Do electrons flow in an electrical circuit?

[sevensages]

The video on youtube on the above link seems to imply that electrons don't flow through a circuit. At 01:29 into the video, the narrator says that he used to teach his classes how electrons move through a circuit by using a clear plastic tube with a chain inside of the clear plastic tube. The narrator would say that power lines are like the clear plastic tubing. He had a chain inside of the clear plastic tubing. He said that the chain was like the electrons in the wire of the power line. He moved the chain back and forth, without moving any link in the chain throughout the entire circuit. Before I watched this youtube video, this was how I understand electrons to travel through a circuit. However, the narrator said that this is wrong. In other words, the narrator used to teach his classes how electrons move through a circuit in a way that was incorrect.

My understanding of the video is that the narrator is saying that electrons don't move in an electromagnetic circuit.

My father says that an electromagnetic field cannot be generated without having electrons moving around in the wire. If the electrons don't move in a circuit, how can an electromagnetic field be generated?

 

[berkeman]

Please be careful not to put too much trust into simplified analogies of how electricity and electronics work.

The key term for you to do a Google search on is the drift velocity of electrons in circuits. In an AC circuit, the electrons indeed do not stray very far from their quiescent locales, since their motion is back and forth (over pretty short distances). In DC circuits of course electrons move through the circuit, albeit a lot more slowly than you would expect until you calculate their drift velocity.

 

[dlgoff]

Please be careful not to put too much trust into simplified analogies of how electricity and electronics work.

The key term for you to do a Google search on is the drift velocity of electrons in circuits. In an AC circuit, the electrons indeed do not stray very far from their quiescent locales, since their motion is back and forth (over pretty short distances). In DC circuits of course electrons move through the circuit, albeit a lot more slowly than you would expect until you calculate their drift velocity.

Searching on Microscopic View of Electric Current gives this fine explanation from hyperphysics:
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/miccur.html

 

[256bits]

My understanding of the video is that the narrator is saying that electrons don't move in an electromagnetic circuit.

That is not what to take home from the video.
It is about power transfer - from a pure physics explanation the field is the explanation, not the electrons themselves.
In most electrical circuits, it is modeled as if the current is doing the 'work,' and the field is paid scant attention.

Ask yourself, if the electrons do not move, how do capacitors become polarized.
How does the phospher on a crt screen become exited so that you see a picture.

 

[dlgoff]

It is about power transfer - from a pure physics explanation the field is the explanation, not the electrons themselves.

Yes. From my hyperphysics link above:

...the actual drift velocity of electrons through copper wires is very slow. It is the change or "signal" which propagates along wires at essentially the speed of light. ...

 

[DaveE]

My understanding of the video is that the narrator is saying that electrons don't move in an electromagnetic circuit.

He didn't say they don't move. He said they don't have to move very far. I thought that was pretty clear with his chain analogy. Watch it again. Another analogy is waves in the ocean. A wave can travel from Hawaii to California, but each water molecule doesn't move very far at all. BTW, all analogies are wrong at some level, that's why they are called analogies instead of theories.

However, that is for circuits, with wires and such. You'll also want to think about how radio waves transfer energy across the vacuum of space, where there are essentially no charged particles. Yes, moving charges created those waves, but they don't have to stay with the waves forever (or at all). You can have e-fields and h-fields without charges. But charged particles, if present, will react to EM fields they experience.

 

[vanhees71]

NB: The drift velocity in a copper wire for typical houshold currents is on the order of 1mm/s only. The energy flow, given by the Poynting vector is due to the em. field.

As usual, the best treatment about the (apparently!) simple case of a DC through a coax cable is given in Sommerfeld, Lectures on Theoretical Physics, vol. 3 (electrodynamics).

 

[sevensages]

He didn't say they don't move. He said they don't have to move very far. I thought that was pretty clear with his chain analogy. Watch it again. Another analogy is waves in the ocean. A wave can travel from Hawaii to California, but each water molecule doesn't move very far at all. BTW, all analogies are wrong at some level, that's why they are called analogies instead of theories.

The links of his chain did not move very far in his chain analogy. He just shifted the chain back and forth about two or three inches. Each link of the chain did not move throughout the entire circuit. So he didn't move the chain very far, yet he still said that the chain analogy is wrong. If the electrons only move very slightly, how is his chain analogy wrong?

 

[sevensages]

In an AC circuit, the electrons indeed do not stray very far from their quiescent locales, since their motion is back and forth (over pretty short distances).

In his chain analogy, he didn't move the chain very far. He only moved the chain about two or three inches in each direction. Then he said that the chain analogy is wrong. Well, if he only moved the chain about two or three inches in his chain analogy, then how is his chain analogy wrong?

In DC circuits of course electrons move through the circuit, albeit a lot more slowly than you would expect until you calculate their drift velocity.

 

[berkeman]

Well, if he only moved the chain about two or three inches in his chain analogy, then how is his chain analogy wrong?

It's wrong only in amplitude, unless he was demonstrating huge AC currents. When you Google displacement current like I suggested and look at the numbers, you will see that the movement of the electrons in a 60Hz AC mains circuit is much less than a few inches per cycle.

 

[A.T.]

how is his chain analogy wrong?

The point of the video is how energy is propagating. This is where the chain analogy breaks down.

 

[sevensages]

It's wrong only in amplitude, unless he was demonstrating huge AC currents. When you Google displacement current like I suggested and look at the numbers, you will see that the movement of the electrons in a 60Hz AC mains circuit is much less than a few inches per cycle.

I googled displacement current like you suggested, and I could not tell the answer from what I saw on google.

If the narrator of the video moved the chain only about a milimeter (or maybe only a nanometer), would the chain analogy be a lot more accurate?

 

[berkeman]

I googled displacement current like you suggested, and I could not tell the answer from what I saw on google.

https://www.uu.edu/dept/physics/scienceguys/2001Nov.cfm

If the narrator of the video moved the chain only about a milimeter (or maybe only a nanometer), would the chain analogy be a lot more accurate?

Yes, much more accurate (see above).

 

[vanhees71]

It's wrong only in amplitude, unless he was demonstrating huge AC currents. When you Google displacement current like I suggested and look at the numbers, you will see that the movement of the electrons in a 60Hz AC mains circuit is much less than a few inches per cycle.

What has "displacement current" to do with the drift velocity of electrons in a wire? Nothing! Displacement current is an old-fashioned name for a term in the Ampere-Maxwell Law, which has been written as (in Heaviside-Lorentz units)
$$\mathrm{\nabla} \times \vec{B}=\frac{1}{c} \vec{j} +\frac{1}{c} \partial_t \vec{E}.$$
The last term is, even today, still called "displacement current". It originates from Maxwell's original ideas using mechanical models to "derive" his famous equations. Today, more than 160 years later, we know a lot more about the structure of Maxwell's equations. Most notably the discovery that it is a relativistic field theory, has taught us to write the equation as
$$\mathrm{\nabla} \times \vec{B} - \frac{1}{c} \partial_t \vec{E}=\frac{1}{c} \vec{j},$$
with the current density (i.e., moving electric charges) as the only source of the electromagnetic field, $(\vec{E},\vec{B})$.

For household AC the displacement current can very often be neglected, at least as long as we deal with phenomena in the "near area" around the circuit, i.e., in a region of extension smaller than the wave-length, $\lambda=c/f \simeq 5\cdot 10^6 \text{m}$ :-)).

 

[berkeman]

What has "displacement current" to do with the drift velocity of electrons in a wire?

You are right, of course. I someplace used the incorrect term and it kept propagating. It should have been drift current, not displacement current. Thanks for the correction!

https://en.wikipedia.org/wiki/Drift_current