I bought 1000 meters of wire to settle a physics debate [video] • Hacker News Board

Re: I bought 1000 meters of wire to settle a physics debate [video]

#121

Post by foxfluff » Sat, Dec 18, 2021, 1:58 AM UTC

lmilcin wrote: post #104

foxfluff wrote: post #90

Earlier quoted context omitted.

"I have never in my life length matched the two lines of a differential pair, including at 10 GHz. Never." - Rick Hartley https://youtu.be/QG0Apol-oj0?t=2765

It is right if you are only interested in making the damn thing work. But if you don't want it to broadcast noise like crazy you want it tight and matched as much as possible. The value of differential pair comes from the fact that they have constant common component. That only works if both components aren't shifted in time. If that happens, you will get strong common signal and say bye to nice quiet differential pa…

Are you saying your differential line is causing EMI if it's not length matched?

Or are you saying that the receiver won't do a good enough job cancelling common mode noise if you're not length matched? For that, I'd like to have a reference. Otherwise I'm not convinced this is more critical than the max skew to keep your crossings in the right region.

Re: I bought 1000 meters of wire to settle a physics debate [video]

#122

Post by lmilcin » Sat, Dec 18, 2021, 2:03 AM UTC

foxfluff wrote: post #121

lmilcin wrote: post #104

Earlier quoted context omitted.

It is right if you are only interested in making the damn thing work. But if you don't want it to broadcast noise like crazy you want it tight and matched as much as possible. The value of differential pair comes from the fact that they have constant common component. That only works if both components aren't shifted in time. If that happens, you will get strong common signal and say bye to nice quiet differential pa…

Are you saying your differential line is causing EMI if it's not length matched? Or are you saying that the receiver won't do a good enough job cancelling common mode noise if you're not length matched? For that, I'd like to have a reference. Otherwise I'm not convinced this is more critical than the max skew to keep your crossings in the right region.

Differential pair minimises EMI by having constant common mode potential over the length of the line (and wires close to each other). Meaning if you look at the pair from afar the average of their voltage at any point in the line is constant. Or you might say that the noise from both lines cancels each other. From afar, it looks like there is no signal traveling the length of the line (to an approximation, because there is necessarily some distance between the wires meaning the cancelling is not perfect).

If the signals are shifted in time, though, that is no longer true. From afar you will see places in the line that have different common mode potential. Where one signal edge lags behind the other the average will no longer be the same as for the rest of the line. From afar this looks like high bandwidth signal travelling down the line.

Re: I bought 1000 meters of wire to settle a physics debate [video]

#123

Post by charcircuit » Sat, Dec 18, 2021, 2:06 AM UTC

lmilcin wrote: post #72

charcircuit wrote: post #67

Earlier quoted context omitted.

>There is NO debate I'm guessing you never checked the comment section.

Bring any topic however widely known and thoroughly proven to a large enough general audience and you will always find people with no actual knowledge that will question it. Try to find a post on HN with a lot of upvotes and comments where everybody would agree on something. If you want to know if there is "physics debate" ask people who know physics, not commenters on Youtube or HN.

Why are random commenters not allowed to have a debate regarding physics?

Re: I bought 1000 meters of wire to settle a physics debate [video]

#124

Post by layer8 » Sat, Dec 18, 2021, 2:06 AM UTC

m348e912 wrote: post #105

MarkusWandel wrote: post #91

Light is slow, not exactly 1 foot per nanosecond but it's a reasonable mnemonic. Think about that, if you had an optical cable running a reasonable (these days) 40 Gbits/second link and it's 3 feet long, that's 120 bits in transit from end to end at any given moment. And that's if it is optical and ignoring the slightly slower speed of lighit in glass; any sort of actual wire is slower. For a transmission line i.e. o…

>>Light is slow I am going to have to go ahead and disagree with you on that

Most stuff in the universe is moving apart faster than light, so yes it’s quite slow. ;)

Re: I bought 1000 meters of wire to settle a physics debate [video]

#125

Post by iraqmtpizza » Sat, Dec 18, 2021, 2:10 AM UTC

it_does_follow wrote: post #100

tpush wrote: post #76

So, I genuinely do not get the "controversy" over the Veritasium video. AFAIU, the entire point of the video was to show that electricity travels not through wires but via electric fields ("through the air" so to speak). To illustrate this, he sets up an experiment with parameters such that the conclusion can only mean that some electricity has traveled to the bulb before any could arrive through/around the wires. He…

> some electricity I think the key part of the controversy is the word "some". In the original Veritasium video it pretty strongly implies that enough energy to light the bulb, i.e. virtually all the electricity, is traveling instantaneously a short distance rather than "through" the wire. A big part of that video is to essentially say you're wrong if you understand electrons moving through the wire as how electricit…

>you're wrong if you understand electrons moving through the wire as how electricity works

in direct current, how does charge move from one terminal to another if it's not carried by electrons (which often act as waves)?

Re: I bought 1000 meters of wire to settle a physics debate [video]

#126

Post by userbinator » Sat, Dec 18, 2021, 2:18 AM UTC

y04nn wrote: post #112

The first signal is a close RF transmission, billions of people rely on it everyday, a current disturbs the electromagnetic field and and an antenna, sometimes thousands of kilometers away, detects a very small variation in the field (-120dbm @ 50Ω = 0.2μV). This is how our cellphones, wifi, GPS, RFID works. So, even if you disconnect the bulb and the switch you can create a current in the bulb by turning on and off…

I think the important point to note about that is it doesn't necessarily have to be a purpose-built "radio transmitter" to emit RF energy; everything which involves moving charges will radiate an EM field. This becomes especially apparent when you read the century-old books on early experiments with radio, and see the extremely crude circuitry they used to generate those waves.

Re: I bought 1000 meters of wire to settle a physics debate [video]

#127

Post by systemvoltage » Sat, Dec 18, 2021, 2:22 AM UTC

Laremere wrote: post #25

0xTJ wrote: post #5

That Veritasium video is one of the most controversial educational videos I've seen. I think one of the issues with it is that it's a bit pedantic, but also ignores other issues, that invites other people to be pedantic about it.

As much as I love Veritasium, I feel the OP video is better than his. Actually doing the experiment is really good, and it honestly feels like he was fishing for another debate like the faster than wind car by giving only a super theoretical and unintuitive explanation. (I don't have any real evidence of that, but it does /feel/ like it.) I also wonder what would happen if, instead of a setup like this: ┏───────light…

You also need to account for the fact that oscilloscope wires would also be longer going to the light bulb. You can imagine this on the extreme case where light and battery are 1 km apart. Now, you need oscilloscope wires to go 1km far to connect to the light bulb and your timing is off.

Re: I bought 1000 meters of wire to settle a physics debate [video]

#128

Post by function_seven » Sat, Dec 18, 2021, 2:24 AM UTC

stormbrew wrote: post #62

paxys wrote: post #11

Earlier quoted context omitted.

That video perfectly summarizes online discourse. I personally found it fantastic as a resource. It was the best and most clear explanation of transmission of energy in an electrical circuit that I have seen. It debunked a lot of myths in my head that we were all taught in school. The thought experiment he proposed was just a hook to keep people interested - and it certainly did the job. Because it is a popular inter…

I'm sorry but no, the original video does clarify some important things that people misunderstand about transmission of electricity, but then it piles on a bunch of other really unclear information with no explanation. It answers a question with about 8 other questions and then offers no help in understanding any of them. You can't just say "there's electricity at the light bulb nearly immediately! (oh by the way it'…

> Especially since the effects he's describing are probably (as mentioned in this video) because of capacitance, and are completely dependent on the 1m distance between the wires across the entire span, a constraint he mentions basically once and then never again, and never says explicitly that it's related to the effect.

YES. This is my biggest gripe with the video. A better shape of wires would be a pair of tangent circles, each one with a circumference of 1 light-second, and a 1-meter section removed at the tangent point. (Hope I'm describing it right. Basically but only the switch and the light bulb 1 meter apart, but get the wires as far apart from each other as possible)

Now would that eliminate capacitance? Or largely so? What amount of current would begin flowing across the bulb at t₀?

After seeing that video and all the reactions, I know more—but not from the original video. My mental model of electricity is still that it "flows along wires", but I now can somewhat separate the movement of charged particles from the energy flux those moving charges propagate. I think.

Re: I bought 1000 meters of wire to settle a physics debate [video]

#129

Post by jeffbee » Sat, Dec 18, 2021, 2:25 AM UTC

foxfluff wrote: post #118

jeffbee wrote: post #106

Earlier quoted context omitted.

I mean ... he's saying right there on the slide there's less than 1mm of length error budget at 10GHz.

And that's gobs. What's your typical space between the lines in a pair? Five mils? You run them side by side and you'd be hard pressed to make a length error significant enough to matter. Yeah you do need to be aware of it enough to ask whether you're good when you're routing a really weird loop.. but for the vast majority of things and sane routing, you can just eyeball it or don't need to care at all. Point is, a l…

I've always followed Howard Johnson's rule of thumb to match them within 1/10 of the rise time, but it's not like it's a lot of work: the software does it automatically.

Re: I bought 1000 meters of wire to settle a physics debate [video]

#130

Post by lamontcg » Sat, Dec 18, 2021, 2:28 AM UTC

gcommer wrote: post #96

tpush wrote: post #76

So, I genuinely do not get the "controversy" over the Veritasium video. AFAIU, the entire point of the video was to show that electricity travels not through wires but via electric fields ("through the air" so to speak). To illustrate this, he sets up an experiment with parameters such that the conclusion can only mean that some electricity has traveled to the bulb before any could arrive through/around the wires. He…

There's two bits of controversy: 1) Slander: He claims that the way EE is taught is a "lie" but leaves out the many ways that standard EE models agree with his result. 2) Clickbait / inadequate explanation: he leaves out many details in order to make his experiment seem far more mind-blowing that it really is.* Critique 1 can be countered by saying he is targeting a lay audience (say, they took 1 high school physics…

The problem is that his video is a mish-mash of two different concepts that were taught perfectly well in my freshman physics course. One is that the electric field travels down a wire at the speed of light while the electrons do not. The other is induced currents by moving charges.

I got the answer "wrong" by watching the video because I didn't consider the tiny amount of current induced in the other wire to be sufficient to turn on the light in any useful manner. If you had replaced the light with a sensitive ammeter and asked me what the first point in time there would be any measurable deflection from zero, then I would have gotten it correct.

Could have also just asked how this works, and the success rate would have gone way up (but he'd have lost all his clickbait):

https://www.amazon.com/Fluke-324-Temperature-Capacitance-Mea...

Ask bad questions, get bad answers. Nothing wrong with how physics/EE is taught.