hi do you remember derek of veritasio made a video posing an electrical question and everyone including me were fuming over it now the question is after i close this switch how long would it take for the bulb to light up well now he made a new video on the subject and i'm back baby derek in his second video acknowledged that his original question was too vague and did a very good job clarifying a lot of details otherwise in my opinion almost all his answers could be correct but well here i am which means i'm not
happy with all the details i feel like we are not exactly on the same page mind you derek and i are friends hopefully he thinks the same and he's one of the greatest youtube science communicators and his presence is very valuable especially since his idea of science is not melting lipsticks rewind needs more science can we do an experiment love that melting lipsticks melting lipsticks melting lipsticks we want more science melting lipsticks this back and forth is not about who wins it is about all of us understanding science better and what better way to understand
things than my sponsor brilliance interactive courses and quizzes you can learn a ton of what you need for free signing up using my link brilliant.org electro boom and the first 200 people can also get 20 of brilliant annual premium subscription anyway i decided to have a call with derek and discuss which would be much faster than making a video and waiting for his response and less embarrassing too in case i'm wrong so let's call you ready so here i have my friend derek muller who is a science communicator filmmaker television personality and inventor and of
course he's a youtuber at veritasium channel with over 12 million subscribers don't you wish you had a friend like him now you can with a low monthly payment of 39.95 just kidding sir so thank you for taking the call i like to focus on two points that i feel i don't agree with misconception number one is thinking that electrons carry the energy from the battery to the bulb misconception two thinking that mobile electrons push each other through the circuit i think those are both um good statements the point about electrons not pushing each other through
the wire is specifically that the mobile electrons so like the the current carrying electrons are not pushing into each other like you know marbles in a tube or like uh water in a hose so regarding the first point the electrons pushing each other uh i think we need to define pushing first see at this point i'm thinking derek might be thinking that pushing is only between the surface of the material and not between energy fields basically an object pushes another object by exerting force to the other one but in atomic level the two objects don't
actually touch as in matter doesn't touch matter because the electromagnetic interactions between their atoms keep them apart so we can define pushing as an object exerting force to another due to the repulsion of the electromagnetic forces between their surface atoms and i guess the gravitational interaction of matters in larger scales yeah uh if you have two two objects and you bring them close together yeah charges in and around the surfaces can apply uh forces on on the other object oh he knows of course he does and i think of course it's important that there's sort
of like a net force right every electron in a wire will exert a force on every other electron in a wire but the argument i am making is that there are equal and opposite forces on every electron such that the mobile electrons are not actually pushing each other around the main thing i like to think about the battery doing is maintaining this distribution of surface charges as in his video here derek explains that there is a gradient of seemingly static charges only on the surface of the conductor and the density of charges inside the bulk
of the wire is uniform with number of electrons and protons being equal so the charges inside the wire don't push each other to move because the conductor is electrically neutral there and the net force on charges from neighboring charges is zero tera claims it is the surface charge gradients that creates fields inside the wire pushing charges forward why do you think that charge distribution is what is the reason for that i think that charge distribution is set up by the fact that there's a battery there and so the battery has an electric field which influences
the charges right beside the battery and their electric field influences the charges right beside them and their electric field influences the charges right beside them and so on and that's what uh sets up the surface charge distribution the way i always imagined how this all worked was that the battery just pushes an electron in the wire by its electric fields and like a train the electrons push each other forward sending the current through it has always worked for me and explains everything but at the same time i don't have a good reason to reject derek's
notion of surface charges why does it even matter either way the charges are pushing each other forward so again back to the definition of pushing i was talking about that it's the atomic level it's the electric fields and magnetic fields between the atoms that keep them apart it seems to me that the electrons reacting with each other is causing them to move in the wire right and to me it's pushing so so the argument that's made um which is not my argument but the argument that i relay to you is this idea that like okay
we have uh electron moving along here another electron moving along here and of course like the actual motion is like this but they're ever so slowly drifting this way if you if you net it out and then there's kind of a question of like okay so why are they net drifting this way is because this one's pushing this one derrick again explains that because of equal density of electrons and protons inside the wire the charges are not pushing each other forward which i still don't quite agree with i think the sea of electrons is a
bit compressed as it is pushing forward through the resistance of the wire and i think that's a fundamental disagreement can i can i read you something this is from a uh physics textbook and there's a heading which says electrons cannot push each other through the wire as we've stated previously there can be no excess charge inside a conductor and we'll see later that this also applies the steady state as well to equilibrium as well as the equilibrium so the number density of mobile electrons inside a metal wire must be equal to the number density of
positive atomic cores the inside of the wire is electrically neutral rather it must be other charges somewhere outside the wire that make an electric field throughout the wire that continually drives the electron current and so this is my explanation for the maintenance of this current is that there is a gradient of charge on the surface creating electric field inside the wire that causes the electrons to go one direction more than the other so do you think so do you think if we have more like a larger electric field through the wire we will have more
current yes see the thing is if there is a charge gradient across the wire it is because the wire is resistive and this creates a voltage across the wire we typically want to avoid because it would waste energy if the wire is more conductive there is a smaller electric gradient and voltage across it what results in more current through the load less electric gradient across the wire means more current which is opposite to derek's notion that the gradient creates current oh that's interesting that's very interesting yes yeah i hear you you say it's bad for
current i say it's the it's the thing driving the current yeah but how come when we remove it we have more current but it's not it's not an apples to apples comparison derek explains we can have a certain resistive wire and place a voltage across it increasing the voltage we increase the electric field gradient across the wire and therefore the current through the wire which is opposite of what i'm saying yeah so i'm saying higher gradient more current you're saying higher gradient less current right but it just depends on how you set up the problem
dammit he's right he's messing with my brain with truth and facts but either way no matter the mechanism of how charges flow in the wire the charges are still pushing each other forward electric and magnetic fields around the wires are created by electrons surface charges or whatever you want to call them in the wire right so why do we say they are not pushing against each other because it's them directly affecting each other i would agree that electrons push electrons but i don't think that mobile electrons push mobile electrons so i guess what i'm saying
is the electrons which are forming the current are not pushing on each other yeah i can send you this chapter of this book which like is basically exactly what i said in the video so is the energy carried by the electrons or by the fields and obviously the energy is delivered by the electrons we all agree that the electron bumps into an ion in the load transfers the energy like that it's a it's a little bit confusing like i i always had this traditional view of how electrons move through the wire maybe it's a good
sign that like there are different mental models right different ways of seeing the world and i think the model that we all learn about voltage and current is like very useful and very intuitive and works 99.9 of the time so it's like why do you have why why even think about this you know well the thing for me is that it works 100 of the time i've never seen a case that it doesn't work that's the problem i mean what if you try to answer the question like a kid asks you like okay these electrons
are coming here and there's a resistor down here and there's a resistor over here and this one is twice as big as that one so half as many electrons go down this path as that path but like how do the electrons know to go like half as many this way and twice as many that way well in my view when you first close the switch a wave of current is pushed through the wire and splits equally in both branches although there are different resistances in each but when the wave hits resistors some of it reflects
from them there is a greater reflection from the higher resistance as the current chants get through as easily which adds up to the initial wave resulting in less current through the higher resistance so initially the charges don't know which branch to go more into but because of this constant feedback from the loads after this transient state passes and things settle less current can go through the higher resistance yeah yeah yeah yeah yeah i don't know is it a worthwhile distinction to say is it the electrons in the wire like in the bulk of the wire
which are creating this electric field or is it electric electrons on the surface of the wire i don't know is that a worthwhile distinction to make uh well it's good to know the truth of course but it seems to me that the traditional way of thinking although it may be wrong doesn't seem to have any effect on the outcome of what you're trying to do in the actual circuit right i mean and i think that's the power of it as a model like it's such an effective way of thinking like why would you ever change
your mind thanks for the call bye i thought it was an open and short case me being so smart and all knowing but damn he makes some good arguments so the charges don't push each other through the wire the thought around charges wouldn't let go until one restless night when i had too much tea and couldn't fall asleep thinking about charges suddenly everything clicked oh shh keep it down derek was right about the surface charges but at the same time i'm still right charges do push each other through the wire i need to talk to
derek again thanks for taking a call again and thanks for the chapter of the book you sent me to i read it last night and for the most part i believe it just agreed with what i thought during my restless night which was good let me explain more clearly how charges flow in the wire let's just use one resistive wire initially not connected anywhere in equilibrium as in positive and negative charges are equally distributed suddenly we place a battery across the wire which with its electric fields pushes some electrons into negative and sucks some from
the positive just remember electric charges are super strong and it doesn't take much to make a lot of current so in reality you would see close to zero charge movement this is exaggerated so you see things better the excess electrons repel each other in all directions but vertically they can only go as far as the surface horizontally a lot of them move forward which again repel each other in all directions some more end up at the surface and the rest move forward similar distribution happens on the positive side too except there the positive charges don't
move but rather electrons are sucked into the positive areas so we end up in a charge distribution like this on a very thin layer of conductor surface and the rest of the wire is electrically neutral like derrick mentioned and charges there have a net zero force on them from the adjacent midwire charges now let's take an electron in the middle and see the surface charge forces on it from four close corners surface is more negative here so we have strong repelling forces surface is less negative here so we have weaker repelling forces this makes a
net forward force on electrons in the middle we see the same forward force on all the electrons inside the conductor so derrick was absolutely right inside charges are pushed forward by the surface charges only all good and dandy so far but i did some extra analysis on the surface charges like this one strong repelling force from these two corners and weaker repelling forces from these two so we have a net force pointing up and right we do the same analysis and right in the middle the force is to the right and in the positive side
it is to the right and bottom it makes sense excess electrons are pushed to the surface on the negative side center is equilibrium and on the positive side they are sucked to the middle but there is also a horizontal force on all the surface charges which means they also move forward through the wire they are not static charges as derek initially pointed out the only thing static about them is their distribution over the surface of course the positive charges don't move to start with we assign a movement to them opposite of the electron movement and
because electrons move this way we assume positive charges move that way like what would stop them from moving forward that's the question is there a reason for them not to move forward when there is a force applied to them that's a very good question uh and i don't know that i have a good answer for you and you're extending that to say well the surface charges are moving as well yeah they are also the current carrying charges right yeah so the author of the textbook wrote to me about this specific question okay it is rather
common for students to ask whether the surface charges contribute to the current obviously the positive surface charges don't drift i don't know anything about whether the electrons are bound or not in surface states but it's easy to see that even if the surface electrons do move they will make an utterly negligible contribution to the current right so the argument the argument is is there are they bound in some sort of surface states i would say no but i mean uh you and the author of the book don't know and there might be a reason for
it that they actually don't drift and i don't know so i guess that's another video for you let's figure that out see the surface charges are a very negligible portion of the total current but they are so strong they create the entire current through the wire and they also seem to be pushing each other forward so the battery does the work by replenishing the surface charges pushing electrons into them and sucking electrons from the other side as they go through the circuit and surface charges act like the conveyor belt that carries the middle charges forward
so i feel like that much force between them compensate for no force in the middle charges [Laughter] in the case of our thought experiment with this battery and a switch the long wires and the bulb when you close the switch why is there any energy at the bulb at all in like 1 over c 1 meter over c seconds right like why is there energy there at all it's because of fields like yes it's and you could say well it's because electrons over here are pushing electrons over there which is also true but they're pushing
each other through a long-range interaction which is a field interaction well it is always the case like electrons never yeah it's always a field interaction it doesn't matter if it's close or far right yeah i guess if it's a close range interaction i'm more comfortable with saying like oh this electron pushed that electron that electron pushed that electron you know if it's a close range interaction yeah you can't you can't discriminate like that it's always fields pushing against each other close or far yeah it is that question of like uh i don't know is is
near field versus far field is that a a worthwhile distinction to make and that's really what this question was meant to tease out it was meant to tease out the the question of like do you think that uh electrons in the wire are are physically sort of carrying the energy or is it the fields around them which carry the energy and i think that's that was the point of the thought experiment right and i i don't disagree with you i just think that both of our thoughts exist together none of them is wrong no i
think electrons and fields are existing together in the same environment right yeah let me ask you this question in a traditional way of teaching circuits you would say the charge comes along uh drops off its energy in the load and then goes back to the battery and you could ask a question of the students like okay so which way are these electrons carrying energy as they come back to this battery and i don't know if the answer would be like well they're not really carrying energy they're at a zero potential now so they're just basically
completing the loop or whatever i'm not sure if that would be the answer but if you look in the field way and you think about pointing vectors around the wires what you see is that both electrons going to the load and electrons coming back from the load are both carrying energy through the fields to the load so i would say there's a qualitative difference at least in how you view the flow of energy uh depending on whether you take a field view or you take your like electrons pushing electrons going around yeah but i can
always refer back to my bike chain uh example right both the chains leaving the front paddle and coming back to the front pedal are contributing to the energy placed in the back wheel right yeah well i mean in a bike when you paddle the top chain is rigid and the bottom chain is slacking but imagine a system that the bottom chain was rigid and could also push or a closed-loop water pipe that water flow could both push and pull both acts of pushing and pulling are transferring energy to the load and that's what electrons are
doing they are on one side you're pushing them into the load on the other side you're pulling them away from the loop in both cases that's nice that's nice i like that any and if we add the idea that like uh the pulling and pushing is done through long-range field interactions then yeah i think we're fully in agreement yeah i think i think that's the case and then we are fully in agreement but yeah i think like the my motivation in making both these videos was to reflect on the fact that i was taught and
i taught circuits in a way that certainly didn't understand these nuances right um and uh you know it was that surprise that led me to to to make the video this excitement yeah well it was an eye-opening conversation for me thanks to derek he is one smart dude but at the end of the day i think my view of charges pushing against each other transferring energy from the source to the load works because i never look at charges as marbles banging against each other i always see charges and fields as one package where fields can
extend forever i would say the only difference between my model and reality was that where i thought charge gradient was distributed throughout the entire wire in reality they were distributed over the surface they still push and pull each other and the outcome is the same and i'm not done i bought a whole ton of cables and i'm gonna do veritasium's original test with the longest loop of wire ever done in the history of mankind on youtube probably so subscribe for that and definitely check my sponsor brilliant to make sure you have all your fundamentals in
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