Simulating Veritasium's electricity riddle - Page 1
Forum section: electronics / projects

author: akku46
author_rank: Newbie
timestamp: February 07, 2025, 06:37:41 pm
content: edit: Fixed image ordering. If you saw the first edition of this post, the images were wrong.Hi all.Not sure if this is the best subforum to post this in, but please move the thread, if it isn't.Veritasium's electricity riddle, which many of you probably know, popped up on another forum, although in a slightly different form compared to the original. I decided to finally simulate it to find out whatactuallyhappens in the circuit. Not sure if anyone already did it to such amount of detail, at least I have not seen anyone doing so, but I'll post my results anyway, in case someone is interested, and also to get some insight whether my simulation looks correct or not.I have seen this being simulated with L/C chains but that method does not accurately represent the experiment in question, so I wanted to use a better approach.The video is here:And the follow-up video with some measurements on a smaller setup is here:The setup: A battery with a switch connected to it, and a lamp 1 meter away. The lamp is connected to the battery and the switch with two wires, 300 000 km long each, each forming a rectangular loop, 150 000 km x 1 m.The switch is closed. How quickly does the lamp light up?Veritasium makes some assumptions such as: the resistance of the wires is neglected, and the lamp lights up immediately when there is any voltage over it.The answer he presents is 1m/cseconds.Last time I attempted to simulate this, LTspice didn't work with my circuit. But the latest version seems to handle this problem nicely.Let's make a few assumptions:The distance between the wires is "small" so the circuit can be modeled as two parallel-wire transmission lines with wire spacing of 1 meter. Wire size must be assumed, but as long as we choose some "reasonable" wire size, it doesn't actually matter very much. Let's say the wire is 2.5 mm2 which is common in electrical installations in Europe, its diameter is about 1.78 mm. Let's also assume that the line is air or vacuum-insulated and its propagation speed isc.With this kind of wire, we get the following parameters:Z0 = 840 ohm, for a lossless line, and this doesn't change very much with small changes to the wire diameterL = 2.809 µH/mC = 3.96 pF/mR = 0.01416 ohm/m, for copper wireSource:https://phys.libretexts.org/Bookshelves/Electricity_and_Magnetism/Electromagnetics_II_(Ellingson)/07%3A_Transmission_Lines_Redux/7.01%3A_Parallel_Wire_Transmission_LineThe battery is clearly 12 volts in the video, and the lamp can be an incandescent bulb and it's modeled simply as a resistance which is not fully accurate but sufficient for now. Lamp size/power must be assumed, but let's try two cases: 12 watts (12 ohms) and 0.17 watts (840 ohms).First, without taking the wire resistance into account.In all of the following simulations, the switch closes at t = 0.2 sec.Simulation of a 12-watt lamp produced a result that actually caught me by surprise, even though I knew that there would be some delay before the lamp gets approximately full voltage (click for bigger images):There is some voltage almost immediately (after 1m/c seconds which is not shown by this simulation), but only about 80 mV. Will it cause the lamp to light up? I would say no, unless it's some kind of a sensitive voltage detector device as assumed in the video.What is more surprising is that it takes several minutes to reach near-full voltage! For example, after 2 minutes there is about 10 volts over the lamp.But if you think how the reflections happen on such a transmission line, this seems logical. This kind of arrangement is a bit confusing to understand - it is much easier if there are just a battery, switch and a lamp in series, and just one loop of wire that connects the other side of the lamp to the other terminal of the battery.With a 0.17-watt (840 ohm) lamp we get faster light-up:4 volts almost immediately (1m/c secs), approximately full voltage in less than 10 seconds.We can repeat this with different lamps and wire spacings. For example, with a 20 000 ohm lamp there is some temporary overvoltage on the lamp before it settles.Next, let's not neglect the wire resistance. With 2.5 mm2 copper wiring it will be megaohms and we already know how well the lamp will produce light in that case.With a 12-ohm lamp:There is a short pulse of about 80 mV but it quickly falls down. The lamp would not even blink, unless it is Mr. Veritasium's strange voltage detection lamp.With a 840-ohm lamp:Much stronger pulse of voltage, but is it long enough to actually cause a the lamp to even blink? A LED lamp maybe, incandescent not.Please correct me if you see any problem in these simulations!It's very easy to get garbage out when simulating if you don't set certain parameters correctly and I'm not exactly an expert in simulations.Using transmission line models in the simulation works when the distance between the wires is "small" so whatever happens inside the line happens "instantly". We can add the slight delay of 1 meter when we discuss the results, but in case of a wider setup, simulation should be done on electrical/magnetic field level.Veritasium has many interesting videos about physics and mathematics problems, but I think this one created more confusion than insight.The video states that the speed of electrons is not important which is true, and that energy flows in the EM field.But it doesn't discuss how much voltage the lamp actually gets, and in what time. We can see that the resistance/impedance of the lamp vs. the transmission line impedance greatly affects this. In the follow-up video, the first voltage was measured and it was not much.The video also states "Energy does not flow in wires", but it does not discuss what happens if you cut open one or both of the wire loops from the far side.It also doesn't take the wire resistance into account which prevents any real-life lamp from producing light. Unless we use some crazy high-voltage high-resistance lamp.But a follow-up question to this issue is: if the wires are located in space, will they get cold enough to become superconductive?

author: ledtester
author_rank: Super Contributor
timestamp: February 07, 2025, 08:01:52 pm
content: Previous 2000 message discussion:https://www.eevblog.com/forum/chat/veritasium-(yt)-the-big-misconception-about-electricity/and some LTspice simulations people tried:https://www.eevblog.com/forum/chat/veritasium-(yt)-the-big-misconception-about-electricity/msg3828668/#msg3828668https://www.eevblog.com/forum/chat/veritasium-(yt)-the-big-misconception-about-electricity/msg3858857/#msg3858857

author: Zero999
author_rank: Super Contributor
timestamp: February 07, 2025, 08:10:30 pm
content: Quote from: akku46 on February 07, 2025, 06:37:41 pmA couple of hints:Label the net names: V(n000x) doesn't mean much.Post the .asc file so we can all have a go at simulating it.Quote from: ledtester on February 07, 2025, 08:01:52 pmDid anyone simulate it?

author: ledtester
author_rank: Super Contributor
timestamp: February 07, 2025, 08:32:47 pm
content: Quote from: Zero999 on February 07, 2025, 08:10:30 pmI updated the message with a few simulations I found.

author: akku46
author_rank: Newbie
timestamp: February 08, 2025, 11:45:35 am
content: Quote from: ledtester on February 07, 2025, 08:01:52 pmLooks like I'm getting equal results as those people if I choose similar impedances (or impedance ratios, load vs. lines). So perhaps I'm at least not doing it terribly wrong.The second post also shows an elegant way to get rid of the problem that I had last time when I tried to simulate this. I tried some other ways back then and the simulator got stuck.Quote from: Zero999 on February 07, 2025, 08:10:30 pmYeah, I should have at least mentioned that the graphs are showing voltage over the resistor ("lamp"). That was what I was interested in.I attached the .asc files. The second one with lossy transmission lines requires either starting with DC sources at 0 ("startup") or skipping the initial conditions calculation ("uic"), otherwise it doesn't produce meaningful results.