# Drift Velocity in AC ?

Discuss Drift Velocity in AC ? in the Electrical Engineering Chat area at ElectriciansForums.net

#### Mark_Djorn

Welcome to ElectriciansForums.net - The American Electrical Advice Forum

Hi All

I hope this topic makes sense for this forum and somebody can help me to understand.

This is what I understand (please allow numerical approximations):
1. Electrons move in a conductor at a speed anywhere near the speed of light.
2. Electrons gain actual velocity (drift) only when supply is ON. Propagation of EM field is about 270.000 km/s.
3. For DC, drift velocity can be calculated.

Now, am I right to say that, in AC, actual average velocity is zero? That's because in a PERIOD, electrons move back and forth. I could calculate the drift in half a period, but in a period would be zero. Correct? This means there is no actual transmissions of electrons from source to load, right?

Under the assumption above, why we keep talking about "power transmission"? As a matter of fact, there is nothing moving from source to load. Electrons simply move back and forth while performing work.

Also, under the considerations above, can anyone explain the concept of "reactive power going back and forth from source to load" in cases where power factor is lesser than 1?

Thank you very much in advance!

#### marconi

##### -
Mentor
Esteemed
Arms
Dear Mark_Djorn: Hello from London. Where are you in Singapore so I can look up your town or city in my atlas?

Your question indicates you are very curious. Be prepared that the answer to your question ranges from A - the simple, to B the frontiers of physics. I can only take you part way between A and B. I have enjoyed pondering your question.

I think first of all one needs to think about what is an electric current?

My favourite physicist is Dr Feynman and this is how he has replied to this question:

Electric current (A flow of electrons or positive charges?) - http://feynman-answer.blogspot.com/2016/08/electric-current-flow-of-electrons-or.html

If this make sense, some sense or no sense then please reply to me.

DPG

#### marconi

##### -
Mentor
Esteemed
Arms
Now I think you should know about the electron sea model of metallic bonding - note how he explains localised and delocalised electrons:

Then you should read how electrons inside a metal are modelled. One early model was the Drude model. Here is a short lecture:

And here is an interactive tool where you can play with temperature and applied electric field to 'see'* electrons in the electron sea* drifting:

The Drude Model - https://pages.pomona.edu/~tmoore/drude/

* As an aside see and sea are homophones - words that sound the same but are spelt differently. It is also an example of alliteration - the see sound repeated.

Last edited:

#### marconi

##### -
Mentor
Esteemed
Arms
MArk_Djorn: I do not know if you are reading my posts so this will be the last unless you respond.

The thought for you today is that an electric current, composed of millions and millions and millions of electrons, is a large scale or macroscopic outcome by them all when they are in an electric field. When an electric field is present, the atomic scale ever present thermal motion of each electron is influenced by a force F = Ee where E is the field strength and e is the charge of an electron. Thus there is random motion caused by thermal motion and also an influence to move in the direction of the electric field. And on top of that there are random collisions with other electrons and nuclei. Thus we talk of a cloud or sea of electrons with a macroscopic net movement or drift - an electric current. Individual electrons move very very fast but the cloud very very slowly.

Last edited:

#### DPG

##### -
Esteemed
Arms
Patron
Doesn't look like he's been on here since the 6th of May.

OP
M

#### Mark_Djorn

Dear Mark_Djorn: Hello from London. Where are you in Singapore so I can look up your town or city in my atlas?

If this make sense, some sense or no sense then please reply to me.
Dear Marconi,

Thank you for your first post. I stay on the east side, Changi.

I had chance to go through the article and, despite it helped to clear the concept of electric current, it did not provide information on the actual drift velocity of electrons in AC.

OP
M

#### Mark_Djorn

MArk_Djorn: I do not know if you are reading my posts so this will be the last unless you respond.

The thought for you today is that an electric current, composed of millions and millions and millions of electrons, is a large scale or macroscopic outcome by them all when they are in an electric field. When an electric field is present, the atomic scale ever present thermal motion of each electron is influenced by a force F = Ee where E is the field strength and e is the charge of an electron. Thus there is random motion caused by thermal motion and also an influence to move in the direction of the electric field. And on top of that there are random collisions with other electrons and nuclei. Thus we talk of a cloud or sea of electrons with a macroscopic net movement or drift - an electric current. Individual electrons move very very fast but the cloud very very slowly.
Ok, let me rephrase my question.
In DC, electrons move and their velocity is called Drift. In DC they move (in average) in one direction.
In AC, they move of a certain space in one direction, and of the same same in the opposite direction.
Hence the drift velocity is zero.
Correct?

OP
M

#### DPG

##### -
Esteemed
Arms
Patron

People were responding with information but you weren't replying. It seemed like you maybe weren't going to come back to the forum.

OP
M

#### Mark_Djorn

People were responding with information but you weren't replying. It seemed like you maybe weren't going to come back to the forum.
You are out of topic

#### DPG

##### -
Esteemed
Arms
Patron
You are out of topic

I was just letting Marconi know that it may be worth making sure the OP was still checking the forum before responding in great detail. Marconi puts a lot of time and effort into his replies and I wouldn't have wanted it wasted.

No offence intended.

Last edited:

#### marconi

##### -
Mentor
Esteemed
Arms
Ok, let me rephrase my question.
In DC, electrons move and their velocity is called Drift. In DC they move (in average) in one direction.
In AC, they move of a certain space in one direction, and of the same same in the opposite direction.
Hence the drift velocity is zero.
Correct?
Not quite correct. The time averaged drift of each electron over all electrons in the cloud in the electric field is zero. Thus the cloud of electrons has a net drift velocity of zero. The electrons have no net displacement along the axis of the electric field when the interval over which the average is taken longer than the period of the ac. They displace as much in one direction of the half cycle as they displace in the opposite direction during the alternate half cycle. Note that during a half cycle the drift velocity increases and decreases as the electric field increases and decreases.

The drift velocity also varies in a unidirectional electric field as in the dc case if the field strength increases or decreases over time.

Thus, we can use an electric field whose strength is a function of time to speed up or slow down the electrons. This is called velocity modulation. The effect is for electrons in a stream to bunch up or space out along their axis of travel. Velocity modulation of electrons caused by changing electric fields is the way microwave electronic tubes work. See:

Last edited:
DPG

#### marconi

##### -
Mentor
Esteemed
Arms
I used the word displacement deliberately because even though the net displacement is zero the electrons never stop moving and will have travelled some distance during the time interval. Zero average drift velocity does not mean the electrons are stationary.

See

Displacement and velocity - Higher - Types of motion - CCEA - GCSE Physics (Single Science) Revision - CCEA - BBC Bitesize - https://www.bbc.co.uk/bitesize/guides/zhxh47h/revision/5

https://byjus.com/physics/distance-and-displacement/

Forgive me if you already knew this.

OP
M

#### Mark_Djorn

Not quite correct. The time averaged drift of each electron over all electrons in the cloud in the electric field is zero. Thus the cloud of electrons has a net drift velocity of zero. The electrons have no net displacement along the axis of the electric field when the interval over which the average is taken longer than the period of the ac. They displace as much in one direction of the half cycle as they displace in the opposite direction during the alternate half cycle. Note that during a half cycle the drift velocity increases and decreases as the electric field increases and decreases.

The drift velocity also varies in a unidirectional electric field as in the dc case if the field strength increases or decreases over time.

Thus, we can use an electric field whose strength is a function of time to speed up or slow down the electrons. This is called velocity modulation. The effect is for electrons in a stream to bunch up or space out along their axis of travel. Velocity modulation of electrons caused by changing electric fields is the way microwave electronic tubes work. See:

Superb answer, thank you. Very insigntful.

DPG
OP
M

#### Mark_Djorn

I used the word displacement deliberately because even though the net displacement is zero the electrons never stop moving and will have travelled some distance during the time interval. Zero average drift velocity does not mean the electrons are stationary.

See

Displacement and velocity - Higher - Types of motion - CCEA - GCSE Physics (Single Science) Revision - CCEA - BBC Bitesize - https://www.bbc.co.uk/bitesize/guides/zhxh47h/revision/5

https://byjus.com/physics/distance-and-displacement/

Forgive me if you already knew this.
Totally clear. They move at high speed but with no net drft velocity

#### marconi

##### -
Mentor
Esteemed
Arms
We often talk about the flow of current in a conductor and make an analogy with the flow of a fluid such as water in a pipe. I thought you would like to listen to this short physics tutorial which has some important remarks about the differences between water flow and current flow. Note well his remarks about an electron being a quantum entity.

Reply to Drift Velocity in AC ? in the Electrical Engineering Chat area at ElectriciansForums.net

Hi All, I am currently learning about AC and DC supplied coil and have been given the scenario below. I understand the first few steps but then...
Replies
1
Views
245
Hi All, During my exercises on paper, we always assume the source from grid has Tension and Currentnin phase. Today, for the first time, I was...
Replies
5
Views
742
So, for a bit of completely pointless fun really, does this antique looking item that was acquired entirely for novelty/aesthetics value work...
Replies
5
Views
1K
Hi I am helping with some village lights and need some advice as to a route forward (our electrician is unavailable for an undefined period) We...
Replies
6
Views
878
I have a Samlex 600 Watt PSW AC inverter. The inverter connects to the battery source by way of the positive and negative clamps from the back of...
Replies
12
Views
960