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

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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

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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.
 

marconi

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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.
 
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marconi

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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.
 
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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 ra

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.
 
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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?
 

marconi

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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:

Radartutorial - https://www.radartutorial.eu/18.explanations/ex19.en.html
 
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marconi

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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
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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:

Radartutorial - https://www.radartutorial.eu/18.explanations/ex19.en.html
Superb answer, thank you. Very insigntful.
I loved also the sources you referenced to enrich your answer
 
OP
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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

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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.

 

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