I made a video about a gigantic circuit with light-second long wires that connect up to a light bulb, which is just one meter away
from the battery and switch, and I asked you, after
I closed the switch, how long will it take for us to get light from that light bulb? And my answer was 1/c seconds. - And his answer is wrong. - We would be able to communicate faster than the speed of light. - That violates causality
and common sense. - This is actually a bit misleading. - Misleading. - Misleading in a way. - Extremely unconvinced. - Naughty Mr. Veritasium has
stirred up a right hornet's nest. - Clearly I did not do a
good job of explaining what was really going
on in the last video. So I wanna clear up any
confusion that I created. So behind me, we have a scaled
down model of this circuit. It is only 10 meters in
length on either side. Obviously that's a lot
shorter than one light-second, but for the first 30 nanoseconds, this model should be
identical to the big circuit, and Caltech has very fast scopes, so we'll be able to see
what's going on in this time. I got a ton of help on
this from Richard Abbott, who works on LIGO, the
gravitational wave detector. Over here, we are going
to put a little resistor, which is gonna be the stand
in for our light bulb, and we're going to measure it with
a scope and see essentially, what is the time delay
between applying a pulse on the other side, basically
flicking the switch, for us to get a voltage
across our resistor. And the magnitude of that
voltage is really important. A lot of people thought
it would be negligible. - The amount of energy supplied
by this is so minuscule. - A tiny, tiny effect, right? - The amount of power you're getting to the lamp over here, it's nuff-all - He meant the light turns on at any current level immediately. - That is not what I meant. - Well, actually, with that assumption, Derek's answer is wrong. The light never turns off no matter the state of the switch. Some electrons will jump the
gap and result in an extremely small continuous leakage current. - Let me be clear about
what I am claiming. Okay, it is my claim
that we will see voltage and current through the
load that is many orders of magnitude greater than leakage current, an amount of power that would actually produce visible light if you put it through
an appropriate device, and we will see that power there in roughly the time it takes the light to cross the one meter gap, but to understand why this happens, we first have to clear
up some misconceptions that I saw in responses. Misconception number one
is thinking that electrons carry the energy from
the battery to the bulb. Let's say we have a simple
circuit with a battery and a bulb operating at steady state. If you zoom in on the light bulb filament, you'd see a lattice of positively
charged cores of atoms, the nucleus and lowest
shells of electrons, surrounded by a sea of negative electrons, which are free to move around the lattice. The actual speed of these
electrons is very fast, around a million meters per second, but all in random directions. The average drift velocity of an electron is less than 0.1 millimeters per second. Now frequently, an electron
will bump into a metal ion, and transfer some or all
of its kinetic energy to the lattice. The electron slows down
and the metal lattice starts wiggling more. It heats up. And ultimately this is
what causes the filament to glow and emit light. So a lot of people will
look at this and conclude the electron carried the energy
from the battery to the bulb where it dissipated its
kinetic energy as heat, but consider, where did the
electron get its kinetic energy from before the collision? It didn't carry that
energy from the battery. In fact, if the circuit has
only been on for a short time, that electron hasn't been
anywhere near the battery. So how was it accelerated
before the collision? The answer is, it was by an
electric field in the wire. The electron repeatedly
collides with the lattice, and loses energy. And after each collision, it is again accelerated
by the electric field. So although it is the
electron that transfers energy to the lattice, the energy
came from the electric field. So where does that
electric field come from? Well, a lot of animations make
it look like the electrons push each other through the circuit via their mutual repulsion. So you might think the electric field comes from the electron behind it. There is the analogy of
water flowing through a hose, or marbles in a tube. This is misconception two,
thinking that mobile electrons push each other through the circuit. That is not how electrons
flow in circuits. The truth is if you
average over a few atoms, you find the charge density everywhere inside a conductor is zero. The negative charge of
electrons and the positive cores of atoms perfectly cancel out. So for each repulsive
force between electrons, there is an equal and opposite
force from the positive ion next to it. These forces cancel out. So mobile electrons cannot push
each other through the wire. So where does the
electric field come from? Misconception number
three is that it comes entirely from the battery. This makes intuitive sense, since the battery is the
active element in the circuit, it has a positive side
and a negative side. So it has an electric field, but this is not the electric
field that all the electrons within the wire experience. Consider that the electric field of the battery is much
larger close to the battery. So if its field were really what's pushing the electrons around, then if you brought the light
bulb close to the battery, then the bulb would glow much brighter. And it doesn't. The truth is that the
electric field in the wire comes both from the
battery and from charges on the surface of the
wires of the circuit. As you go along the wire
from the negative end of the battery to the positive end, there is a gradient of charge
built up on its surface, starting with an excess of electrons, through to roughly no
charge in the middle, as we'll see the steepest
charge gradient is actually across the load to a
deficiency of electrons, the exposed positive cores
of atoms on the surface of the positive end of the wire. All these charges and the
charges on the battery create the electric field
everywhere inside the wires. They also create an electric field in the space around the wires. These surface charges were
set up almost instantaneously when the battery was
inserted into the circuit. You might think you'd
have to move electrons a significant distance to
create this charge distribution, but that is not the case. Even a slight expansion or
contraction of the electron sea, with electrons moving on
average, the radius of a proton, can establish the surface charges you see. So the time for the charges to move is completely negligible. The speed of the setup process is limited only by the speed of light. Once that surface charge
distribution has been established, the battery does continuous
work to maintain it, by moving electrons through the battery against the Coulomb force. In the load, the electric field created by all the surface
charges, accelerates electrons, which dissipate their energy
in collisions with the lattice. So the battery is putting
energy into the field, which electrons take out
and transfer to the load. An electrical engineer
who made a response video, Ben Watson, came up with a good analogy. The battery is like a shepherd. The surface charges are the sheep dogs responding to his orders. And the mobile electrons are the sheep, guided by those barking dogs. The surface charge description
of electric circuits is omitted from most textbooks, but there is a great treatment of it in Matter and Interactions
by Chabay and Sherwood. They also have a VPython
simulation where you can see the positive surface charge in red, and negative surface charge in blue. You can see how this
entire charge distribution creates a net electric field
shown by the orange arrow, everywhere in and around the circuit, everywhere inside the wire, the electric field has the same magnitude and its direction is along the wire. This is really showing
you the electric field in the center of the wire, but
it's depicted on the surface so you can see it. In this circuit, all the conductors are
made of the same material, but the segment at the bottom has a much narrower cross section. So it represents a resistor. Since the cross sectional area is smaller, the electron drift velocity
through the resistor has to be higher so that it
can carry the same current as everywhere else in the circuit. Now, drift velocity is
directly proportional to electric field. So the electric field must be
largest inside the resistor. And this is achieved by
having the steepest gradient of surface charges here. You can also see the contribution
to the net electric field from the battery in magenta, and the contribution from
surface charges in green. Far from the battery, most of the electric field
is due to surface charges, whereas close to the battery,
it has a greater contribution and the field due to surface charges is actually in the opposite direction to the field from the battery. So to sum up, electrons
don't carry the energy from battery to bulb, nor do they push each
other through the wire. They are pushed along
by an electric field, which is created by
charges on the battery, and charges on the surface of the wires. With this view of circuits, things that might have
previously seemed mysterious, make a lot more sense. Like if electrons leave a
battery at the same rate, and with the same drift
velocity as they return, how do they carry energy from the battery? The answer is they don't. They are accelerated by the electric field before each collision with the lattice. At a junction, how do the
correct number of electrons go down each path? Well, they're guided
by the electric field, which extends everywhere
throughout the circuit. The fields are the main actors, extending everywhere
throughout the circuit, and the electrons are just their pawns. So how does this apply to the big circuit? When the battery is
connected into the circuit, even with the switch open,
charges rearrange themselves. On the negative side of the battery, there is an excess of electrons on the surface of the
wires and the switch. On the positive side, there
is a deficiency of electrons. So positive charges built up
on the surface of the wires. The charges rearrange themselves
until the electric field is zero everywhere inside the conductor. This electric field is due
to all the surface charges and the charges on the battery. There is an electric
field outside the wires due to these charges, but
it's zero inside the wires. We now have the full potential difference of the battery across the switch. And no current is full flowing,
except for leakage current, which I'll assume is negligible. When we close the switch, the surface charges on
both sides of the switch neutralize each other on contact. And at that instant, the electric field inside the
conductor is no longer zero, and current starts flowing
through the switch. Simultaneously, the new electric field from the modified surface charges radiates outwards at
essentially the speed of light. And when it reaches the bulb, the electric field inside
it is no longer zero. So current starts to flow here too. This is why I said the bulb
lights up in 1/c seconds, because the bulb was one
meter from the switch, and the change in the
electric field travels out at the speed of light. As some of you pointed out, the answer should have been
one meter divided by C. And I apologize for the
casual use of units. If you were to move the switch, then the bulb would take a different amount of time to emit light, which just depends on the distance between the switch and the bulb. In response to my original video, Ben Watson simulated
a model of the circuit using software from Ansys called HFSS. It provides a complete
solution to Maxwell's equations in three dimensions. Now have worked with Ben and Ansys to make these simulations. When the switch is closed, you can see the electric
field radiate out, and as it hits the far
wire, it generates current. The electric field is to the right. So the electrons flow to the left. This simulation shows the
magnitude of the magnetic field, which falls off pretty
rapidly as it crosses the gap. But then a magnetic field
appears around the far wire, and this magnetic field is created by the current in that wire. To me, this suggests that it
really is the electric field, and not the changing magnetic field that creates the current through the load. Some commenters on the original video claimed my answer of
three or four nanoseconds violates causality. I guess they were thinking that the bulb would only go on if the
circuit were complete. And it wouldn't if the
circuit were broken somewhere, which could be up to
half a light second away. So it seemed like I was saying, we could get information about the status of the whole circuit, even
half a light second away, in just nanosecond seconds. But that is not what I was saying. What I should have stated explicitly, is that the bulb goes on regardless of whether the
circuit is complete or not. The current flows through the load due to the electric field it experiences. To illustrate this, Ben added
a wire below the circuit that is completely disconnected from it. You can see is that its response to the changing electric
field is virtually identical to that of the top wire, at least up until the
signal reaches the far end and reflects back. This is why my answer
doesn't break causality. At least initially, connected
and disconnected wires behave exactly the same. Using this software, you can also simulate the Poynting vector that is the cross product of
electric and magnetic fields. In the last video, I showed
how the Poynting vector indicates the direction of energy flow. And after the switch is closed, the Poynting vector
points out of the battery, across the gap to the other
wire, whether connected or not, because energy is carried
by fields and not electrons, it can go straight across the gap. So you might ask, why
do we need wires at all? Well, we don't, I mean,
phones and toothbrushes charge without wires connecting them to a stream of electrons, and researchers have
demonstrated remote charging using the energy from WiFi signals. Wires are more efficient
because they channel the fields and hence the energy from source to load. Here's another angle
on the Poynting vector. And you can see once there
is current in the top wire, the fields around it carry
energy in both directions. Now, of course, the
Poynting vector also points parallel to the first wire, carrying the energy around the circuit as most people would expect. But again, note how the energy
is carried outside the wires, not in the wires. Now admittedly, thinking
about circuits this way is complicated. And since nobody wants to
solve Maxwell's equations in three dimensions just to
analyze a basic circuit, scientists and engineers
have worked out shortcuts. For example, Ohm's law, voltage equals current times resistance, is just the macroscopic result
of all the surface charges, their electric fields
and zillions of electrons bumping into zillions of metal ions. You can simplify all that physics into a single circuit element, a resistor, and the basic quantities
of current and voltage. This is called the lumped element model, lump all the spread-out
multi particle and field interactions into a few
discrete circuit elements. And we use this technique every time we draw a circuit diagram. So our original diagram of
the big circuit is flawed because fields between the wires are important to the problem, but there are no circuit elements to indicate these interactions. To fix it, we need to add
capacitors all down the wires. These capture the effect
charges on one wire have on the other. If there are negative
charges on the surface of the bottom wire, for example, they'll induce positive charges on the surface of the top wire. Also, since these wires are long, they're gonna create
significant magnetic fields around them, which resist
changes in current. So we model this with inductors
all the way down the wires. We could also add resistors, making what electrical
engineers would recognize as the distributed element
model for a transmission line. But we're assuming that these
wires are super conducting. So this is how we could model a super conducting transmission line. This diagram offers another
way of understanding why current flows through
the load almost immediately. When you first apply a
voltage across a capacitor, current flows as opposite charge builds up on the two plates. In the short time limit, a
capacitor is a short circuit. It acts just like an ordinary wire. Once it's charged up,
no more current flows, but by this point, the next
capacitor is charging up. And then the next one,
and then the next one. And so we have a loop of
current that is expanding out at roughly the speed of light. This is of course, just
another way of talking about the effect the electric field that the bottom wire has on the top wire. One reason it's useful to
look at the circuit this way, is because you can use
the values of inductance and capacitance to calculate
the characteristic impedance of the transmission lines. You can think of this as the resistance to alternating current
that a source would see when sending a signal down the wires. The characteristic impedance
is equal to the square root of inductance divided by capacitance. And for our circuit, I measured the capacitance and
the inductance of the lines, - 11.85, call it, micro Henry's. - So we got a characteristic
impedance of about 550 Ohms. To maximize the power
delivered to our load, we want its resistance to
equal the sum of the other impedances in the circuit. So that's why we picked
a 1.1 kilo-Ohm resistor. Now, I hope you're convinced
that current will flow as soon as the electric
field reaches the far wire. The question is, how much? Are we gonna see an appreciable voltage even with these lines a meter apart? That's what it seemed like a
lot of people were doubting in the last video. So that's really what we
want to find out here. Okay, so now we're
putting a pulse in there. - Yep. Well looky, looky, Derek. - So what do we got
that yellow one is our- - Got a fraction of the
applied voltage overshoot. And then- - So it looks to me
like the initial voltage that we're getting is about- - Five volts per division. So it looks like about five volts, roughly four or five volts. - The green curve rising
up to around 18 volts is the source voltage. And the yellow line is the
voltage across the resistor. So after just a few nanoseconds, this voltage rises to around four volts. Since the resistor was a kilo-Ohm, that means four milliamps of current are flowing in the resistor, before the signal goes all
the way around the circuit. So we were transferring
about 14 milliwatts of power. This is what 14 milliwatts
of light actually looks like. So, yeah, it's not a fully on bulb, but it is visible light and
way more than you would get from just leakage current. Now, some of you may argue, it's unfair to use a little
LED when I showed a bulb and car battery in the original video, but those items were for
illustrative purposes only. The clue that this is
actually a thought experiment is the two light-seconds
of super conducting wire that stretch out into space. This is not an engineering
question about how best to wire up a light bulb in your bedroom. The question was intentionally vague. And if you want to
choose circuit components such that the bulb never goes on, you are welcome to do that and I support your conclusion. Just to me, the most interesting way to approach this problem is to ask, how could you make the
light go on fastest? I was worried that those
long wires would pick up all the radio waves passing through, and we wouldn't even be able to see the signal for that noise, but you can see clearly on the graph that the signal is way
above the noise level. Alpha Phoenix set up a kilometer of wire and got a very similar result. - 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. - YouTuber, ZY, simulated the
transmission line circuit, and found that even with
realistic assumptions, he transferred 12 milliwatts
to the load straight away. - Derek is actually more correct than we give him credit for. So, I believe that he's
correct on all counts. And the question is neither deceptive or requires like technicalities. - So everyone agrees that a steady, small, but way-bigger-than-leakage-current signal flows through the load in the first second after the switch is closed. Is it enough to emit light? Yes, if you use an LED. But the point of the thought experiment was to reveal something
that's normally hidden by the way that we think about
and teach electric circuits. You know, we use voltage and
current and lumped elements because they're more convenient than working with Maxwell's equations, but we shouldn't forget
that the main actors are actually the fields. They are what carry the energy, and you don't have to take my word for it. This is Rick Hartley, a veteran printed circuit board designer. - I used to think in terms
of voltage and current. And I used to think that
the energy in the circuit was in the voltage and
current, but it's not. The energy in the
circuit is in the fields. The most important thing you need to know is that when you route a trace, you better define the other
side of that transmission line, because if you don't, those
fields are gonna spread and they're gonna leave
you an unhappy individual. - I think one of the things
that I'm most excited about the circuit's video
was the response videos I saw by so many people, especially people with
far better credentials in electrical engineering than me. I really enjoyed watching those videos. So I feel like my circuits
video was kind of like, a mistake on my part in certain ways that I didn't delve deep enough into this part of the problem. I honestly didn't think that this was the focus of the video, but clearly everyone who
watched it did, so that's on me, but by making that mistake, and by not going deep into my explanation, I invited seemingly a
whole bunch of other people to make explanations,
which I thought were great. And some people like
Alpha Phoenix even took up the challenge and set up his
own version of the experiment. So, frankly, I'm just really
excited at what came about, even though I do acknowledge
that this was my fault in the first place. Like I should have done
a better explanation, but by not doing so, you know, there are a lot of great
explanations out there. And that's what I love. So I'm gonna recommend a whole bunch of electrical engineering YouTubers to you in case you wanna check those out because they're a lot of great channels, and you should really see how
they think about electronics, and how they explain this circuit. Hey, this video was
sponsored by Brilliant, the website and app that
gets you thinking deeply about concepts in math,
science, and computer science. Brilliant is sponsoring a
lot of our videos this year, because they know someone
who makes it to the end of a Veritasium video is
exactly the sort of person who would love to learn with Brilliant. And they have a great course
on electricity and magnetism, which methodically steps
you through an introduction to E&M with questions, simulations,
videos, and experiments. I really think this is
the best way to learn because the sequence of
steps is so well thought out. The difficulty builds gradually. And by asking you questions, you are forced to check your
understanding at each step. If you need help, there's always a useful
hint or explanation. You know what sets Brilliant
apart is their interactivity. You can learn calculus or machine learning or computer science fundamentals all in this very active way. So I encourage you to go over
to brilliant.org/veritasium and just take a look at their courses. I will put that link
down in the description. And if you click through right now, Brilliant are offering 20% off an annual premium subscription to the first 200 people to sign up. So I want to thank Brilliant
for supporting Veritasium. And I wanna thank you for watching.
Having seen the two videos and several response videos, I now understand absolutely nothing about electricity. Thanks.
If you want a great answer from the internet, post the incorrect or vague answer.
If only we could figure out how magnets work.
5/10
He acknowledges that the original video had flaws, but does double down on the incorrect idea that the bulb lights up nearly instantaneously. Sure, if you design an antenna to transmit a non-negligible amount of power across the gap, you can do that, but for the regular wire/battery setup of the original video the bulb will not light. “Illustrative purposes” is, in my opinion, a cop out.
I think it would have been better to just skip trying to justify the earlier video and just better explain what is going on with the transients and steady state.
A few months ago Derek made a video on how electricity actually works. It received a lot of attention from engineers, physicists, and geeks on the internet, and was characterized as everything from misleading to completely wrong. In this video he revisits the claims made in that video with a new experiment done at Caltech.
EE here. This is a very complicated way of describing how antennas work presented as how electricity in general travels. The induced current at the bulb would not be nearly maximum until the field travelled all the way from the closed switch along the wire. The. early current would be some lesser amount due to antenna-like radiation. Right?
His original video wasn't controversial it was misleading. He used wrong units and showed an example of very specific behavior/property in such a way that it makes you think you knew nothing about electricity. That's why so many electrical engineers went on to explain what is actually happening.
I can't help but think he's just trying to capitalize now on the whole mess previous video stirred up.
I think this video is super problematic> I normally like veritasium but these two videos are incredibly misleading. He basically takes a victory lap for most of it, explaining details of electric circuits and imo often phrasing them in a way to make himself look good.
The main issue is that his original video and this video heavily imply that the electric field crossing the gap is the main way that electricity works, and that there is this big misconception about circuits. His original title was "The big misconception about electricity" and this video title is "How electricity actually works" yet what he describes is a small interesting side effect of circuits, not the main way you deliver power to a device. Spoilers for his actual experiment below
His own experiment demonstrates this perfectly. You get a small amount of voltage (not negligible) that crosses the gap at distance to resistor / speed of light. This makes sense, its an antenna. However this is not the main mode that changes the voltage across the resistor, it takes the distance across wires / speed of light for that to arrive. If you were to move the bulb farther from the switch but keep the wires the same length, this short term effect would disappear but the long term effect would remain perfectly constant. What he describes in more an interesting epiphenomenon and is in no way the "main" way that electric power in any of your devices work, otherwise as you moved a wired bulb around it would change intensity.
Veritasiums conclusion is well defined and well backed up and I do not disagree with it at all ... but... his explanation does not sit well with me (and I am not an idiot, I have a relevant degree too).
He is interested in being aware of a more accurate interpretation of how energy is transferred and he keeps saying something like "it is not the electrons but the fields that transfer energy" and this (I believe) is essentially moving to almost the exact same mistake on the other side of the issue.
The better answer, in my opinion, is to remember that the Electron and its field are the exact same thing. Every interaction we have with an electron is through the fields that surround it, even down to the electron colliding with the lattice (just not EM fields). So saying its not the electron pushing is also wrong, the better realization is to remember how the electron pushes other electrons and at what range it can push them, and this mechanism is as he describes, it just it is as much the electron doing it as it is the field because they are the same thing.
This is, in my judgement, just wave particle duality in another manifestation,