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I'm trying to understand how the electricity works in my house. I live in the US. I know that at the transformer, the primary winding induces a current in the secondary winding. The secondary winding is center tapped with a "neutral" line. The neutral line is grounded to the Earth. Here is the diagram:

enter image description here

Here are my questions:

1) When the circuit is closed, are hot 1 and neutral both "hot"? That is, are they both carrying a current of 120V that is alternating back and forth?

2) When the circuit is open, there is no current flowing. So, what are the electrons in hot 1 doing? Are they still oscillating back and forth? In other words, when a conductor is "energized" but no current is flowing through it, how are the electrons behaving?

enter image description here

3) When the circuit is open, the neutral is not "energized" like hot 1. Why not? I know it has to do with it being grounded to the Earth, but I have not found an explanation of what is happenening at the level of the electron. Are the electrons alternating back and forth in and out of the Earth instead of through the neutral wire?

user3080392
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    When would you expect N to ever become hot? Ie >50= LV threshold? – Tony Stewart EE75 Mar 26 '19 at 16:40
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    The definition of hot and neutral has nothing to do with the amount of **current** they carry, it is because of their **voltage** with respect to ground. The word "energized" doesn't have any real meaning here. – Elliot Alderson Mar 26 '19 at 16:46
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    “Hot” refers to voltage potential not current. However if you broke a Neutral connection to a motor, it would arc and then become hot. – Tony Stewart EE75 Mar 26 '19 at 17:12
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    It doesn't seem like anyone has yet pointed out that there are some basics about how electricity works that you might not be aware of. If the answers here aren't helping you much, then it might be because some of the basics about electricity aren't clear to you. One way to address that would be to look for online resources or books that discuss the fundamentals, like the relationships between voltage, current, and resistance. – Todd Wilcox Mar 26 '19 at 19:14
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    " I know that at the transformer, the primary winding induces a current in the secondary winding" - no, it doesn't. Do you think the transformer somehow knows what circuit is connected to it, and magically starts inducing a current when it notices that you just closed the switch, and stops when it notices you opened it again? It induces a voltage, but not a current! – alephzero Mar 26 '19 at 23:15

4 Answers4

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When the circuit is closed, are hot 1 and neutral both "hot"? That is, are they both carrying a current of 120V that is alternating back and forth?

First, "a current of 120 V" doesn't make sense. The units of current are amperes or amps. Volts are a unit of voltage (potential difference), not of current.

You can have a current of 1 amp, or a voltage of 120 V, but a current of 120 V is as impossible as a length of 20 pounds or a price of three meters.

The neutral wire carries current. The amount of current is determined by the load. But its potential (voltage) remains very close to ground, so if you touched it while your feet were on the ground, there wouldn't be a very large voltage applied across your body, and therefore we don't say that this line is "hot". (Don't actually try touching the neutral line because there are a variety of ways your wiring could be faulty that could make the neutral line hot and dangerous)

When the circuit is open, there is no current flowing. So, what are the electrons in hot 1 doing? Are they still oscillating back and forth?

If there is a parasitic capacitance (there is) between the hot line and either the neutral line or the actual earth, then to a certain extent, the electrons in the hot wire will indeed flow back and forth charging and discharging this capacitive "load".

When the circuit is open, the neutral is not "energized" like hot 1. Why not?

The neutral line is not energized whether there is or isn't a load. Because it's tied to actual earth ground, it can't develop a potential relative to ground, and so if you touch it or connect a load between it and ground, it won't produce any current through that load. It can't produce any potential relative to ground because if there were a potential relative to ground a large current would flow (momentarily) and equalize the potentials.

When the circuit is open, a small current might flow in the neutral line due to parasitic capacitive "loads" I mentioned above.

The Photon
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    People in the USA often use "current" colloquially and incorrectly when they mean "voltage", e.g. "mains current". I blame Edison. – Rich Mar 27 '19 at 03:52
  • @The Photon Could you check if my "explanation" at the bottom, which is posted as an answer is correct? Thanks. – user3080392 Apr 15 '19 at 11:54
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  1. When the circuit is closed, both conductors are carrying a current which is dependent on the load. The phrase 'carrying a current of 120V' is nonsensical. It is not the case that hot and neutral are both hot. Hot is always hot (120V) and neutral is always neutral (0V) regardless of whether the circuit is closed. For instance, you can touch the neutral wire with the circuit closed and you will not get injured HOWEVER do NOT actually try this experiment because there is non-zero risk that something is wrong with your wiring and you can get hurt. Conversely, you can touch the hot wire with the circuit open and you will certainly get injured.

  2. Ignoring secondary effects like capacitance to ground, the electrons are taking a rest when there is no current flowing. Of course, they are still physically moving due to momentum, thermal energy, etc. but they are not moving to and fro under the effect of an electric field as they would if the circuit was closed.

  3. When the circuit is open, the neutral has no current flowing through it. The electrons are doing what they would be doing if you had a piece of wire laying on your table, connected to nothing. I'm not sure why you'd think that electrons would be going back and forth into the ground? If you stick a piece of metal in the ground, do electrons continually go back and forth from the metal to the ground? Even if the circuit was closed, electrons wouldn't be going back and forth into the Earth.

As an aside, (this may be an unpopular/controversial opinion) but I don't believe you need to know what's happening at the electron level to understand basic electricity. Even at the advanced level, the electromagnetic waves that the oscillating electrons are generating are far more consequential than the electrons themselves. It only becomes really necessary to ask yourself 'what are the electrons doing?' when/if you study semiconductor physics. Even then, you don't need a PhD in semiconductor physics to successfully design a circuit including diodes and transistors (which is good, because that stuff can get seriously complicated at an advanced level). Focusing too much on the electron when one first begins to learn about electricity leads one to such incorrect conclusions as 'electrons travel at the speed of light in a wire', 'conventional current flow is incorrect', etc.

pr871
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  • I'm still confused. So what you're saying is that hot 1 always has 120V in it and neutral always has 0V in it whether there is a current flowing through it or not? So, at the electron level, what distinguishes hot 1 from neutral? Does hot 1 have more electrons in it? Both wires are "receiving" the same effect of the primary winding, but the electrons in hot 1 are somehow different than those in the neutral. – user3080392 Mar 26 '19 at 17:48
  • @The Photon If the system was not grounded, then hot 1 and neutral would essentially be identical. The system is grounded through the neutral, so therefore hot 1 and neutral become different. I'm having trouble conceptualizing what this difference is at the electron level. What about the electrons in hot 1 make them unique from neutral? – user3080392 Mar 26 '19 at 18:00
  • I think you are worrying too much about the lives of individual electrons. Compared to the amount of electrons that flow when you turn on a light or your stove, there is an *absolutely minimal* number of electrons that are flowing back and forth to establish the 120VAC potential between neutral and hot -- and for the purposes of this question, there are the *same* number of electrons flowing in the neutral wire as in the "hot" wire. – TimWescott Mar 26 '19 at 18:06
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    Yes, hot1 always has 120V and neutral always has 0V regardless of whether or not there is current flow. Most of the time (but not always!) voltage is the independent variable and current is the dependent variable. The voltage exists independently of the current. – pr871 Mar 26 '19 at 18:10
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    Again, your insistence on looking at things at the electron level is doing you more harm than good. Consider that in the 17th-19th century, incredible scientific advancements in the field of electricity and magnetism were made without anyone having heard of something called an 'electron'. Forget about the physical electrons (for the time being) and focus on macroscopic quantities like charge, voltage, current, etc. – pr871 Mar 26 '19 at 18:10
  • I'm focusing on the electron because that helps me understand what is going on at the most basic level. All these internet tutorials and videos show the movement of electrons, losses and gains of electrons, and other basic behavior of electrons. The books I read on electricity start off by looking at electricity at the electron level. Something is making hot 1 different than neutral fundamentally, in terms of electrons, and I can't grasp what. – user3080392 Mar 26 '19 at 19:56
  • If the system wasn't grounded, hot 1 and neutral would be the same physically, both would have 120V. Yet when we attach neutral to the Earth, the neutral changes. If I touch it I don't get electrocuted, if I wave a non touch voltage tester in front of it, it doesn't beep. How does the 120V disappear? What happened to the electrons? – user3080392 Mar 26 '19 at 20:05
  • All charged particles within an electric field have electric potential energy (unless they exist at a point of 0V). An electric field is generated within the transformer secondary according to Faraday's law. A charge distribution exists along the length of the hot and neutral wires, extending the electric field within the space between the wires. The electrons on the 120V side have more (or less) electric potential energy than the electrons on the 0V side depending on what point in the ac cycle we're considering. – pr871 Mar 26 '19 at 21:14
  • If you cut the neutral AND close the switch, then there no longer exists an electric field between the electrons on the hot wire and the electrons on the neutral wire (because the hot and the neutral now form an equipotential surface and the voltage gradient within the wire must be 0 because there is no current flow). So before the neutral is cut, the electrons in the hot and neutral had different electric potential energy. After the neutral is cut, they have the same electric potential energy. That's how the 120V 'disappears'. – pr871 Mar 26 '19 at 21:14
  • "What are the electrons doing?" really just means (if you exclude thermal motion) "What is the current doing?" and current is definitely something you should understand to understand electricity or electronics. – user253751 Mar 26 '19 at 23:02
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    @user3080392 If you really insist on understanding "how the electrons behave," you will have to learn FAR more about electromagnetism than you currently know. For example, you need to understand now capacitors work (including the fact that the earth is effectively a capacitor), and that when the switch is open, the geometrical configuration of wires still behaves like an antenna, creating electromagnetic waves in space. But none of this is necessary to "know how electricity works in your house" unless *you* insist on going down this rabbit hole. – alephzero Mar 26 '19 at 23:07
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    @user3080392 I think you're confusing voltage and current. [This answer](https://electronics.stackexchange.com/questions/72875/do-electrons-actually-flow-when-a-voltage-is-applied) is probably a good starting place. Voltage is potential energy in the electric field, just like having a brick on a table is potential energy in the gravitational field. It can sit there at 120 V and be doing nothing, just like the brick sits on the table doing nothing. A current doesn't flow until a load connects hot 1 and neutral. – Katie Kilian Mar 26 '19 at 23:54
  • One place the 'brick on a table' analogy falls apart is with AC power: the _potential_ is moving from +120V to 0 to -120V and back again, 60 times a second. Bricks don't have their potential energy change while they're sitting on a table like that. But if you think of electrons in the classical sense as being little spheres, no electrons are moving: Just the potential -- the voltage -- is changing. – Katie Kilian Mar 27 '19 at 00:01
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    Electrons are _not_ little spheres. As other people have said, what electrons are is actually very complicated and not necessary to understand -- thankfully! Frankly, what they are is not even perfectly understood by physicists (well, their behavior is _very_ well understood, mathematically speaking, but what the equations actually mean at a fundamental level is [open to interpretation](https://en.wikipedia.org/wiki/Interpretations_of_quantum_mechanics)). – Katie Kilian Mar 27 '19 at 00:02
  • @pr871: "you will certainly get injured." - citation needed. My father touched a live wire in his youth (at 240V - in the UK) and it stung a bit, but there was no permanent injury. (Of course, I still recommend avoiding electric shocks - there is a significant risk of injury.) – Martin Bonner supports Monica Mar 27 '19 at 10:20
  • @MartinBonner No need to argue about the definition of the word 'injured'. For the purpose of this conversation, 'injured' can mean anything from "ow, that hurts" to death. – pr871 Mar 27 '19 at 13:13
  • @pr871 Could you check if my "explanation" at the bottom, which is posted as an answer, is correct? Thanks. – user3080392 Apr 15 '19 at 11:55
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What you are missing here is that voltage is not current, it is pressure. The 120V side of the circuit has a lot of pressure, enough so that under bad conditions you can be killed by it. (Note that it actually takes current to kill, voltage only matters in that it must push hard enough to send enough current through you. Thus extreme voltages without the current behind them can hurt but won't kill--that's why you don't die from a static electric snap and why a teacher of mine from long ago didn't die when he made a mistake with 300,000V.)

Lets try looking at it with water instead of electricity. Go outside, consider the garden faucet. Inside the pipes there is probably something in the ballpark of 60 pounds per square inch of pressure. (This can vary considerably depending on where you are, though.) This is the equivalent of your hot wire.

Outside the faucet there's no pressure, that's your neutral. Open the faucet, water comes spitting out--that's current. You still have pressure in the pipes, though, and you still don't have pressure outside the pipe. The pressure of the water isn't enough to harm you, though--it's more like a 9V battery than mains power.

Loren Pechtel
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    Voltage is even called "tension" in some languages, and in some old texts and specialized fields in English as well. – Hearth Mar 27 '19 at 13:33
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Based on the responses and brainstorming, I developed an explanation:

The primary winding produces an electromagnetic field that continuously alternates polarity. At each pole of the secondary winding, electrons are either drawn to the pole by the electromagnetic field, causing an overall negative charge, or repelled from the pole causing an overall positive charge:

enter image description here

enter image description here

The Earth has an overall balanced charge to it, so if the neutral pole of the secondary winding is connected to the Earth, there will be a potential difference between it and the Earth. Electrons will flow to the Earth and the overall charge of the neutral pole of the secondary winding will balance out to match the Earth:

enter image description here

enter image description here

If I am standing on the Earth and touch the neutral wire, there will be no potential difference between the Earth under my feet and the neutral pole of the secondary winding. So, no current will flow and I won't get shocked:

enter image description here

If, however, I touch the hot 1 wire, which is connected to the pole where the overall charge is either negative or positive, there will be a potential difference between the pole and the balanced charge under my feet. Current will flow and I will be electrocuted:

enter image description here

In a closed circuit, there is a potential difference between the hot 1 pole and the neutral pole, so current flows:

enter image description here

user3080392
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  • Everything from "If I am standing on the Earth" onward is okay. The part before that is not horrible, but it isn't really clear on every detail. – The Photon Apr 15 '19 at 15:47
  • I'm not sure why you insist on coming up with your own explanation of how transformers work when transformer operation has been well understood for over 100 years. I would say that your explanation of electrons being attracted to magnetic poles, which causes a charge separation that establishes a potential difference is wrong. Electrons with zero net velocity are not affected by magnetic fields. Plus, the magnetic field is largely confined to the core and does not reach out along the length of the wires. – pr871 Apr 15 '19 at 17:38
  • My impression is that you want to come up with an electrostatic explanation for a phenomenon that is inherently electrodynamic. Transformer operation cannot be explained by Gauss' law; you need Faraday. The potential difference between the hot and neutral wire is driven by a circulating electric field within the transformer coil that's generated by time-varying magnetic flux through the core. This electric field gives rise to an electromotive force that will cause current to flow if a load is attached. Even without a load, a tiny current will flow due to capacitance between the wires. – pr871 Apr 15 '19 at 17:40
  • Now there will be a charge separation due to this capacitance given by Q = CV. There will also be a charge density along the length of the wire due to the non-zero current flow. The actual picture of what this looks like in terms of electrons is extremely complex and the best way to visualize it would be to maybe use a field-equation solver. I haven't done this and don't plan to, but I'm assuaged by the fact that knowing the exact potential distribution around and throughout a transformer is wholly unnecessary for the purpose of understanding how a residential electric system works. – pr871 Apr 15 '19 at 17:41