# A different angle of Why do you need a line 2 on an AC circuit?

• timmeister37
In summary, this conversation is about the need for a line 2 on an AC circuit to bring current back to the power source. The reason for this is because current needs to enter and leave the power source simultaneously, similar to how a bicycle chain works. The diagram provided shows that it is not possible to have the current flow in both directions on the same wire. There is no way to make current circulate in a single wire moving in both directions at the same time.
timmeister37
TL;DR Summary
This is looking at why one needs a line 2 on an AC circuit from a different angle. Why cannot the entire bicycle chain be on line 1?
Preface to thread: I am darn well aware of the fact that on an AC circuit, there has to be a line 2 after the load to bring current back to the power source. This thread is about WHY does there have to be a line 2 after the load to bring current back to the power source. I hope that I don't get a bunch of people acting like they are teaching me something new and tell me "There has to be a line 2 to bring current back to the power source." Darn it. I said that in the preface of the OP!

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Back in March of this year, I created a thread titled "Why do you need a line 2 on an alternating current circuit. PF member anorlunda gave me the excellent answer that one has to have a line 2 on an alternating current circuit because of the following: "Current does not leave the power source on line 1 unless the same amount of current returns to the power source instanteously.

On that post #17 of that thread back in March, PF member phinds gave me the following excellent analogy: "Think of current as a bicycle chain. It either all moves or none of it moves."

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The reason for the creation of this thread is that I want to know why you cannot have the line 2 on the exact same wire as line 1. Below this sentence is Diagram 2:

The switch on diagram 2 is closed. The green wire on Diagram 2 is just ONE WIRE. The green wire on Diagram 2 only has one prong that plugs into an electrical outlet not two. The orange line on Diagram 2 is NOT A WIRE. The orange line on Diagram 2 is the current. The green wire on Diagram 2 is not one cord with two internal wires. The green wire on Diagram 2 is JUST ONE WIRE.
Anorlunda told me that in a circuit, current has to both leave the power source and enter the power source simultaneously. Phinds told me that current travels in a circuit like a bicycle chain, either it all moves or none of it moves. My diagram 2 follows both what anorlunda told me and what phinds told me about circuits. Why cannot the current flow in a circle (like a bicycle chain) both to the power source and away from the power source and to the load ON THE SAME WIRE as I depicted in diagram 2?

A standard electrical wire is a three-dimensional object, not two dimensional. I don't see why there is not room for current to flow in different directions on the same wire.

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P.S. when i first created this diagram 2, the line that represented the current was red. Now, for some unknown reason the line changed from red to orange. Therefore, I edited this to say orange instead of red.

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davenn
Such is the danger of "sorta like" analogies. The electrons are in fact not really like a chain (or a group of trunk- to-tail circus elephants)
There is no inducement for the electrons to travel down the wire and back...they will simply take the short route and bypass the wire completely. You do understand that the electrons are coming and going at the same instant? The power supply will see a short circuit and the lamp will see nothing at all...

timmeister37
timmeister37 said:
Summary:: This is looking at why one needs a line 2 on an AC circuit from a different angle. Why cannot the entire bicycle chain be on line 1?

The switch on diagram 2 is closed. The green wire on Diagram 2 is just ONE WIRE. The green wire on Diagram 2 only has one prong that plugs into an electrical outlet not two. The red line on Diagram 2 is NOT A WIRE. The red line on Diagram 2 is the current. The green wire on Diagram 2 is not one cord with two internal wires. The green wire on Diagram 2 is JUST ONE WIRE.
Look at your figure. You have drawn one current upwards and one downwards, so they are going in the opposite direction. Since they are of the same magnitude and in the opposite direction, the net current is - zero.

Dale and davenn
hutchphd said:
There is no inducement for the electrons to travel down the wire and back.
That's the answer. What could you do to the single wire to make current circulate in that wire moving both directions at the same time? The answer is there is none.

[Note: in case you read about eddy currents; no they are not the answer either.]

etotheipi, timmeister37 and russ_watters
Svein said:
Look at your figure. You have drawn one current upwards and one downwards, so they are going in the opposite direction. Since they are of the same magnitude and in the opposite direction, the net current is - zero.

Is what Svein what correct? I think that what Svein wrote is incorrect.

On an AC circuit with a line 1 and a line 2, the line 1 current is traveling in the opposite direction as the line 2 current, with line 1 traveling from the power source to the load and with line 2 traveling from the load back to the power source, yet the current will still light up the light bulb.

davenn
anorlunda said:
That's the answer. What could you do to the single wire to make current circulate in that wire moving both directions at the same time? The answer is there is none.

[Note: in case you read about eddy currents; no they are not the answer either.]

Let's pretend that the switch is closed in diagram 1. What induces the current in diagram 1 to travel in line 1 from the power source to the load and in line 2 from the load and back to the power source?

timmeister37 said:
View attachment 266499

Let's pretend that the switch is closed in diagram 1. What induces the current in diagram 1 to travel in line 1 from the power source to the load and in line 2 from the load and back to the power source?

The power source provides the inducement. Usually in the form of a voltage. It is critical to note that the power source has two terminals, not one. In your post #1, you sketched a power source with only one terminal.

etotheipi
anorlunda said:
The power source provides the inducement. Usually in the form of a voltage. It is critical to note that the power source has two terminals, not one. In your post #1, you sketched a power source with only one terminal.

Would it be true to say that a circuit without a line 2 (like the circuit in the OP) would have no inducement because one line can only connect to one terminal (opposed to two terminals) on the power source?

anorlunda, please address what PF member Svein wrote in post #3? Is Svein correct?

I could, instead of writing the number ##0## always write it as $$(1-1)$$ or more provocatively $$(137-137)$$ It would be silly and it would still be zero. @Svein is correct that most people would call a wire with two opposing currents zero current. In most situations (unless otherwise noted) the wire is presumed of negligible size.

davenn and russ_watters
timmeister37 said:
Would it be true to say that a circuit without a line 2 (like the circuit in the OP) would have no inducement because one line can only connect to one terminal (opposed to two terminals) on the power source?
Rephrased;
The first thing that occurs in a “circuit” is that a voltage difference propagates from the source towards the load. A voltage difference requires two separate conductors, with an electric field between them.

How can a voltage difference travel in parallel up one wire, while that voltage difference is "short circuited" by that same wire?

etotheipi and berkeman
timmeister37 said:
I don't see why there is not room for current to flow in different directions on the same wire.

It isn't a matter of "room". It's a matter of what potential difference the electrons are responding to.

timmeister37 said:
Would it be true to say that a circuit without a line 2 (like the circuit in the OP) would have no inducement because one line can only connect to one terminal (opposed to two terminals) on the power source?

You're getting close here, yes. Think about what I said above about potential difference. Your statement that one line can only connect to one terminal on the power source, whereas two lines can connect to two different ones, is correct. Can you see why that makes a difference as far as the potential difference the electrons will respond to is concerned?

timmeister37 said:
Is Svein correct?

His statement is correct as far as it goes, but it doesn't fully address the issue. See above.

berkeman
hutchphd said:
I could, instead of writing the number ##0## always write it as $$(1-1)$$ or more provocatively $$(137-137)$$ It would be silly and it would still be zero. @Svein is correct that most people would call a wire with two opposing currents zero current. In most situations (unless otherwise noted) the wire is presumed of negligible size.

If one could magically induce a current through one wire (I know it would take magic to do it), I don't see why the light bulb would not light up. The current (magically) flowing through the light bulb would be exactly the same as the current not-magically flowing through the light bulb in a circuit with both a line 1 and a line 2.

I am confused.

davenn
For the same reason you can not drive your car both ways on a one lane road at the same time.You can not go both ways at the same time.

Dale
timmeister37 said:
If one could magically induce a current through one wire (I know it would take magic to do it), I don't see why the light bulb would not light up. The current (magically) flowing through the light bulb would be exactly the same as the current not-magically flowing through the light bulb in a circuit with both a line 1 and a line 2.

I am confused.
You are confused because magic doesn't work in reality?

You remember why your previous thread was locked and why you had a temporary vacation from the PF as a result. This thread is now locked temporarily. Send me a PM to explain in detail why this thread is different, and why it should remain open for discussion please. Thanks.

EDIT/Update -- After a long set of PMs (and separate conversations between the OP and @PeterDonis and @Dale the OP is finally understanding the situation and is happy with the result. Thread can stay closed. Thank you everybody for pitching it to help the OP understand this.

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timmeister37, davenn, etotheipi and 3 others

## 1. Why is line 2 needed on an AC circuit?

Line 2 is needed on an AC circuit because it provides a return path for the flow of electricity. In an AC circuit, the flow of electricity alternates between positive and negative directions, and line 2 allows for the completion of this alternating current.

## 2. Can't the circuit just work with one line?

No, a single line cannot provide a complete circuit for an AC circuit. Without a return path, the electricity would not be able to flow continuously and the circuit would not function properly.

## 3. What is the purpose of having two lines instead of one?

The use of two lines in an AC circuit allows for a more efficient and stable flow of electricity. It also helps to balance the load on the circuit and prevent overloading on a single line.

## 4. Is line 2 always necessary for an AC circuit?

In most cases, yes, line 2 is necessary for an AC circuit to function properly. However, there are some specialized circuits that may only require one line, but these are not common in household or commercial electrical systems.

## 5. Is line 2 the same as the neutral wire?

No, line 2 and the neutral wire are not the same. Line 2 carries the current from the power source, while the neutral wire carries the returning current back to the source. However, in some cases, line 2 and the neutral wire may be connected at the same point in the circuit.

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