hi derek of veritasium made a video where he steps into my territory electronics and poses a question at the beginning and gives an answer at the end and his answer is wrong you should watch derek's video for full context of course his video is great and highly educational but his question wrapped me in ways that i didn't like it wasn't just a physics question it was a tricky question see if you want to raise your iq you get an book and a great deal from my link in the description thanks to my sponsor audible more
at the end but a trick question is designed to trick not to test your knowledge something like a baby is hungry and will die now if not fed and the mother is nowhere to be seen what do we do i don't know we strap on a fetch breast and feed the baby no dummy open the closet door the mommy's in there the hell derek's question was imagine you have a giant circuit consisting of a battery a switch a light bulb and two wires which are each 300 000 kilometers long that is the distance light travels
in one second well more like this now the question is when we flip the switch does it take half a second one second two seconds one over c seconds or none of the above for the lamp to turn on c is the speed of light and one over c comes from the one meter distance between the wires over c meters per second and so the value of 1 over c would be the value of time per second we also make some assumptions like the wires have to have no resistance and the light bulb has to
turn on immediately when current passes through it so i jump right to it wires with no resistance is possible we make it out of superconductive material put it in the shadow of a giant umbrella or earth keep it super cold and super conducting the light must turn on immediately as soon as there is current he's left his lamp is a regular one that can be incandescent or led and draws a certain level of current when it's on it takes some time from when there is operating current into the lamp when it's on but let's assume
it's a special lamp and it will be fully on as soon as there is operating current and of course it won't turn on if there is not enough current through it for operating currents the electric waves would have to travel the entire length of the loop at the speed of light so it could take at least one second or more depending on the after i close the switch the light bulb will turn on in roughly one over c seconds what [Music] and my iq was shattered that was when i understood when he said and the
light bulb has to turn on immediately when current passes through it he means the light turns on at any current level immediately you little derrick it was a trick question wasn't it so that's what the mystery black box is for something that turns on the lamp immediately at any current level well actually with that assumption derek's answer is wrong the correct answer is e none of the above the light never turns off no matter the state of the switch when the switch is off over time the voltage across it will be the same as the
battery voltage which means there are electric fields across the switch contacts which pull on electrons some electrons will jump the gap and result in an extremely small continuous leakage current derrick never said there is no leakage current and it is a real thing the light turns on at any current so it's always on and i win not gonna lie derek's answer triggered me just a bit so i was thinking about it for over a week every day and night which is good because it brought back a lot of lost memories thanks derek let's dive into
derek's video because i don't quite agree with every point he makes here he says in school we have been taught something wrong that in an ac circuit the flow of electrons is like a chain moving back and forth delivering energy to the load if it's the electrons that are carrying the energy from the power station to your device then when those same electrons flow back to the power station why are they not also carrying energy back from your house to the power station well i'm sawing wood pushing and pulling both acts of me putting energy
into the saw cutting the wood saw is a vessel to transfer energy like electrons in a wire just because i pull doesn't mean that i'm gaining energy these are the lies you were taught about electricity come on lies that electrons themselves have potential energy i think this is debatable and depends on your frame of reference hopefully the rest of my video clarifies better that they are pushed or pulled through a continuous conducting loop well in a closed circuit with no capacitive or inductive brakes they are running in a closed loop and that they dissipate their
energy in the device they do help dissipate the sources energy into the load those analogies can make sense so i don't think calling them lies is accurate my claim in this video is that all of that is false i think derek is highly exaggerating which is appealing to the general audience but i think professionals expect a bit more accuracy then he talks about the true flow of energy from the source to the load that segment is highly inspired by a video from science asylum i had seen a couple of years back which derek also credited
to another great youtube channel you should subscribe to in that segment he talks about the pointing vector not pointing pointing the scientists who came up with it in an electrical circuit the pointing vector shown as s indicates the flow of power with a unit of watt per square meters and is equal to a constant times the cross multiplication of electric and magnetic field vectors so using the right hand rule if the direction of fingers match the e vector and they curve towards the b vector thumb shows the direction of power flow or energy per second
so direct shows that in a closed loop of a circuit where we have electric fields between the wires and magnetic fields around the wires according to the pointing vector the electrical energy in fact flows in the space around the wires by the electromagnetic fields from the source to the load which is true but what's shown here is again exaggerated it shows all vectors of energy at the same strength we know that in a straight wire the magnetic field strength is reversely proportional to its distance from the wire so a field 10 centimeters away is 100
times weaker than a field one millimeter away from the wire yes maybe the electric fields are kind of uniform between the wires but the magnetic fields are much stronger closer to the wires so especially for dc the energy flow looks more like this with energy much more concentrated close to the wires and at much smaller levels further from the wires ac is a different beast though as the frequency rises the energy will find and radiate through many different parasitic capacitive and inductive pathways to the load or to the environment let me show you how crazy
the world of ac is here i have a loop of wire connected nowhere just an led at the end of it and a five megahertz signal coming out of the scope and if i connect the signal only to the led either on this side or the other side it turns on with no grounds connected this also shows it's the fields and not the electrons that carry the energy again the frame of reference is important a counter argument is already in derek's video the current inside the wires creates a magnetic field outside the wires so if
flow of electrons creates magnetic fields then why do we say inside the wires electrons just oscillate back and forth but they do not carry the energy yes fields carry the energy but the current inside the wires creates a magnetic field outside the wires so to conclude that but what we've learned in this video is it's not really what's happening in the wires that matters directly contradicts with the current inside the wires creates a magnetic field outside the wires electric fields are an inherent property of electrons or protons for that matter creating electric fields around the
wires motion of these charges creates magnetic fields around the wires and then the fields carry the energy so to say it's the fields and not the electrons that carry the energy sounds inaccurate okay enough of the side quest let's focus on the main question of long wires i'm going to provide some detailed information that is guaranteed to blow your minds so save your minds now if you have to and just watch my sponsor segment at the end but if i see a drop in my viewership you're just a bunch of traitors i checked the analysis
dr olsen and dr abbott provided to derek in his video i also did a ton of simulations and analysis myself and i was happy to see that my results match theirs i also double checked a few things with dave from eev blog to make sure i'm not completely on crack thank you dave for the videos on the subject subscribe to his channel too but i'm not going to say things the same as other people now am i let's go super basic and replace these super long wires with their equivalent impedances we don't know what they
are and we don't care as long as there is no significant other way for different parts of the circuit to exchange energy if current goes in here same current must come out of here almost immediately limited by the speed of light so derek is right that as soon as we close the switch we have current through the lamp in one meter over c seconds because the wires are only one meter apart no matter how long the wires are in the loop but how much current it depends on these components now let's dive deeper the circuit
is symmetrical on both sides and whatever happens on one side happens on the other side in the opposite direction so let's just focus on one side the switch has been off long enough that negativity has spread all over the wires only a tad bit of positive charges here and as we close the switch suddenly a burst of positivity travels in the wire at the speed of light after the burst of current travels one meter at the speed of light the new electric fields finally reach the opposite wire and start pulling on the electrons up here
which means right then we have current flowing into the lamp what is charges pulling charges between two surfaces it's a capacitor current running through the wire creating magnetic fields and soul self-inductance it's an inductor this is how we engineers deal with fields around the circuits and lump them into more digestible components so this long length of wire can be modeled by many stages of inductors and capacitors each for a defined length of wire and as the wave travels into this network it encounters more and more of these stages the inductance blocking the current and capacitance
sucking the charges is why the burst of current flattens out as it travels through the line for the moment let's look at the pointing energy vectors shortly after we throw the switch and the waves of fields appear in the wire at the speed of light they still don't know what we have at the end of the wire all they see is a uniform network of impedances if we draw the energy vectors around the loop wires we see that all of them are pointing into the loop and nothing comes out as if this is a giant
resistor sucking all the energy in only after half a second traveling through the length of wire these waves hit the short circuit at the end some of them reflect and travel for another half a second to add up to the energy going into the lamp so the waves can travel and reflect multiple times and these lines being half a light second long it can take one second to multiple seconds before the light receives the full power of the source at which point the entire length of wire is at the same potential level so no electric
fields connecting them only electric fields connecting the positive side to negative side a line like this is called a transmission line and if we ignore the wire resistances and the leakage between the wires and the line goes to infinity then the impedance of this circuit is in fact a resistor equal to the square root of two of these l's divided by a c but if this truly goes to infinity then the impedance seen from this point forward is no different from the impedance at the beginning so we can delete the rest of the line and
replace it with a single resistor equal to the line impedance a transmitter would see the same line as if the energy only goes in with no reflections which means a receiver would be absorbing the entire energy now if the transmitter also has the same impedance matched with the line we would have the maximum energy transfer between the transmitter and receiver but that's another story so back to our direct lines right when we close the switch and the waves haven't reflected from the back yet this line looks like a resistor at the input so as soon
as the switch closes and the waves propagate at the speed of light the voltage of the lamp jumps to its known level sharp with no overshoots or further delays this is not the full battery voltage though because of these resistors and the only way to get rid of these resistors is to make the length of connecting wires zero so if you have any length of wire initially it is seen as a resistor until the waves reflect from the end and bring the information back that we were tricked it's a short circuit let me show you
a simulation the simulation does not actually represent circuit dimensions or effects of light speed but correctly demonstrates the lc propagation delays in the circuit don't watch if you have diarrhea here i tried to model 40 kilometers of wire round trip or 20 kilometers of transmission line i roughly calculated inductors and capacitors based on one kilometer segments of the line assuming my wires are one centimeter thick one meter apart supply is 10 volts and lamp is 100 ohms so like a one watt lamp reasonable running the simulation this is the voltage across the lamp you see
when i turn on the battery the voltage rises slowly and takes maybe over 15 milliseconds to settle close to the battery voltage and this is just a 20 kilometer transmission line and that's not the whole story if i zoom in you see the voltage is actually rising in steps this is the first step very flat around 65 millivolts indicating that the line is acting like a resistor here the supply turns on and the waves travel all the way and hit the short circuit here and return all the way carrying the information and we enter the
next step so any length of transmission line is a momentary resistor at high speeds that's not the entire story if i zoom in right at the beginning you see initially i have a spike of voltage close to the battery voltage because my supply rises at one nanosecond and then it slowly ramps up into the first step both the spike and the ramp into the step are inaccuracies due to my model i put the line capacitance first that acts like a short circuit at high speed that creates the voltage spike and then discharges then the inductance
causes the ramp into the first step this doesn't affect the overall result though but the truth is the inductors and capacitors exist on the line at the same time the simulation would be much more accurate but almost impossible if we made our segments much smaller maybe every one micrometer instead of one kilometers until the circuit would approach a true resistance property all these oscillations are also due to inaccurate model anyway if i change the inductance and capacitance value so that the line impedance matches half the lamp impedance or both lines together match the lamp resistance
and simulate you see the lamp voltage would jump right away to half the supply voltage there would be no ringing and after the wave would complete a round trip over the line the voltage would jump to the full supply voltage this is the best case for derek's question the lamp voltage would jump right away to a decent voltage at the speed of light and after a one second round trip over 300 kilometers it would jump to the full voltage but with more reasonably picked values for our case where nothing matches properly it will take multiple
steps for the lab voltage to settle and each one of these steps is one second so where would you pick your voltage and current threshold to turn the lamp on low after one second high after 10 seconds or to achieve your one over c second time you just say any current turns your lamp on and forget to address the existence of leakage current that would keep the lamp on forever so was derek's answer wrong yes on a minor technicality and because he tried to trick me into giving the wrong answer but his question raises a
great insight into electromagnetic waves i guess if you also watch my video as a compliment what would happen if lines were giant circles well you should visit my sponsor audible no more excuses like i have to cut all this wood and i have no time to read books you'll use my link audible.com electroboom or in us text electroboom to 500-500 and this season for a limited time you'll get 60 percent of first three months that's only 5.95 a month and you start to listen to any audio title from the largest selection of audiobooks hands-free so
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