I bought 1000 meters of wire to settle a physics debate

two weeks ago as i filmed this derrick over on veritasium proposed a physics thought experiment and today i have made that into a real experiment he proposed a simple circuit where you have a battery a switch and a light bulb except the wires that connect these components together are really really really long and stretch far away in both directions such that if the power from the battery when you flip the switch had to go all the way down the wire and back you'd actually be able to see a delay before the light bulb turned on if you had to wait for light speed effectively to make that round trip as i watched that video i was extremely unconvinced i think that his answer that you don't need to wait for the light speed delay is technically correct if you use his very carefully crafted definitions like any current passing through the light bulb constitutes the light bulb turning on now the bulb won't receive the entire voltage of the battery immediately but i think that there's a whole lot more to this question and your intuition about the answer is probably not quite as wrong as you've been led to believe so i'm about to fire this circuit up i have an oscilloscope that can do very precise timings and i have about a kilometer of copper wire stretched out in both directions here and we're going to see if there's a light speed delay when you flick that switch [Music] all right so full disclosure i'm filming this over a couple days and it got cold so if you notice me wearing a coat and then suddenly not wearing a coat and then wearing a coat again that's why okay so first of all i need to explain to you how i've set up this circuit and measurement apparatus because it's going to look a little bit different than derek's cartoon switch and light bulb first difference no switch when you actually close a mechanical switch you've got two pieces of metal that are attached to wires that sort of slam into each other but when they slam into each other they have a tendency to sort of bounce a little bit which means that as you flip a switch if you zoom in on that in time it's actually going on and it sort of bounces and it means that your signal gets really really noisy considering that i'm trying to see the faintest of transients on a really long line i wanted it to be as good and smooth and even as possible so i'm using an electronic switch i have a transistor here that is going to start five volts flowing in this wire as soon as it receives a pulse second change no light bulb so i mean i'm using a resistor and to be fair an incandescent light bulb is basically just a resistor but more than that by measuring the voltage across this resistor with two oscilloscope probes i can find out exactly how much current is flowing through that resistor resolved in time so i can flick this switch start this thing recording and know exactly how long it takes for any current to start flowing through that bulb or resistor light bulb whatever the last critical elements here are of course the wires themselves and i have stretched out a kilometer of wire in two big 500 meter loops on both sides i say a kilometer it's approximately a kilometer i actually the best measurements of the length of the wire so far have come from the oscilloscope not from the wheel that i've been walking with if you look at the light speed delay like how long it takes light to move from here to here directly through space it's you know half a nanosecond i'm not going to be able to see that time delay from flipping this switch to this thing turning on with this oscilloscope uh because my switch itself takes you know 20 or 30 nanoseconds to turn on so it's just it happens too fast to see but for signals to go all the way down that 500 meters of wire and come back and to go all the way down that 500 meters of wire and come back that takes a lot longer and that should be on the order of one and a half to two microseconds which is an eternity to an oscilloscope like this so we will be able to see what's going on very clearly how much current gets there right away how much current we have to wait for how much bouncing there is in the lines and how long it takes for ohm's law to come into effect like how long does it take for steady state dc to take over so without further ado let's hook it up and try it out there's so much noise oh got it this is this is not how this should be done oh look at that so the light bulb turns on a little bit and then after one light speed delay the light bulb turns on the rest of the way let's confirm that okay so this cursor right there when we see the the light bulb turn on most of the way is 1.6 microseconds 1.6 microseconds times c is 480 meters that's perfect we got 500 meters going out that way and we got 500 meters going out that way that is our light speed delay awesome okay so i am really excited about this graph right here but i realize that not everybody watching is probably used to interpreting oscilloscope traces so i'm gonna take two minutes and i'm going to explain what's going on here an oscilloscope in a nutshell is really just an exceptionally fast volt meter like a regular multimeter voltmeter like this you can put the probes across something and you can see how like if a battery is dead or something like that because you can read how many volts there are this is all this is doing except it's plotting it over time all of these graphs are a voltage or potential plotted on the y-axis versus time on the x-axis so if we follow this yellow trace right here it rides along and then at this particular point in time the voltage rises from zero all the way up to 5 volts this yellow trace is probing the battery and switch together if you imagine connecting a voltmeter across the battery and switch with the switch open these probes are actually attached to the same end of the battery albeit with a really long wire so the potential difference between the probes is zero however after we close this switch now this probe is immediately connected to this end of the battery and even ignoring light speed stuff the wire is resistive enough that we see the full voltage of the battery across these probes in this case 5 volts for the interested this particular set of traces was made with this complete hookup now what's this white line that i was specifically so excited about well strictly speaking it's on an oscilloscope which means that it is a voltage plotted against time however i've chosen this voltage in the circuit very carefully this is actually the voltage drop across the resistor that is representing the light bulb at the far end of the circuit and because i know the resistance of that resistor it's one kilo ohm and i know ohm's law v equals ir or in this case i equals v over r i can calculate the current flowing through the light bulb as a function of time and that's basically what this is plotting now current is literally a measure of how many electrons per second are flowing through a particular electrical component so in this case we see that after about a microsecond there's somewhere in the vicinity of a fifth of a volt across this one kiloohm resistor which equates to a current of about 200 microamps which might not sound like very much but it's somewhere in the vicinity of quadrillion electrons per second traveling through that resistor now that makes it sound like a lot but if we wait another few microseconds until the circuit reaches steady state we get a significantly larger current 1.7 milliamps or 10 quadrillion electrons per second so if you were watching the video just to find out if derek was wrong and you made it this far here's the data you get a tiny little bit of current immediately almost immediately probably not quite as fast as he said but after you wait for the signal to go all the way down the wire and all the way back you get a much larger signal that shows up and actually turns the light bulb on in my mind if a light bulb is capable of turning on with this much current it's probably going to you know burn out and explode with this much current so if this is your situation you can turn on a light bulb with that little current you've done a terrible job at designing your circuit but the fact remains there is a tiny little bit of current right there and it's flowing and it's flowing almost immediately which is weird so to help imagine this bonus current let's take a look at the circuit from the top down when you flip the switch you start current flowing in the wire and the electrons in the wire start moving around in a loop that's what electricity is but if we account for the light speed delay you could imagine that all of these electrons don't start moving at the same time there's a wave that travels through them that gets each electron started on its way and only when that wave reaches the light bulb do the electrons and the light bulbs start moving and turn the light bulb on this extra current that we see immediately then can't come from this primary wave down the wire it doesn't have time to get there it has to be something else it's actually a second wave that starts at the light bulb almost immediately flowing a very small amount of current through that light bulb and then it travels down the wire to meet the first wave but only once the first wave makes it all the way back to the light bulb does the bulb turn on all the way so how does current flowing in this wire start current flowing in this parallel but effectively disconnected wire i'd say that there are two possibilities capacitance and induction and because most transmission line models ignore the cross inductance at least for today i will too but to understand how these wires act like a capacitor we need to know a little more about why electrons flow yes when you have a wire like this that's just a bare wire hanging out in the air the power that is being transmitted through this wire is being transmitted through the electric field around the wire you can't just have a battery that flows current out into nothing and then somewhere else you're drawing power out and a light bulb turns on because of the magic of the pointing field like wires are important wires are the guides for electrons and for our power i actually really liked the chain in a tube example that derrick used in his video because you need to make almost no changes to that to make it extremely accurate so i have this set up here i've covered a bunch of things with paper so it's a little easier to see these wires are basically invisible when you stretch them out over a field which makes it one hard to set up and two very hard to film if you see this whole bunch of stuff you should see battery and switch and this you should just imagine as light bulb and everywhere these probes show up really accurate clock other than that we just have these wires that stretch out two this way in a big loop and two that way in a big loop so for a moment entertain the idea that electrons are actually little charged hard spheres i know that that's not technically what they are if you want to go into the full wave function but for a lot of cases you can gain a lot of intuition by thinking about them as actual physical particles so if we imagine that this wire right now is full of electrons say it's like one line of electrons the trick to metals is that in metals due to some really cool solid state physics electrons are actually able to move long distances and when i say long distances i don't mean like down a transmission line i mean like millimeters because most electrons in most solids even most of the electrons in a metal are actually restricted to moving within a few angstroms you know every atomic nuclei is only spaced out by a couple nanometers and if an electron is never allowed to leave its own atomic nucleus then you know it's it's hardly going to move at all so to move millimeters is enormous and that's what we're leveraging when we use wires to transmit electric power now when i connect this circuit up again imagine that this is just battery and switch when i flip that switch the battery starts pumping electrons from one side to the other that's all a battery does is basically remove an electron from this wire and set it at the end of this wire then we've got too many electrons over here this electron that we just placed the battery pumped into this wire is going to repel the next electron in the chain the next negatively charged thing and then that electron is going to be it's going to move a little bit and it's going to repel the next electron which is going to move a little bit and all of them are going to start moving at once you know once that that ripple that wave of motion reaches the end of the line over here we're actually doing the opposite we're pulling electrons out of this wire so that we can put them over here which means that we don't have enough electrons over here so it actually ends up taking on a positive charge because there aren't enough electrons to shield out the positive static electric fields from all of the atomic nuclei and that means that nearby electrons are actually attracted to this region another way to think about it is that this electron used to be repelled equally by both of these electrons on both sides of it and when we take this one away it's only being pushed from this side so it ends up moving in towards the battery and both of those waves of motion travel down the line at approximately the speed of light because the interaction between electrons is actually modulated by photons by light like how does this electron right here know that this electron is too close to it so it wants to start moving that way it knows because of the electromagnetic field and the electromagnetic field is modulated by photons and information about the electromagnetic field travels at the speed of light so what does this say about the chain in a tube model well i i would say that the chain in the tube model can be made extremely accurate if you say that the the chain links are actually bigger than the tube the tube still constrains the motion of the chain links like you can't have an electron leave the wire but the electron the sphere of influence of the electron is actually much larger than the wire so this electron need not only interact with the one right next to it you can draw if you if you drew all these electrons as dots you could draw springs between all of them and considering that they're only allowed to move in a line here you know the farther away springs are weaker and and so on but they all really do still interact with each other which begs the question can this electron right here interact with an electron in this wire and the answer to that is yes and that is the absolute core of this thought experiment now real experiment and why we just saw the effects that we did if you think about this top down view again the spot where this wavefront lives actually has too many electrons in it electrons are moving towards this point from one side but they aren't moving away from this point on the other side so we must be accumulating a little bit of negative charge right here and that little pocket of negative charge is going to have a net electric field likewise on the other side we have a region that's depleted of electron meaning that this little spot is actually positively charged and it emits its own net static electric field now imagine an electron sitting on this wire it's free to move and it's simultaneously repelled from this pocket of negative charge and attracted to this pocket of positive charge so it accelerates this way and passes through the light bulb it's not a lot of current but it's almost immediate because as soon as these waves leave the battery we get this charge imbalance that reaches across the air gap with electric fields and pushes electrons around without the far ends of the wire even knowing okay so i just ran all the way down this is the end of the line this is the wire that goes to the light bulb this is the wire that goes to the switch two empty wires down here without changing any of the triggering or anything i'm gonna fire this again and i would expect that we still see this blip but then it drops back down to zero now yep there we go yeah it sort of fluctuates around and then it drops back down and the current through the uh near side also drops down once it charges up the capacitor that is the two wires it stops that's really cool so for the first 1.6 microseconds of this circuit being on it is bound by the laws of relativity to look exactly like the trace from when the wires were fully connected we have to wait 1.6 microseconds for the oscilloscope and the resistors and all the bits that we're measuring to be able to learn that the end of the wire doesn't exist and then they start behaving differently but there's a bunch of stuff that happens after that light speed delay that is really interesting so this green trace right here is showing the current through one of the resistors that's right next to the battery and switch and that says that there's some current flowing at the beginning but then that current actually sort of dies off after a while so here we have the battery tirelessly pumping electrons from one side to the other except eventually all those electrons pile up because we've cut the end of the wire they have nowhere to go and the battery stops being able to pump electrons the most interesting thing about this plot is actually the fact that the white trace reverses briefly that means that we push a whole bunch of current through the light bulb but then when it runs into the ends of the wires that are cut that current actually sort of sloshes back and we get electrons that are bouncing back and forth on the line like a wave through a slinky i don't want to get into how electronic resonators work because i don't know enough to do that topic remote justice but i do know that you need both capacitance the ability to basically store charge and resist changes in potential and inductance the ability to give charges inertia and resist changes in current and the combination of those two things can actually result in a circuit that bounces back and forth and back and forth and that's what we're seeing right here but yeah without um calling up einstein and saying that we found that information can go faster than light we are always going to have the first 1.6 microseconds look exactly the same i think that's really cool okay so if you disconnect both ends of the wire you still get that transient and then it sort of bounces around and eventually goes away to zero but it never actually turns on the light bulb all the way because you can't flow current through a wire that's disconnected now i want to see what happens when the two loops of wire are different lengths okay i'm sorry i really honestly hate to do this i also just because i want to be done editing but i've been editing and editing and it looks like it's going to be some 45 minute monstrosity so i decided to break the rest of the experiment variants off into their own part 2 video i had so much fun with this project and i actually learned a lot by fiddling with the geometry of the wires and trying out different things and different variations on the experiment i got some results that i really wasn't expecting like at all and i want to be able to take the time to properly explain all of those weird results in a second video as one of the now many replies to derek's thought experiment i hope that this video has been more enlightening than confusing there are a lot of things going on here it is a deceptively complex problem and there are a bunch of pieces of physics that i think are important that didn't even make it into this video so i hope that the capacitive wires hypothesis that i have proposed here makes sense and i hope that you'll stick around and join me for the part 2 video i will plan to see you there [Music] [Music] you

Channel: AlphaPhoenix

Views: 85,226

Keywords:

Id: 2Vrhk5OjBP8

Length: 22min 48sec (1368 seconds)

Published: Fri Dec 17 2021

Reddit Comments

[u/MrFette]

Here is the video referenced in the title.

I went from understanding Veritasium's explanation in his video, to then hearing that his explanation wasn't exactly correct from ElectroBoom's response, to now feeling like the explanation in this video is explaining the phenomenon in the way Veritasium said professors incorrectly explain it to students using the chain and tube demo.

I am far from an electrical engineer or physicist, and feel like, after all three videos, I'm just as clueless as to how it works as I was before watching any of them, haha

[u/sahukhala]

The problem with the Veritasium video is that it made it sound like all current directly travel through space between the battery and the light bulb.

[u/BigHaircutPrime]

I think the problem with Veritasium's video is that Derek constructs this super ideal scenario that conveniently ignores important variables, and then uses that to prove his theory. While a lot of this stuff confuses me, the key seems to be this "bulb turns on at any current" detail. Once you look at things from a realistic perspective, Derek's theory falls apart.

I'm loving all of these explanations. I just wish that someone could make a video that builds upon itself in a "explain it like I'm 5" way and doesn't just throw a bunch of technical terms 95% of people won't get.

[u/Pathfinder24]

I think for all practical purposes veritasium is wrong. There is some disturbance communicated through the field, but it is not the power from the battery, which is shown now to come later due to travel time through wires.

The experiment I asked for in the original video and I'll repeat here is to have two such circuits adjacent. That way one battery switch can be flipped and we can observe if the disturbance is seen on both bulbs or only the connected bulb. If it was seen only on the "correct" bulb that would raise questions about how the bulbs have non local information about their distant connectedness.

I think at the core of my skepticism is that the bulb cannot turn on before the ends of the wire have current without violating locality or causation.

[u/seanalltogether]

I think his explanation starting at 15:30 is the real a-ha! moment for me. Electron flow is constrained by the wire, but an electrons sphere of influence is much greater then the wire. When an electron is forced to move, it will push on all other electrons in its sphere of influence, in this case its sphere of influence includes electrons in the other wire that's close by.

I would assume the greater the voltage, the greater the sphere of influence, but I'm not sure.

[u/TheMalcore]

Tycho made a really great and concise break-down of Veritasium's explanation as well.

[u/iamamuttonhead]

This dude, IMO, wins the video war. I suspect ElectroBoom loves this video if he's seen it. In any case, this video has made me entirely comfortable with the phenomena that are the core of this thought experiment. Well done AlphaPhoenix!

This is entire debate really exposes the value and benefits of the internet. We started with Veritaseum's very interesting thought experiment where he does a good job of explaining some difficult ideas (for us non-ees and non-physicists) but sort of finesses some of the details to arrive at his conclusion. Then ElectroBoom explains why some of the shortcuts/assumptions that Veritaseum makes are sort of designed to support his conclusion. Then AlphaPhoenix comes in and actually does a real damn experiment and explains everything. Before the internet there was just no good way of exposing the world to this kind of discussion in such little time. I've always been somewhat mystified by electricity and these three videos have done a great job of helping me to understand it better.

[u/eqleriq]

it's just proof of how and why there aren't generalist "scientists" and you need specialized information and practice to get the semantics of your ideas explained clearly so they don't sound like non-answers to trick questions.

The Veritasium video, to an EE or really anyone focusing on electrical physics, sounds like someone loosely paraphrasing very specific things and coming to vague conclusions as a result.

All for the sake of cramming an entire couple centuries worth of knowledge in a specific field into a publicly consumable short video.

Here's an idea: not every physics concept can be handwaved down into a format that increases clicks and viewing time

[u/what_comes_after_q]

Why does he not include the transistor in the model? He is saying it's a switch, which is functionally true, but it has a big caveat in the model. There is going to be current flowing from the base to the gate of the transistor. I can almost guarantee that the initial current he is seeing is the transistor current.

Veritasium is also mostly wrong. The way to think of AC current, think of waves in a pond. If you start splashing in the pond, your waves will start propagating. This is what you will see in the wire. The power plant will start making the AC waves. The wave will propagate over time down the wire just like the waves across the pond. It will propagate at the speed of light in the medium.

It is absolutely correct that the field is what is providing power, but the analogy is a bit confusing. It's the field because the field holds energy in the same way gravity is what makes objects heavy, not the object itself. However, you still need the object to have gravitational potential energy. Electrons are the carrier particle for EMF. You need to move electrons to generate magnetic field. This is actually described in maxwell's equations.

In short, this is a silly argument.