Understanding the Flow of Electrons in AC Circuits

[Wrichik Basu]

In DC, we have learned that the electrons move from their position through the conductor, resulting in a current. They leave their positions on the application of a potential difference, and move or flow in the direction of higher to lower potential, thereby causing a current.

But what about AC? Say the frequency is 50Hz. What is altering in AC? It is said that the sine graph of AC is positive for 50 times and negative for 50 times in a second. It is also said that the direction of AC keeps changing. Does this mean that the live will become the neutral in our house wires? But that doesn't happen. Then what is the thing that changes direction 50 times a second? Is their actually any flow of electrons in AC, or the electrons just vibrate about their mean positions, resulting in a current? If so, how can a current result from harmonic vibrations of electrons?

 

[FactChecker]

These are not "vibrations of electrons". 50 Hz gives enough time for electrons to move very far back and forth between atoms. The frequencies to vibrate atoms is much higher. Water resonates at about 22 GHz.

 

[Wrichik Basu]

These are not "vibrations of electrons". 50 Hz gives enough time for electrons to move very far back and forth between atoms.

Can you elaborate a bit on this part? What does back and forth in a conductor mean?

 

[FactChecker]

50 Hz gives you current in one direction for a long time before it switches to the other direction. So however you want to visualize electric current, you can consider 50 Hz as being DC for a long time in one direction, then DC for a long time in the other direction.

PS. I am using "long time" as a relative term. Relative to KHz or GHz.

 

[Wrichik Basu]

50 Hz gives you current in one direction for a long time before it switches to the other direction. So however you want to visualize electric current, you can consider 50 Hz as being DC for a long time in one direction, then DC for a long time in the other direction.

How does the direction change physically? If it was flowing from A to B (say) at first, does it now flow from B to A after a change? If the direction changes completely, then shouldn't the live wire in our household become a neutral wire and vice-versa?

 

[FactChecker]

How does the direction change physically? If it was flowing from A to B (say) at first, does it now flow from B to A after a change?

Yes.

If the direction changes completely, then shouldn't the live wire in our household become a neutral wire and vice-versa?

Well, it averages out to neutral, but it is only 0 voltage for the instant that it is switching between + and -. All other times, it is hot. (The voltage is really a sine wave so the voltage does decrease smoothly to 0 when it is switching rather than making a sudden jump between positive and negative.)

 

[Wrichik Basu]

Well, it averages out to neutral, but it is only 0 voltage for the instant that it is switching between + and -. All other times, it is hot. (The voltage is really a sine wave so the voltage does decrease smoothly to 0 when it is switching rather than making a sudden jump between positive and negative.)

If it averages out to neutral, and if current actually flows from both live and neutral wires, then why do we get a shock on touching the live and not the neutral, as the neutral should have a current as the direction of flow of electrons changes physically?

 

[CWatters]

But what about AC? Say the frequency is 50Hz. What is altering in AC?

AC stands for Alternating Current but actually both the voltage and the current is alternating polarity/sine.

It is said that the sine graph of AC is positive for 50 times and negative for 50 times in a second.

Correct. The voltage of one wire relative to the other does change polarity (sine) 50 times a second.

Does this mean that the live will become the neutral in our house wires?

In some countries the two wires dangerous wires (for want of a better term) are called Live and Neutral. The Neutral is typically but not always connected to Earth somewhere but for now let's consider it to be our reference voltage (0V). The voltage on the live wire relative to 0V will change sign so it goes positive and negative. In the UK where the average voltage is 240V it actually goes down to about -330V and upto +330V fifty times a second. That causes the current to change direction fifty times a second.

 

[Wrichik Basu]

In some countries the two wires dangerous wires (for want of a better term) are called Live and Neutral. The Neutral is typically but not always connected to Earth somewhere but for now let's consider it to be our reference voltage (0V). The voltage on the live wire relative to 0V will change sign so it goes positive and negative. In the UK where the average voltage is 240V it actually goes down to about -330V and upto +330V fifty times a second. That causes the current to change direction fifty times a second.

Doesn't a negative value of voltage indicate a physical change in direction of flow of current? In that case, current should now flow through the neutral wire to the device and then back through the live wire. How does connecting the neutral to Earth allow such a case?

 

[CWatters]

If it averages out to neutral, and if current actually flows from both live and neutral wires, then why do we get a shock on touching the live and not the neutral, as the neutral should have a current as the direction of flow of electrons changes physically?

As I said above the Neutral is frequently connected to Earth (0V). However for safety reasons you should always treat the neutral wire as dangerous because one day there might be a fault and can become dangerous.

Doesn't a negative value of voltage indicate a physical change in direction of flow of current? In that case, current should now flow through the neutral wire to the device and then back through the live wire.

Correct.

How does connecting the neutral to Earth allow such ans case?

0V is more positive than say -100V.

Consider a 9V DC battery. If you connect the +ve terminal to Earth (0V). Then the -ve terminal becomes -9V. It will still work just fine.

 

[CWatters]

Going for lunch. Back later.

 

[FactChecker]

A neutral or ground wire can be kept at the same potential as the ground (and your body). So you should never get a shock from it no matter what the voltage of the AC hot wire is. (Please don't take this too literally. If you don't wire things correctly, you can get killed by a neutral wire.)

 

[Wrichik Basu]

@CWatters then as you are saying, the neutral at both the dynamo station and at our homes are connected to the earth. When there is a negative voltage, then the current reverses direction and flows through the earth, through the neutral, device, and back to live?

 

[Wrichik Basu]

A neutral or ground wire can be kept at the same potential as the ground (and your body). So you should never get a shock from it no matter what the voltage of the AC hot wire is. (Please don't take this too literally. If you don't wire things correctly, you can get killed by a neutral wire.)

I understood the concept of earthing it, but why should we Earth it? Can't it be carried back to the dynamo station, like the phase wires are brought to our homes?

 

[FactChecker]

I understood the concept of earthing it, but why should we Earth it? Can't it be carried back to the dynamo station, like the phase wires are brought to our homes?

It can be done either way and different countries do things differently. A return voltage wire requires twice as much wire.

 

[Wrichik Basu]

@FactChecker then as you are saying, the neutral at both the dynamo station and at our homes are connected to the earth. When there is a negative voltage, then the current reverses direction and flows through the earth, through the neutral, device, and back through live?

 

[jim mcnamara]

Here is an explanation that might help -- I hope:
https://www.uu.edu/dept/physics/scienceguys/2001Nov.cfm

 

[Wrichik Basu]

Here is an explanation that might help -- I hope:
https://www.uu.edu/dept/physics/scienceguys/2001Nov.cfm

It doesn't address my query regarding change in direction of current after half cycle.

 

[jim mcnamara]

It is what happens when:
1. you flip a switch
2. current reverses

 

[CWatters]

There are different ways to model the behaviour of electrons in a wire depending on how much physics you have studied. One model is to think of electrons like a cloud of bees that wiz about in random directions. When a voltage is applied the cloud drifts along in one direction but the bees still wiz about in random directions within it. When the voltage is reversed the direction of the cloud drifts in the other direction. The speed with which the cloud drifts is very slow, typically only a quarter of a millimetre per second. So in 1/50th of a second they don't drift very far.

People then ask how come when I switch on a light it turns on virtually instantly? To explain that consider a garden hose that is already full of water. When the tap/faucet injects a little more water all the water in the pipe moves and excess water comes out of the other end virtually instantly.

These models aren't the best but until you learn about the quantum theory of the atom they are good enough.

 

[CWatters]

@FactChecker then as you are saying, the neutral at both the dynamo station and at our homes are connected to the earth. When there is a negative voltage, then the current reverses direction and flows through the earth, through the neutral, device, and back through live?

It's a bit more complicated. Most houses are connected to a local transformer so it looks more like...

For 1/50th of a second current flows...
Local transformer -> Live -> Appliance -> Neutral -> Local transformer

For the other 1/50th of a second...
Local transformer -> Neutral -> Appliance -> Live Local transformer

What happens between the power station and the transformer is more complicated and I would suggest we don't get into that at the moment.

 

[Wrichik Basu]

For the other 1/50th of a second...
Local transformer -> Neutral -> Appliance -> Live Local transformer

Fully understood everything. That's what I wanted to know. But tell me something: why don't I get a shock or the line tester doesn't respond when touched to the neutral wire of a running appliance, because in the second half cycle, the neutral "behaves" like the live, having a current in it.

 

[Nugatory]

Fully understood everything. That's what I wanted to know. But tell me something: why don't I get a shock or the line tester doesn't respond when touched to the neutral wire of a running appliance?

Because there is no potential difference between the neutral and the earth; the neutral is tied to ground at some point near you, never farther away than the local transformer. Thus, the neutral stays at zero relative to the ground while the live wire's potential varies sinusoidally from positive to negative relative to ground.

If something goes wrong with that the local connection between neutral and ground, the neutral will develop a voltage relative to ground and then it will shock you (and cause other strange things to happen throughout the system). These "loose neutral" situations are very dangerous and properly done installations go to great lengths to avoid any possibility of such failure.

 

[Wrichik Basu]

Because there is no potential difference between the neutral and the earth; the neutral is tied to ground at some point near you, never farther away than the local transformer. Thus, the neutral stays at zero relative to the ground while the live wire's potential varies sinusoidally from positive to negative relative to ground.

If something goes wrong with that the local connection between neutral and ground, the neutral will develop a voltage relative to ground and then it will shock you (and cause other strange things to happen throughout the system). These "loose neutral" situations are very dangerous and properly done installations go to great lengths to avoid any possibility of such failure.

OK, but as @CWatters mentioned, why is their a current in the second half cycle? Is it because there is a potential difference between the transformer and the appliance, due to which there is a current in the neutral wire, even though it is at no potential difference? But how is that possible?

Or is it something like this: as the potential in the live becomes negative, and Earth is always at 0V, and as 0>-ve, a current flows from Earth to neutral, through the appliance, and then through the live. But if I touch the neutral, then I'm already earthed, so I do not get any shock.

Please confirm if this is the correct explanation.

 

[CWatters]

Ok so the Neutral stays at 0V all the time. The live alternates between +330V and -330V. Current flows from which ever is the more positive at that time to the more negative.

In the second half cycle 0V is more positive than -330V so current flows from 0V (neutral) to -330V (live).

 

[Wrichik Basu]

Ok so the Neutral stays at 0V all the time. The live alternates between +330V and -330V. Current flows from which ever is the more positive at that time to the more negative.

In the second half cycle 0V is more positive than -330V so current flows from 0V (neutral) to -330V (live).

Yes, as I mentioned just above your post in a reply to @Nugatory. Thank you a lot, everyone. Problem solved. I've at last understood after a long time of trouble.