AC and polarity of hot and neutral wires

  • #1

Main Question or Discussion Point

I'm a bit confused as to how only one wire changes it's polarity in an alternating circuit. I also checked out a previous thread (https://www.physicsforums.com/threa...ut-keep-the-live-hot-and-neutral-cold.752605/) and didn't find a sufficient answer. I found this image online (http://hyperphysics.phy-astr.gsu.edu/hbase/electric/hsehld.html) and this is how they seem to perceive household circuits to be designed (in the most basic sense) :

image.gif


I did a little more research into the topic and didn't find a good answer but I saw that only the hot wire changes it's polarity while the neutral wire supposedly stays at zero volts and acts as a return path for the current. I'm sorry if this is elementary but this is ultra confusing to me because I fail to understand how one wire (the hot wire) can change the direction of the current?? Doesn't changing the direction of the current imply that there are two paths for it to take? I thought the picture above made some sense because at least it could change direction and the neutral wire would acquire a voltage but since that is apparently not how it works,I have no idea what to believe. Maybe the above picture is only true for appliances that require no grounding?

P.S. If there is any way you could draw a circuit diagram for how the current actually alternates in the hot wire that would be greatly appreciated!
 

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  • #2
meBigGuy
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I guess you need a basic understanding of AC voltage and current.

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/acres.html may help a little.

Think of the top of the resistor as connected to the HOT, and the bottom to the neutral. The neutral is just chosen as the reference and defined as 0 volts. You could just as easily define the HOT as 0 volts and say the neutral is the AC source. (but that's not a convenient way to think about it since neutral is basically (well almost) ground)
 
  • #3
Please bare with me on this topic. Believe it or not I've taken my fair share of electronics courses at uni, but this is something they just glossed over and has been troubling me for a while.

I understand how AC voltage and current works, I just don't understand how it works when it comes to the way they actually deliver power. Are you saying that the HOT is actually somehow made up of two wires? If not, then I don't understand how one wire can provide both paths for the current to switch back and forth (alternate). In the link that you posted, the AC is produced by sending current out one side of the source and then sending the current out the other side for equal amounts of time. However, in order to do that the neutral wire would need to read above 0V at some point.
 
  • #4
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However, in order to do that the neutral wire would need to read above 0V at some point.
Why?

How do you "read" 0V on a single wire?
 
  • #5
Why?

How do you "read" 0V on a single wire?
Read the potential between a point on the wire an ground? I'm saying that it would have to read above 0V (if the AC produces a current in the neutral wire) the same way the wire in the voltage in the HOT wire reads above 0V when the AC produces a current in it.
 
  • #6
meBigGuy
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not exactly sure where your confusion lies.

There are two lines, hot and neutral. Neutral is, by agreement, defined as zero volts. Hot's voltage is always measured relative to neutral. Current flows through the load between Hot and Neutral due to the voltage on Hot (relative to neutral). The voltage on Hot varies at 60Hz (in US) between positive and negative voltage, which causes the current between hot and neutral to also vary between positive and negative. The RMS value of the voltage is 120V, which means for a sine wave the peaks are +170 and -170 volts. So, the voltage on HOT (relative to neutral) is changing between +170 and -170 volts at 60Hz, in the shape of a sine wave.
 
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  • #7
Yupp I understand everything you're saying and I agree. I guess my question is: How is it possible for the voltage to oscillate on a single line of wire? I thought the entire idea behind alternating current is you have current going in two directions, and I'm just confused how that's possible with one wire. If the current reverses it's direction solely in the hot wire, wouldn't it be drawing current away from the load at some points?
 
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  • #8
755
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Read the potential between a point on the wire an ground? I'm saying that it would have to read above 0V (if the AC produces a current in the neutral wire) the same way the wire in the voltage in the HOT wire reads above 0V when the AC produces a current in it.
Why would you expect the potential between a point on a wire and ground to change if there is no current flowing to/from this wire and ground?
 
  • #9
Right and I agree! So my question is: How is it possible for the voltage to oscillate on a single line of wire? I thought the entire idea behind alternating current is you have current going in two directions, and I'm just confused how that's possible with one wire. If the current reverses it's direction solely in the hot wire, wouldn't it be drawing current away from the load at some points?
 
  • #10
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Right and I agree! So my question is: How is it possible for the voltage to oscillate on a single line of wire? I thought the entire idea behind alternating current is you have current going in two directions, and I'm just confused how that's possible with one wire. If the current reverses it's direction solely in the hot wire, wouldn't it be drawing current away from the load at some points?
Jason,
Again, voltage is not defined on a single wire. I think this is where you are getting confused. Current is a different story, I can look at a single piece of wire and declare that it has 1A flowing through it.
Voltage is potential difference between two conductors. When you say "voltage oscillates" you need to think "voltage between A and B oscillates" with A and B well defined.
 
  • #11
I understand that voltage is defined by two points, and in this case the voltage in the hot wire is being measured with respect to the neutral wire (which is grounded). So then to clean up my question, I am asking: What is physically happening when the voltage between the hot wire and the neutral wire is oscillating? Is there any current being drawn through the neutral wire, and if so, how is it constantly being held at 0V then? And if there is no current being drawn through the neutral, then how is the voltage between the hot wire and the neutral wire being changed from positive to negative at all? Shouldn't it just be one polarity?
 
  • #12
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Is there any current being drawn through the neutral wire,
Yes, there is equal and opposite current flowing on hot and neutral. See Kirchhoff's current law.

...and if so, how is it constantly being held at 0V then?
It is not "held" at 0V. It is defined to be 0V. Or you can say that ground is defined to be 0V and neutral is bonded to it. Because voltages are relative, we can assign 0V any way we want. I assign 0V to my PCB ground planes even though I know they are not equipotential to earth ground. You can consider the negative electrode of the battery in your flashlight to be 0V even though it has no relationship to earth whatsoever.
 
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  • #13
Okay got it. Just one last question. The last part I am confused about is just that I read online that in many homes, the neutral wire is actually connected to ground. In that case, does the equal and opposite current still apply?
 
  • #14
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Your problem lays in fact that you do not understand what voltage is.
First the circuit with Battery, as you should know each battery has two metal terminals. One is marked positive (+), the other negative (-).
And to measure the voltage we need two point in the space. One of this point is treat as a reference point (GND). We have a very similar situation when we try to measure a height of an object. We need a reference point. The most common reference point is "above mean sea level". But when you measure the height of your kitchen table in your house you pick the floor as a your reference point.
Try read this
http://www.ittc.ku.edu/~jstiles/312/handouts/312_Introduction_package.pdf (start from page 3).

Here you have some examples
rys0.1.JPG

0.1.PNG

As you can see on the second example. If our ground is point A ( we connect the black test lead of a voltmeter to point A ) we can measure all the voltage in our circuit.
For example the voltage at point B is 3 volts higher than the voltage at point A. Voltage at point E is 12 volts higher than the voltage at point A.
But if we change the position of a ground in the circuit. For example to point C, we will have this situation:
Voltage at point B is negative because now the voltage at point B is minus 3 volts lower than the voltage at point B (GND).

As for the AC current we simply measure all the voltage with respect to the neutral wire (neutral wire is our reference point).
And voltage continuously change his value. Form 0V to 120Vrms then from 120Vrms back to 0V. And again from 0V to -120Vrms and from -120Vrms back again to 0V. And start all over again, the wave repeats it self every 16.7ms/ 60Hz.
prąd.PNG
 
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  • #15
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The last part I am confused about is just that I read online that in many homes, the neutral wire is actually connected to ground. In that case, does the equal and opposite current still apply?
"the equal and opposite current still apply"?? What are you trying to say here ? Current always flow within the closed loop. So connecting one of terminal of a voltage source to actually ground do not change anything. We do it because of a safety reasons. But for the current nothing has change (current must return to the source). Still all the current for the positive half cycle (show in RED) is "send" by a voltage source via a hot wire must return to the voltage source via neutral wire. But for negative halt cycle (show in green) the "neutral" wire is now "sending" the current to the load and current "returns" via a hot wire.
 
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  • #16
sophiecentaur
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The word "hot" is not very helpful - although I can see why it's used - it is only relevant for some safety reasons. Fact is that you need two paths between generator and load and they are often 'floating', rather than one of them being near Earth potential. The Neutral is not really a "return path" for the current; it is just one of a pair of conductors. As far as the functioning of appliances is concerned, the absolute potentials (wrt Earth) of the terminals is irrelevant. It's the Potential Difference that causes current to flow through them. This PD (difference) will alternate from positive to negative, in an AC circuit but the circuit could, if you wanted, be operating at an absolute potential of many hundreds of volts (filament supplies in some transmitters and some measuring equipment works like this)

Also, the notion of a current loop is a bit too simplistically treated in this thread. It is quite possible for current to be shared between Earth and Neutral conductors in some fault conditions. All that counts is for Kirchoff 1 to be obeyed at any junction in the circuit.
 
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  • #17
meBigGuy
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The current flows out of HOT and into NEUTRAL (through the load) while the voltage on HOT is POSITIVE. When the voltage on HOT momentarily goes to zero there is no current flow. Then, when the voltage on HOT goes negative the current flows out of NEUTRAL and into HOT (through the load). The current can flow into or out of NEUTRAL (through the load) depending on whether HOT is positive or negative voltage. The current can flow in either direction through the load. In almost all cases it doesn't matter to the load which way the current flows, which is why it can accept AC voltage directly, like a toaster or a filament light bulb. (we will skip the exceptions for now).
 
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  • #18
jim hardy
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Sophie - Glad to see you back ! :smile:
 
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  • #19
dlgoff
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... hot wire changes it's polarity...
Just to make sure you (and other readers) don't get voltage "polarity" confused with "polarized" receptacles from the Hyperphysics link you provided, the other picture shows the neutral plug blade which is wider than the hot plug blade; hence it's polarized.

recept.gif
 

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