7 ms·

this is an interesting video (to me anyway!!) on the speed of electrons vs fields The Big Misconception About Electricity.. https://www.youtube.com/watch?v=bHI

this is an interesting video (to me anyway!!) on the speed of electrons vs fields The Big Misconception About Electricity.. https://www.youtube.com/watch?v=bHIhgxav9LY https://www.youtube.com/watch?v=bHIhgxav9LY

Dumble 5y ago
https://www.youtube.com/watch?v=2Vrhk5OjBP8 https://www.youtube.com/watch?v=2Vrhk5OjBP8
jaywalk 5y ago
That video is hilariously wrong, which seems to be a common theme for Veritasium lately. I can't tell if he legitimately believes what he's saying, or if he produces these seemingly-correct-but-not videos to drive up engagement and views.
- scrollaway 5y ago
  Care to share the hilarity with the rest of the class?
  - aemreunal 5y ago
    Disclaimer: I don't understand nearly enough about the physics of electricity, but this was a popular response video by ElectroBOOM that Veritasium also responded to in the comments: https://www.youtube.com/watch?v=iph500cPK28 https://www.youtube.com/watch?v=iph500cPK28
    - mannykannot 5y ago
      To be clear, you will not find that Veritasium is wrong here (whether hilariously or otherwise); it supports the essential features of the original claims.
      - tsimionescu 5y ago
        He is extremely misleading, if not technically wrong. The Poynting vector has nothing to do with why some small amount of current is temporarily induced into the light bulb. The actual electrical energy flows along the wires (inside and out). In an unrelated phenomenon, you transmit some tiny amount of electrical power directly through the air/vacuum between the battery and the bulb. In a different configuration, or if you inserted certain kinds of reflective materials, you could block (most of) this energy from ever reaching the bulb, without any change whatsoever in the current flowing along the wires. Also, if you submerged the whole circuit (or just the space around the battery and bulb) in a dielectric material with a low speed of light (say, in a piece of rock), you would get a vastly slower and weaker current, with no effect on the current flowing through the wires. Even the part where he explains the problem with the chain metaphor is wrong. If you actually had a mechanical chain and an engine moving it back and forth, you could extract energy from the movement of the chain either by exploiting friction (to heat up something, just like a resistor does) or by using gears that resist movement in the opposite direction (e.g. slipping the chain) to achieve movement in a single direction. There's nothing all that mysterious about how we extract energy from electricity, at least in practice. In fact, you could even get an equivalent of the small induced curent: if you recreate his extremely long circuit with a motor instead of the battery and an extremely sensitive motion detector instead of the light bulb 1m away, you would detect some motion [1m/speed of sound in material connecting them] seconds after the motor is started, assuming you are not floating in a perfect vacuum; and then vastly higher motion after 300,000km/speed of sound in chain seconds later. Nothing more mysterious going on with the EM field.
        mannykannot 5y ago
        Of course, if you do different experiments, you get different outcomes - no-one is suggesting otherwise. > Nothing more mysterious going on with the EM field. That's precisely the point here - it all works out exactly in accordance with Maxwell's laws, via transmission line theory - see also my other reply (https://news.ycombinator.com/item?id=29934233 https://news.ycombinator.com/item?id=29934233)
        5y ago
  - Tostino 5y ago
    Reply video here: https://youtu.be/iph500cPK28 https://youtu.be/iph500cPK28 Edit: apparently too slow, sorry for the noise.
- tom-thistime 5y ago
  Have you read the slides cited in the description on YouTube, and found the error? There's both a theoretical argument (which I haven't followed), and lab results using a 15m (?) transmission line. Both seem to confirm the claim in the video. If they're wrong, I'd really like to understand where the error is. E&M can be a little tricky, even for experienced circuit designers.
  - sebzim4500 5y ago
    The slides look correct to me but the video massively misrepresents what is actually going on. For example, you could cut the wire at both ends and the answer would not change, so the fact that there is a 'circuit' is irrelevant.
  - tsimionescu 5y ago
    The video gives a wrong impression that most current is flowing along the Poynting vector. In fact, only a tiny amount of current is flowing directly, and the vast majority is flowing along the wires, as expected. The tiny amount flowing directly through the air is much better understood as an antenna effect, unrelated to regular current flow, and has nothing really to do with the Poynting vector.
    - tom-thistime 5y ago
      I agree that current doesn't follow the Poynting vector. Power follows the Poynting vector. That's true for antennas and for transmission lines.
      - tsimionescu 5y ago
        Yes, but most power is in fact following the Poynting vector along the wires, not the direct Poynting vector between the battery and the bulb, as the video misleadingly suggests (but doesn't outright claim).
        mannykannot 5y ago
- pja 5y ago
  His presentation is not very clear & imo the way he answers the question is not actually helpful in actually understanding what’s going on, but he is correct that current will flow in the far side almost immediately. As is made clearer in the documents he shared which contain his discussions with actual physicists, that current will be much smaller than the final current that flows all the way round the circuit, but it is there nevertheless. (In fact that current will initially flow whether or not the two halves of the flat ring are connected - if you think about it things have to work like this, because the middle of the system cannot “know” whether the far ends are connected until light has had time to get there & back. Relativity is absolute.) Take a look at the video made by YouTuber AlphaPhoenix where he constructs the entire thing on a smaller scale and demonstrates the effect with an oscilloscope: https://www.youtube.com/watch?v=2Vrhk5OjBP8 https://www.youtube.com/watch?v=2Vrhk5OjBP8 One way of viewing what’s going on here is to consider the two sides as a pair of large dipole antennas. When the switch (or in AlphaPheonix’s case, a transistor) is turned on, a pulse of electrons is pushed down one antenna, setting up a transient wave in the electromagnetic field around it which induces a current in the opposite direction in the “receiving” antenna on the other side. This happens regardless of whether the wired are connected in a loop or not! Physics is great.
  - deleted 5y ago
    [deleted]
  - marcan_42 5y ago
    I found the AlphaPhoenix video a bit disappointing because he didn't clearly explain that the capacitive interaction between both ends is also light speed limited, and very dependent on distance, i.e. the geometry of the test. Had the test been done in a big round loop, current wouldn't start flowing at the far end immediately. Otherwise you'd have made faster than light communications. I'm also not even sure if his testing methodology is solid; he was getting a ton of common mode junk in the target measurement probes, and subtracting it out in the scope. That works in steady state, but when trying to measure fast transients like this it depends on things like probe skew and more. It's just not clean enough for me to be able to trust the results are caused by the explanation he gave. Measuring subtle phenomena like this with long transmission lines involved with a single ground referenced scope and no true differential probes... ick.
krupan 5y ago
ElectroBOOM replied to this: https://youtu.be/iph500cPK28 https://youtu.be/iph500cPK28
deleted 5y ago
[deleted]

Dumble 5y ago

https://www.youtube.com/watch?v=2Vrhk5OjBP8 https://www.youtube.com/watch?v=2Vrhk5OjBP8

jaywalk 5y ago

That video is hilariously wrong, which seems to be a common theme for Veritasium lately. I can't tell if he legitimately believes what he's saying, or if he produces these seemingly-correct-but-not videos to drive up engagement and views.

scrollaway 5y ago
Care to share the hilarity with the rest of the class?
- aemreunal 5y ago
  Disclaimer: I don't understand nearly enough about the physics of electricity, but this was a popular response video by ElectroBOOM that Veritasium also responded to in the comments: https://www.youtube.com/watch?v=iph500cPK28 https://www.youtube.com/watch?v=iph500cPK28
  - mannykannot 5y ago
    To be clear, you will not find that Veritasium is wrong here (whether hilariously or otherwise); it supports the essential features of the original claims.
    - tsimionescu 5y ago
      He is extremely misleading, if not technically wrong. The Poynting vector has nothing to do with why some small amount of current is temporarily induced into the light bulb. The actual electrical energy flows along the wires (inside and out). In an unrelated phenomenon, you transmit some tiny amount of electrical power directly through the air/vacuum between the battery and the bulb. In a different configuration, or if you inserted certain kinds of reflective materials, you could block (most of) this energy from ever reaching the bulb, without any change whatsoever in the current flowing along the wires. Also, if you submerged the whole circuit (or just the space around the battery and bulb) in a dielectric material with a low speed of light (say, in a piece of rock), you would get a vastly slower and weaker current, with no effect on the current flowing through the wires. Even the part where he explains the problem with the chain metaphor is wrong. If you actually had a mechanical chain and an engine moving it back and forth, you could extract energy from the movement of the chain either by exploiting friction (to heat up something, just like a resistor does) or by using gears that resist movement in the opposite direction (e.g. slipping the chain) to achieve movement in a single direction. There's nothing all that mysterious about how we extract energy from electricity, at least in practice. In fact, you could even get an equivalent of the small induced curent: if you recreate his extremely long circuit with a motor instead of the battery and an extremely sensitive motion detector instead of the light bulb 1m away, you would detect some motion [1m/speed of sound in material connecting them] seconds after the motor is started, assuming you are not floating in a perfect vacuum; and then vastly higher motion after 300,000km/speed of sound in chain seconds later. Nothing more mysterious going on with the EM field.
      - mannykannot 5y ago
        Of course, if you do different experiments, you get different outcomes - no-one is suggesting otherwise. > Nothing more mysterious going on with the EM field. That's precisely the point here - it all works out exactly in accordance with Maxwell's laws, via transmission line theory - see also my other reply (https://news.ycombinator.com/item?id=29934233 https://news.ycombinator.com/item?id=29934233)
        5y ago
- Tostino 5y ago
  Reply video here: https://youtu.be/iph500cPK28 https://youtu.be/iph500cPK28 Edit: apparently too slow, sorry for the noise.
tom-thistime 5y ago
Have you read the slides cited in the description on YouTube, and found the error? There's both a theoretical argument (which I haven't followed), and lab results using a 15m (?) transmission line. Both seem to confirm the claim in the video. If they're wrong, I'd really like to understand where the error is. E&M can be a little tricky, even for experienced circuit designers.
- sebzim4500 5y ago
  The slides look correct to me but the video massively misrepresents what is actually going on. For example, you could cut the wire at both ends and the answer would not change, so the fact that there is a 'circuit' is irrelevant.
- tsimionescu 5y ago
  The video gives a wrong impression that most current is flowing along the Poynting vector. In fact, only a tiny amount of current is flowing directly, and the vast majority is flowing along the wires, as expected. The tiny amount flowing directly through the air is much better understood as an antenna effect, unrelated to regular current flow, and has nothing really to do with the Poynting vector.
  - tom-thistime 5y ago
    I agree that current doesn't follow the Poynting vector. Power follows the Poynting vector. That's true for antennas and for transmission lines.
    - tsimionescu 5y ago
      Yes, but most power is in fact following the Poynting vector along the wires, not the direct Poynting vector between the battery and the bulb, as the video misleadingly suggests (but doesn't outright claim).
      - mannykannot 5y ago
pja 5y ago
His presentation is not very clear & imo the way he answers the question is not actually helpful in actually understanding what’s going on, but he is correct that current will flow in the far side almost immediately. As is made clearer in the documents he shared which contain his discussions with actual physicists, that current will be much smaller than the final current that flows all the way round the circuit, but it is there nevertheless. (In fact that current will initially flow whether or not the two halves of the flat ring are connected - if you think about it things have to work like this, because the middle of the system cannot “know” whether the far ends are connected until light has had time to get there & back. Relativity is absolute.) Take a look at the video made by YouTuber AlphaPhoenix where he constructs the entire thing on a smaller scale and demonstrates the effect with an oscilloscope: https://www.youtube.com/watch?v=2Vrhk5OjBP8 https://www.youtube.com/watch?v=2Vrhk5OjBP8 One way of viewing what’s going on here is to consider the two sides as a pair of large dipole antennas. When the switch (or in AlphaPheonix’s case, a transistor) is turned on, a pulse of electrons is pushed down one antenna, setting up a transient wave in the electromagnetic field around it which induces a current in the opposite direction in the “receiving” antenna on the other side. This happens regardless of whether the wired are connected in a loop or not! Physics is great.
- deleted 5y ago
  [deleted]
- marcan_42 5y ago
  I found the AlphaPhoenix video a bit disappointing because he didn't clearly explain that the capacitive interaction between both ends is also light speed limited, and very dependent on distance, i.e. the geometry of the test. Had the test been done in a big round loop, current wouldn't start flowing at the far end immediately. Otherwise you'd have made faster than light communications. I'm also not even sure if his testing methodology is solid; he was getting a ton of common mode junk in the target measurement probes, and subtracting it out in the scope. That works in steady state, but when trying to measure fast transients like this it depends on things like probe skew and more. It's just not clean enough for me to be able to trust the results are caused by the explanation he gave. Measuring subtle phenomena like this with long transmission lines involved with a single ground referenced scope and no true differential probes... ick.

krupan 5y ago

ElectroBOOM replied to this: https://youtu.be/iph500cPK28 https://youtu.be/iph500cPK28

deleted 5y ago

[deleted]