8 ms·

A point not mentioned by the article: the electrons in a metal at room temperature are already moving very quickly due to their thermal energy (at the order of

A point not mentioned by the article: the electrons in a metal at room temperature are already moving very quickly due to their thermal energy (at the order of 100km/s) — much faster than the speeds quoted in the article, which is what's called the "drift velocity". This thermal motion is essentially random, and the electrons constantly scatter off the nuclei every which way, so it cancels out and doesn't create a net current. So, it's less than the electrons gently move under the influence of an electric field, and more that it introduced a slight bias in the existing thermal motion. E: To clarify in case it may have been unclear, this is unrelated to the speed of propagation of the electric field, which as the article says is the speed of light.

Nimitz14 2y ago
This feels like a very nice intuition to have thank you for explaining!
sfn42 2y ago
Is this related to how some materials become superconductors at low temperature? Does the slowing down of this electron flux improve the material's ability to conduct electricity or is there some other mechanism at play?
- khold_stare 2y ago
  Superconductivity is fascinating. I don't know how people were able to come up with the explanations. Crudely, the reduced temperature means less jiggling of the metal lattice. This in turn makes it possible for the nuclei to be pushed around by electrons to form essentially sound waves (phonons) in the lattice (think of the lattice compressing and expanding due to interplay with electrons). At a certain temperature and therefore a certain frequency of lattice oscillation, electrons pair up to form "Cooper pairs" - they move in concert due to the lattice movement. What's crazy is that cooper pairs become a sort of pseudoparticle, and their quantum behaviour is different to regular electrons. Cooper pairs have integer spin (as opposed to half-integer spin), so they no longer obey the Pauli exclusion principle and all the electrons in the entire material basically form one giant condensate that extends through the whole material and can all occupy the same lowest energy quantum state.
  - jaybrendansmith 2y ago
    That is the BEST explanation of superconductivity I have ever heard.
    - khold_stare 2y ago
      Thanks for the kind words! For anyone curious to dive deeper into the crazyness that is quantum mechanics I can highly recommend a few resources: - Sean M Carroll's work, in particular his Biggest Ideas in the Universe books: https://www.preposterousuniverse.com/biggestideas/ https://www.preposterousuniverse.com/biggestideas/ - Artur Ekert, basically the father of Quantum Cryptography has an amazing course for free on youtube: https://www.youtube.com/@ArturEkert https://www.youtube.com/@ArturEkert . It's a very precise and understandable explanation of quantum computing, and some of the math that is involved with quantum mechanics. - If you have hours to spare, watch Richard Behiel's videos on Youtube. He's like the 3Blue1Brown of Quantum Physics. His latest video on superconductivity and the Higgs Field is almost 5 hours long (!!!) https://youtu.be/DkH1citHtgs?si=-yQNYDu9TlTpE1A0 https://youtu.be/DkH1citHtgs?si=-yQNYDu9TlTpE1A0 . It builds on his other videos, so I'd recommend starting at the beginning.
  - mncharity 2y ago
    > all the electrons in the entire material basically form one giant condensate Very not my field, but perhaps that's "all the paired electrons"? Brief ai-ing (do we have a verb yet?) suggests only some small fraction of conduction electrons form pairs, let alone all the rest.
    - genewitch 2y ago
      "Slop pass"?
    - itishappy 2y ago
      Only Cooper pairs can condensate, but in a superconducting material at the critical temperature Cooper pairs account for nearly all free electrons.
  - 2y ago
- movpasd 2y ago
  Electrons are actually delocalised in a metal: rather than point particles bouncing around the nuclei like a pinball, they're more like waves that ripple and diffract around them. This means that to good approximation, the electrons pass right through each other. Because of this, I don't expect the electron motion to affect resistance much. What definitely affects resistance is the vibration of the nuclei lattice, in which thermal energy is also stored. This vibration makes the electrons more likely to scatter. This means even in a non-superconducting metal, resistivity drops as you get colder. The special thing about superconductors is that there's a temperature where the resistivity suddenly drops to zero. (If you look up "superconductivity resistance against temperature", you'll see some graphs showing what I mean.) I don't know exactly the details of why this happens, but it has something to do with Cooper pairs. Electrons in these states are also sensitive to being knocked out and bumped up to regular conducting states by thermal noise.
pkoird 2y ago
Just to clarify, the high speed at which electrons generally move around in metal is called "Fermi velocity". Like you said, since it's random, on average it cancels out to 0. When applying an electric field, the electrons achieve a non-zero average velocity which is called the "drift velocity".
- movpasd 2y ago
  I hadn't heard of the term "Fermi velocity", but if it is the classical velocity derived from the Fermi energy, that should only be a lower bound on the average speed of the electrons in the metal. That said, I don't remember the orders of magnitude here, the relevant question being how cold "room temperature" is with respect to the Fermi level -- a quick Google suggests quite cold, in which case the Fermi velocity should be a very good approximation. In which case, it's interesting to realise that the motion of the electron gas comes mainly from Pauli exclusion rather than thermal noise! It's not a result I would have expected.
tzs 2y ago
It probably should be noted that the reason the drift velocity is so much lower than velocity due to thermal motion is that electrons cannot move very far before they collide with an atom which changes their direction. The mean distance they move in a copper wire between collisions is about 0.00000004 meters. At 100 km/s it would take 0.4 picoseconds to travel that distance.
- vlovich123 2y ago
  > electrons cannot move very far before they collide with an atom which changes their direction And yet we know it’s mostly just empty space. I’m assuming it’s more because of the electromagnic force being particularly strong at those scales rather than a straight up “collision” right?
  - bavell 2y ago
    Isn't a straight up physical "collision" just a strong interaction between the electromagnetic field of multiple particles?
    - vlovich123 2y ago
      I believe particle colliders really overcome the forces and have the particles touch and annihilate. But sure, at our level it’s all normal low energy electromagnetic stuff and nothing every really touches anything else.
      - Filligree 2y ago
        I would really argue that "electromagnetic stuff" is what "touches" means in the first place. Atoms aren't really empty. They're electron clouds with an extremely dense core of protons and neutrons, but the electron clouds are what we care about.
        vlovich123 2y ago
        I think that’s just arguing over the imprecision of language. Are we referring to the human sense which is the electromagnetic field or are we referring to the abstract mathematical concept of whether two line segments touch which is what the particle accelerator does or matter/antimatter. Both definitions are valid imho.
cogman10 2y ago
> which as the article says is the speed of light. This is pedantic because there's practically no difference. But just to be pedantic, it's not the speed of light and I'd argue it's not usually even close to the speed of light. In communications we are talking about anywhere from 60% to 80% the speed of light through most mediums.
- sightbroke 2y ago
  Confused. Are you saying that, if we took a light bulb (off) and a metal rod (0 charge) beside on another. Then were somehow able to turn the light bulb on and apply a charge to the that rod at the same time. While also having a detector that can sense a photon and a change in electric field some equal distance away from the bulb and rod. Then the photon (from the bulb) would reach our detector before the detection of the change in electric field (from the rod)? Let's suppose the medium is just plain air, and not particularly humid.
  - Spivak 2y ago
    Yes, you can see one of the best people do a demonstration of it https://m.youtube.com/watch?v=2Vrhk5OjBP8 https://m.youtube.com/watch?v=2Vrhk5OjBP8 It's close enough to c that you should just use c but it can be observed that it's less.
    - sightbroke 2y ago
      https://en.wikipedia.org/wiki/Electromagnetic_radiation https://en.wikipedia.org/wiki/Electromagnetic_radiation "In a vacuum, electromagnetic waves travel at the speed of light, commonly denoted c." I would expect that in air, that the photon from the light source and the perturbance of the electric field from the charge to reach the detector at the same time.
      - mr_toad 2y ago
        Yes, essentially. The perturbation of the electrical field causes EM radiation (radio), which moves at the speed of light.
lenkite 2y ago
Btw, Grok explained the point you brought up rather well. I am personally finding AI to be better at explaining concepts than writing code with imaginary API calls. "Random Motion: Even without current, electrons are jiggling around at high speeds (~10⁶ m/s at room temperature) due to thermal energy. The electric field just adds a slight bias to this chaotic motion, resulting in the net drift."
- whatshisface 2y ago
  The difference is that if you ask it to write code, you'll find its mistakes, but if you ask it to explain neutron stars, you won't.
  - deleted 2y ago
    [deleted]
  - andrekandre 2y ago
    interestingly this is also true with most popular internet/youtube personalities...
    - UncleMeat 2y ago
      Another good reason why we don't consider "watched a bunch of youtube videos" to be a credentialed education.
codelion 2y ago
that's a really helpful clarification about drift velocity vs. thermal motion... it's easy to get those mixed up. the analogy i always think of is a crowded dance floor - everyone's moving fast, but not really going anywhere until there's a general push in one direction.
veunes 2y ago
Kind of like a chotic crowd suddenly leaning slightly in one direction rather than an orderly march
fsckboy 2y ago
>electrons constantly scatter off the nuclei every which way, so it cancels out and doesn't create a net current no, not because it's every which way. it doesn't create net current because if, randomly, net charge moves in some direction, the resultant electric field will put pressure on the random movement to bring it back to equilibrium, 0.

Nimitz14 2y ago

This feels like a very nice intuition to have thank you for explaining!

sfn42 2y ago

Is this related to how some materials become superconductors at low temperature? Does the slowing down of this electron flux improve the material's ability to conduct electricity or is there some other mechanism at play?

khold_stare 2y ago
Superconductivity is fascinating. I don't know how people were able to come up with the explanations. Crudely, the reduced temperature means less jiggling of the metal lattice. This in turn makes it possible for the nuclei to be pushed around by electrons to form essentially sound waves (phonons) in the lattice (think of the lattice compressing and expanding due to interplay with electrons). At a certain temperature and therefore a certain frequency of lattice oscillation, electrons pair up to form "Cooper pairs" - they move in concert due to the lattice movement. What's crazy is that cooper pairs become a sort of pseudoparticle, and their quantum behaviour is different to regular electrons. Cooper pairs have integer spin (as opposed to half-integer spin), so they no longer obey the Pauli exclusion principle and all the electrons in the entire material basically form one giant condensate that extends through the whole material and can all occupy the same lowest energy quantum state.
- jaybrendansmith 2y ago
  That is the BEST explanation of superconductivity I have ever heard.
  - khold_stare 2y ago
    Thanks for the kind words! For anyone curious to dive deeper into the crazyness that is quantum mechanics I can highly recommend a few resources: - Sean M Carroll's work, in particular his Biggest Ideas in the Universe books: https://www.preposterousuniverse.com/biggestideas/ https://www.preposterousuniverse.com/biggestideas/ - Artur Ekert, basically the father of Quantum Cryptography has an amazing course for free on youtube: https://www.youtube.com/@ArturEkert https://www.youtube.com/@ArturEkert . It's a very precise and understandable explanation of quantum computing, and some of the math that is involved with quantum mechanics. - If you have hours to spare, watch Richard Behiel's videos on Youtube. He's like the 3Blue1Brown of Quantum Physics. His latest video on superconductivity and the Higgs Field is almost 5 hours long (!!!) https://youtu.be/DkH1citHtgs?si=-yQNYDu9TlTpE1A0 https://youtu.be/DkH1citHtgs?si=-yQNYDu9TlTpE1A0 . It builds on his other videos, so I'd recommend starting at the beginning.
- mncharity 2y ago
  > all the electrons in the entire material basically form one giant condensate Very not my field, but perhaps that's "all the paired electrons"? Brief ai-ing (do we have a verb yet?) suggests only some small fraction of conduction electrons form pairs, let alone all the rest.
  - genewitch 2y ago
    "Slop pass"?
  - itishappy 2y ago
    Only Cooper pairs can condensate, but in a superconducting material at the critical temperature Cooper pairs account for nearly all free electrons.
- 2y ago
movpasd 2y ago
Electrons are actually delocalised in a metal: rather than point particles bouncing around the nuclei like a pinball, they're more like waves that ripple and diffract around them. This means that to good approximation, the electrons pass right through each other. Because of this, I don't expect the electron motion to affect resistance much. What definitely affects resistance is the vibration of the nuclei lattice, in which thermal energy is also stored. This vibration makes the electrons more likely to scatter. This means even in a non-superconducting metal, resistivity drops as you get colder. The special thing about superconductors is that there's a temperature where the resistivity suddenly drops to zero. (If you look up "superconductivity resistance against temperature", you'll see some graphs showing what I mean.) I don't know exactly the details of why this happens, but it has something to do with Cooper pairs. Electrons in these states are also sensitive to being knocked out and bumped up to regular conducting states by thermal noise.

pkoird 2y ago

Just to clarify, the high speed at which electrons generally move around in metal is called "Fermi velocity". Like you said, since it's random, on average it cancels out to 0. When applying an electric field, the electrons achieve a non-zero average velocity which is called the "drift velocity".

movpasd 2y ago
I hadn't heard of the term "Fermi velocity", but if it is the classical velocity derived from the Fermi energy, that should only be a lower bound on the average speed of the electrons in the metal. That said, I don't remember the orders of magnitude here, the relevant question being how cold "room temperature" is with respect to the Fermi level -- a quick Google suggests quite cold, in which case the Fermi velocity should be a very good approximation. In which case, it's interesting to realise that the motion of the electron gas comes mainly from Pauli exclusion rather than thermal noise! It's not a result I would have expected.

tzs 2y ago

It probably should be noted that the reason the drift velocity is so much lower than velocity due to thermal motion is that electrons cannot move very far before they collide with an atom which changes their direction. The mean distance they move in a copper wire between collisions is about 0.00000004 meters. At 100 km/s it would take 0.4 picoseconds to travel that distance.

vlovich123 2y ago
> electrons cannot move very far before they collide with an atom which changes their direction And yet we know it’s mostly just empty space. I’m assuming it’s more because of the electromagnic force being particularly strong at those scales rather than a straight up “collision” right?
- bavell 2y ago
  Isn't a straight up physical "collision" just a strong interaction between the electromagnetic field of multiple particles?
  - vlovich123 2y ago
    I believe particle colliders really overcome the forces and have the particles touch and annihilate. But sure, at our level it’s all normal low energy electromagnetic stuff and nothing every really touches anything else.
    - Filligree 2y ago
      I would really argue that "electromagnetic stuff" is what "touches" means in the first place. Atoms aren't really empty. They're electron clouds with an extremely dense core of protons and neutrons, but the electron clouds are what we care about.
      - vlovich123 2y ago
        I think that’s just arguing over the imprecision of language. Are we referring to the human sense which is the electromagnetic field or are we referring to the abstract mathematical concept of whether two line segments touch which is what the particle accelerator does or matter/antimatter. Both definitions are valid imho.

cogman10 2y ago

> which as the article says is the speed of light. This is pedantic because there's practically no difference. But just to be pedantic, it's not the speed of light and I'd argue it's not usually even close to the speed of light. In communications we are talking about anywhere from 60% to 80% the speed of light through most mediums.

sightbroke 2y ago
Confused. Are you saying that, if we took a light bulb (off) and a metal rod (0 charge) beside on another. Then were somehow able to turn the light bulb on and apply a charge to the that rod at the same time. While also having a detector that can sense a photon and a change in electric field some equal distance away from the bulb and rod. Then the photon (from the bulb) would reach our detector before the detection of the change in electric field (from the rod)? Let's suppose the medium is just plain air, and not particularly humid.
- Spivak 2y ago
  Yes, you can see one of the best people do a demonstration of it https://m.youtube.com/watch?v=2Vrhk5OjBP8 https://m.youtube.com/watch?v=2Vrhk5OjBP8 It's close enough to c that you should just use c but it can be observed that it's less.
  - sightbroke 2y ago
    https://en.wikipedia.org/wiki/Electromagnetic_radiation https://en.wikipedia.org/wiki/Electromagnetic_radiation "In a vacuum, electromagnetic waves travel at the speed of light, commonly denoted c." I would expect that in air, that the photon from the light source and the perturbance of the electric field from the charge to reach the detector at the same time.
    - mr_toad 2y ago
      Yes, essentially. The perturbation of the electrical field causes EM radiation (radio), which moves at the speed of light.

lenkite 2y ago

Btw, Grok explained the point you brought up rather well. I am personally finding AI to be better at explaining concepts than writing code with imaginary API calls. "Random Motion: Even without current, electrons are jiggling around at high speeds (~10⁶ m/s at room temperature) due to thermal energy. The electric field just adds a slight bias to this chaotic motion, resulting in the net drift."

whatshisface 2y ago
The difference is that if you ask it to write code, you'll find its mistakes, but if you ask it to explain neutron stars, you won't.
- deleted 2y ago
  [deleted]
- andrekandre 2y ago
  interestingly this is also true with most popular internet/youtube personalities...
  - UncleMeat 2y ago
    Another good reason why we don't consider "watched a bunch of youtube videos" to be a credentialed education.

codelion 2y ago

that's a really helpful clarification about drift velocity vs. thermal motion... it's easy to get those mixed up. the analogy i always think of is a crowded dance floor - everyone's moving fast, but not really going anywhere until there's a general push in one direction.

veunes 2y ago

Kind of like a chotic crowd suddenly leaning slightly in one direction rather than an orderly march

fsckboy 2y ago

>electrons constantly scatter off the nuclei every which way, so it cancels out and doesn't create a net current no, not because it's every which way. it doesn't create net current because if, randomly, net charge moves in some direction, the resultant electric field will put pressure on the random movement to bring it back to equilibrium, 0.