutral point!!!!
     
  6. Nov 9, 2015 #5

    sophiecentaur

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    They are only 'names'. As far as the appliance is concerned, the two wires do exactly the same job all the time.
    It is only the Potential Difference between the wires that counts when you are using the electrical Power. It just so happens (for completely different reasons) that the Neutral conductor is kept with its Potential Difference to Earth near zero. As a consequence, the PD relative to ground of the other (live) wire goes positive and negative.
    No. The same 'amount of' current is flowing through each wire it's just flowing in different directions. The Earth / Ground wire never carries any of the supply current (except when there is a fault condition).
    This is just the same as when you connect up a battery and a bulb; the same current flows into the - terminal of the battery as flows out of the + terminal.
    You should be careful about what you are saying here. I'm not sure what your message is but it could be read as implying that the currents in live and neutral are not the same at all times - which is not true (except when something's wrong with the system).
     
  7. Nov 9, 2015 #6
    Yes, it is difficult to be perfectly clear at every level. I assume people understand current around a two terminal path is the same value at every point and the wording just may be a little sloppy.

    For example, this corrective statement is not true:
    That correction isn't true. In a perfectly functioning USA home system with zero reactance in the loads, the feeder neutral can actually have any current up to the difference in current between the two hot wires. If one hot is 75 amps and the other 35 and the system is perfect with zero power factor and no leakage currents, the neutral exiting the box to pole will be at 40 amperes. If the load is perfectly balanced, neutral current is zero at the service entrance.

    This is more involved than a couple posts to get interpretation error free. It actually would take a few pages of text and drawings. The short form is the house is a center tapped power and load system, with a grounded center tap. That center tap is called a neutral, not because of current, but because of voltage with respect to the earthing or ground system. The neutral from pole to mains box almost never carries the same current as the hot wires, because the hots are 180 out-of-phase and the load is split between hots with the center of the split going to neutral.

    The only universal statement we can make about the neutral is the neutral should always be at a very low voltage (within a few volts) of earth anywhere in the system. The fact voltage is at or near neutral to earth is the sole reason it is called "neutral". A neutral in a perfectly functioning USA power mains can have any amount of current, since it is the center tap return of a 240 volt center tapped winding and load.
     
  8. Nov 9, 2015 #7

    sophiecentaur

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    I agree with you but with one proviso. People mostly post questions about the live and neutral wires that feed their appliances. The answer follows k1.
    For people who want to know about polyphase systems, the answers are not so simple. IMO, one absolutely has to make it clear when a post goes polyphase. Many questioners are hardly aware about the ramifications of the US two phases in the home or the three phase system used for distribution.
    School level is always (?) single phase.
     
  9. Nov 9, 2015 #8
    The house feed is single phase. It is a single phase center tapped 240 volt winding. It is called single phase because it is one phase.

    Distribution systems are three phase. In early two-phase systems, phase were in quadrature. The two phase 90 degree shift was improved to three phase, with 120 shift between phases. A standard US home in a populated area runs off one phase of three phase distribution, these lines typically are three hot primary wires up at the top with a neutral below. Each house typically feeds between one wire and the neutral. More rural areas with lighter loads have a single top hot wire and a neutral lower on the pole, with the transformer again between the hot wire and the street neutral.

    A home 240 system is not polyphase or dual phase. It is single phase 240V center tapped.
     
  10. Nov 9, 2015 #9

    sophiecentaur

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    You could re-word that as "I call it".
    If you refer the two 120V live wires to the Neutral, they are in antiphase (180°). Why is that not 'two phase' if 120° gives you three phase? The way it is produced is immaterial.

    The UK uses single phase 240 V in all domestic situations. Now that really is single phase.
     
  11. Nov 9, 2015 #10

    jbriggs444

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    It turns out that the term "two phase" is already spoken for. https://en.wikipedia.org/wiki/Two-phase_electric_power
     
  12. Nov 9, 2015 #11

    sophiecentaur

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    As are the terms, wheel, modulation, drug and a few others. Unless there is something very singular about two sinusoids that are 180° out of phase, I would say that you can have two, quadrature phases, two phases at 50°, 75° or 179° and you would still have a two phase system.
    Why not look at the OP and ask yourself whether these contributions to the thread could have helped or confused the questioner? I am as guilty as the next man (and you happen to be the next man, in this case) of adding superfluous information to a basically simple threads so mea culpa on many occasions.
    AC means two wires to most people in this world and the thread was started on those lines. Historical names that were given to ways of producing electrical power aren't of great interest to modern school-level questioners.

    Sorry. Reading this through again it looks far more harsh than I intended. Mea culpa all over again.
     
  13. Nov 9, 2015 #12

    jbriggs444

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    I agree that the discussion of mains power and of one, two or three phase is irrelevant to the original question. And that as the next man, I stand guilty as well.
     
  14. Nov 9, 2015 #13

    sophiecentaur

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    No. I'm Spartacus. :smile:
     
  15. Nov 9, 2015 #14
    To revisit the OP is the actual meaning of AC Alternating Current. Which is what the current does on the label and is why neutral can be as live as the positive when the current alternates.
     
  16. Nov 9, 2015 #15

    sophiecentaur

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    They are carrying the same current but not normally at the same potential wrt Earth.
     
  17. Nov 11, 2015 #16
    Thanks all of you. you have answered the question but my question was different.
    I want to know that is there current flow in both hot (live) and neutral wire in our home ? or it is in single wire?
    if we check neutral wire with tester, it will not show current.
    But in AC current we study current changes direction. Also the polarity of terminals of AC source changes. Then according to this current mus flow in hot and neutral wire. Am i right ?
     
  18. Nov 11, 2015 #17

    sophiecentaur

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    That is not right. Your 'tester' will only respond to the Elecric Field near that wire; it is not a current meter. You could not measure the current except by breaking the circuit and inserting an Ammeter in series (or using a more modern 'tong' meter, which I doubt you have). If you had been able to measure the actual Current, then you would have found exactly the same value through the live and neutral wires. An AC source doesn't have a "polarity" because the PD across both terminals will alternate in sign at 50 (/60)Hz.
    This has all been stated earlier on in this thread and your comments in #4 have been answered. Your last question is effectively just repeating your original question. Try to take on board what has been written already and don't try to impress your original ideas onto those answers. You need a re-think, if you want to get hold of this.
    There is a lot of rubbish talked about AC (even in the hallowed walls of PF) and you need to be careful about what you take notice of.
    A good text book (not a technician's instruction manual - which can be surprisingly dodgy) should put you right, with this and most of elementary Physics ideas.
     
  19. Nov 11, 2015 #18
    I am not trying to impress my ideas on any thing. I am just trying to remove my confusion. All the answers which are given to the thread, may be are correct and give the correct answers, difficult for me to understand. That's why i questioned again. I don't know what makes you feel that i am impressing you.
     
  20. Nov 11, 2015 #19

    sophiecentaur

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    Bad wording on my part but you are not letting the new information you are being given help you overcome your intuition. You are confusing Volts with Current, I think. You cannot measure current easily so I was assuming that your 'tester' was a normal mains tester that just looks at volts. Your conclusion that the currents are different (on the strength of your evidence) is not founded on any evidence. Does that make sense?
     
  21. Nov 11, 2015 #20

    russ_watters

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    I'm sorry I didn't see this thread before because there is too much off-topic discussion that makes it confusing and some of the on-topic responses just aren't correct. I'll re-focus:
    None of that is correct.

    The "neutral" wire is so-called because that's exactly what it is: neutral. It is connected to the ground and therefore its voltage with respect to the ground is always (supposed to be) just about zero. The "hot" or "live" wire has an oscillating voltage with respect to this constant neutral: sometimes positive, sometimes negative. It pulls and pushes the electrons through the circuit.

    A properly functioning neutral should not electrocute you because it doesn't carry a voltage with respect to the ground: it is "after" the load regardless of the direction of current flow, so it requires nearly zero voltage to push/pull current through it (but for safety reasons, you still should not touch it).

    But the circuit is still a single loop, regarless of which direction the current is flowing. The live wire is pulling or pushing the current through the circuit, but in all cases, the current is the same everywhere in the circuit (it has nowhere else to go).

    Hopefully my above addresses most of what is wrong with this (mainly that the current must always be the same everywhere in a circuit), but there is an additional possible source of confusion: What kind of tester? Most people don't ever check the current in their homes because it is difficult and the equipment is expensive. They only ever measure voltage. So if you measure the voltage of the neutral with respect to ground, it will always be zero whereas (in the US) the hot is 120V. If you measure the current, it will always be the same in the neutral and hot wires.

    So, some more specific issues:
    No! The hot/live wire is not "positive", it is alternating between positive or negative. The netural wire is always zero, so it cannot ever "be as live as the [hot]".

    I ignored most of the multi-phase discussion because it isn't relevant, but just to point out the difference between single and multi-phase, it is the neutral wire itself: multi-phase power doesn't have one. For split-phase (when using both) or three phase, both/all three wires are hot.
     
    Last edited: Nov 11, 2015
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