No, Changing Electric Fields DON'T Cause Magnetic Fields; The Real Origin of Electromagnetic Waves

changing electric fields don't cause magnetic fields and just as importantly changing magnetic fields don't cause electric fields hi everyone welcome to atoms and sports and today I want to talk about some misconceptions I hear about electromagnetic radiation and lights and actually I want to tell you about how the entirety of the physics of electromagnetism is more straightforward than a lot of people seem to think let me start by sort of laying out a way of thinking about electromagnetism and lights that I'm gonna say isn't that great now not everything I'm about to say is wrong although some of it is but it's making things a lot more clunky and complicated than they need to be so let's lay out these ideas I want to attack the idea is that electromagnetism is basically all about three fundamental objects you have your electric charges which create electric fields you have your electric currents which create magnetic fields and finally there's a third very special object called an electromagnetic wave or electromagnetic radiation which is kind of its own independent thing these waves are often illustrated with a diagram like this what this is basically showing is that if I stood at a certain spot as one of these waves passed me by that I would detect an electric field whose Direction was oscillating up to down up to down up to down up to down and I would detect a magnetic field whose Direction is oscillating left to right left to right left to right and if I sing about one of these Maxima to stalk because hey everyone needs a hobby I'd find that this Maxima of this peak travels at the speed of light and then that's one of the big reveals of electromagnetism it's not a coincidence that this electromagnetic radiation travels at the speed of light it is light that's what light is it's one of these sinusoidal electromagnetic waves but in fact it's often said that these electromagnetic waves aren't just independence but the reason they exist is because they're also self creating as the story goes electric and magnetic fields interact with each other specifically if I have at a spot a changing electric field that changing field then creates a magnetic field changing electric fields create magnetic fields on the flipside if I have a changing magnetic field that then creates an electric field so it's said that these perfect little sinusoidal objects propagate out forever across the cosmos because they're basically the perfect exploit to these interactions they're cheat code if you will to this way that electric and magnetic fields interact with each other and create and reinforce themselves and end misconception so what I want to do today is maybe sort of tear down a lot of these ideas but maybe just to quickly summarize the idea as I'm kind of going after one electric fields are caused by electric charges magnetic fields are caused by electric currents - electric and magnetic fields created by these sources then interacts with each other three there is a perfect solution to these interactions a very special field arrangement that has the very special property of reinforcing and creating itself for these factory assembled fields these cheat codes in the way fields interacts are called electromagnetic radiation or lights and they travel the universe in this beautiful elegant sinusoidal electromagnetic wave shape now before we set them straight let me just point out some immediate issues with these ideas that you might realize let's call these sandy Chex even if these ideas aren't a correct reflection of our reality are they at least internally consistent first of all if I look at one of these sinusoidal electromagnetic waves what I see he is an electric field and a magnetic field kind of moving in step with each other but hold on that's an issue because if one of these is causing the other then we'd expect some sort of I guess time delay we'd expect say the electric field to change first and then that change to cause some change in the magnetic field but that's not what we see they're changing perfectly in step with each other which is definitely fishy if our misconception is right so suspicious note number one if changing electric fields cause magnetic fields and vice versa why are electric and magnetic fields in electromagnetic waves in perfect step with one another secondly if I take two beams of light say from a laser or from a flashlight and I do what the Ghostbusters tell you never to do and cross the streams sure at the point of crossing I might see something crazy like an interference pattern or something like this but after that the beams continue totally unchanged totally unaffected the electromagnetic waves clearly aren't changed by crossing each other this is why I can't build a lightsaber no matter how much my 10 year olds earned let's be honest 30 year old self wants one if electromagnetic fields allegedly affecting caused one another it seems awfully strange that two light beams can pass through one another without them having any effect on one another so suspicious motion number two if electric and magnetic fields interact why don't elect row magnetic waves interact thirdly isn't it weird to say that currents create magnetic fields I mean electric charges are like fundamental things in our universe you have electrons yeah protons yeah moons and so on but electric wires electrical currents are just like a human technology in fact electric currents are really just a rebranding of electrical charges in a specific scenario to see that this is the case imagine two infinite lines of charge one line of positive charge and one line of negative charge sitting on top of each other now if either of these charges were by themselves they'd surround themselves with an electric field because charges create electric fields but because at every point along our line there are equal numbers of positive and negative charges the net charge is zero and therefore the electric field is zero of course in this case we're both lines were stationary there's no magnetic field either this is basically the case of a neutral wire if there's no current to make a current all I need to do is then start moving one of these line charges at a constant speed it's really the idea of electrical currents is just sort of a cockamamie scheme to create a scenario where I have charges that are moving but no actual net charges so even though there are charges there they're cancelled so it's a purely magnetic situation because of that this can be made pretty clear if I just show you the magnetic field around a single moving charge so you see a single charge moving with a constant speed sort of which just dresses itself with a magnetic field as it moves so suspicious notion number three forget about currents electric and magnetic fields don't really come from different places in reality any old charge which is moving has both an electric and main field around it just like an electromagnetic wave so what's special about electromagnetic waves it's okay then we've identified a fair amount of dubiousness around these ideas but then how does it work well let's say I have an electric charge just moving on some crazy trajectory through space we don't care why it's moving that way or how it's moving that way all we want to know is that at any given moment what is the electric and magnetic field I will detect at some arbitrary point let's call that point a if you can figure that out you've essentially figured out all the physics of electromagnetism that may seem like an exaggeration but really it's not and that's because as we'll see electromagnetic fields don't interact at all unlike our misconception this means that the electromagnetic field due to a large complicated object made of billions of particles is really just the sum of the individual fields of each particle so if you know for one particle you know it for a billion particles so with that in mind all of electromagnetism can just be summed up like this the electromagnetic field detected at a given point in space is just the time-delayed effect of the position velocity and acceleration of a charge at an earlier time that's a bit wordy but let me show you what I mean what sort of way of thinking about this is that a charge is like a beacon and at every moment it is broadcasting in all directions two pieces of information its current velocity and how that current velocity is changing in other words its acceleration and at every moments moments a moment that's all it does however these broadcasts it's sending out its only travel at a set finite speed the speed of light to make this concrete let's imagine taking a snapshot of our particle with its arbitrary trajectory at a specific moment in time at this particular moment it has a certain particular velocity which is information not just about its speed but also the direction of its motion and it also has a particular acceleration and these two things are basically the contents of the transmission it's going to broadcast at this moment now let's look at what happens that broadcast after one second after one second this broadcast has travelled a distance of one light second a light second is just a distance of how far light travels in a second so here's super useful so this circle I've drawn here is showing a circle of one light-second radius let's say after two and a half seconds the message is now here and it has just reached our point a now here are the two big points one when this happens the electromagnetic field at a will be completely dictated by this charge velocity and acceleration information that had just received into this isn't the charges velocity and acceleration right now it's its velocity and acceleration two and a half seconds ago it's a time delayed update in fact let's put this even more concretely when a point a receives one of these messages from the charge that message contains three pieces of information the position of the charge when the message were sense as well as its velocity and acceleration at that time there isn't a rule which we'll call the lynard we hertz rule that takes as inputs these three quantities and spits out a corresponding value for the electric field actually because leaner v hertz is hard to say let's just call it the LW rule in general the way this rule works if the velocity acceleration aren't zero the direction of the electric field will not point in the direction of the time to light position it'll be pushed off in a slightly different direction whenever that happens whenever the resulting electric field doesn't point in the direction of the time to light position there will be a magnetic fields if I look at the direction of those two vectors when they don't point the same direction I can then draw a third vector which is perpendicular to both of them and that is going to be the direction of the magnetic fields and its strength as determines by how large the separation between those two vectors was now that may seem complicated but the fact is that's actually the whole thing so let's maybe summarize if I have a charge at any given moment a point a is receiving messages from the charge about its current position velocity and acceleration however because of the finite speed of lights when a receives those messages they're out of dates so a doesn't respond to where the charge is now but where it was when it gets one of these time delayed messages it explicitly determines its electric field through this mysterious LW rule if the electric field that it spits out dozen points in the same direction as the time position there will also be a magnetic field and that points perpendicular to those two vectors done that's the whole shebang that's all of electromagnetism that might not seem as super satisfying because it all seems to kind of rest on this mysterious l w rule and I'll get to that but already we can understand some of the issues with the ideas in the beginning of this video all electromagnetic fields in the universe via electromagnetic radiation or knots are 100% just the results of receiving out-of-date status updates about the position velocity and acceleration of charges so very importantly there is no causation no interaction between electric and magnetic fields at all in fact there is a real totally valid way of thinking about the physics of electromagnetism where electromagnetic fields themselves are basically just red herrings they're mr. X at the end of the day the only real detectable effect of electromagnetism is that charges can exert forces on other charges the only complexity is the form of those forces and the fact that their effects is time-delayed electric and magnetic fields are just glorified mathematical bookkeeping devices invented to keep track of this time-delayed force of charges on charges you may go to a science lab and find a device that is called an electric or magnetic field detector but these devices don't really detects electric or magnetic fields and said they are devices that have charges inside them and what they really measure is how the charges inside the device are affected by the charge you're measuring so in a sense the device is false advertising it's impossible to directly measure an electromagnetic field fields are created by charges and their motion and their effects are only observable when they produce forces on other charges at some later time okay so let's get back to this LW equation when I first talked about it I kind of just said it was like a magic box it takes in a time-delayed charge position velocity and acceleration and it spits out an electric field vector the reason I was so vague is because the equation itself is kind of complicated let me just throw it up now don't worry if you have equation phobia I'm not expecting anyone to get anything out of this I just wanted to show that it's a big honking complicated equation but looking through the complexity it roughly just has two terms or two separate parts partner Marwan depends on the time delay distance and velocity of the charge in the past we'll call this the position velocity field part number two depends on the same things but also the acceleration we'll call this the acceleration field though it's actually more common to call the second part the radiation field to see why let's go back to our little particle Broadcasting it's a little message and imagine I took one of these concentric circles and walked around in telling up the strength of the electric field at each point actually more correctly we should actually tell you up the energy in the electric fields when I do this I'm basically determining the total energy of the field at a certain distance from a source of the field then imagine I went to another one of these circles one that is farther out and I do it again and again I get a final total energy tally if I compare these two tallies one from a point closer to the charge and one from a point farther from a charge there are really only three ways that they can compare if the field is greater the farther out I go well then I broke in physics that means that the field is actually conjuring energy from some Eldrick void gaining energy as it goes farther and farther outs it also means that if we add up the entire energy of the fields it'll be infinite so you know step one create an infinite energy field step to extract infinite energy step three profits this is what we call in the biz an unphysical possibility another option is that the tally gets less and less as I go farther out this means that the energy in the field is tapering off to nothing at greater and greater distances what this means is that the field carries energy but that energy isn't going anywhere it's kind of just hanging around the vicinity of the charge like a wet towel an analogy I've always liked is a field like this is like flies swarming around a garbage truck the garbage truck is the charge and the energy of the field like this is bound to stick close to it we'll call this a dressing field because it kind of just augments or dresses the area around the charge final option is that the Tahlia doesn't change at all it's the same total no matter how far away you do your tally for field like this even if you're infinitely far away the total energy field is the same in a very real sense such your field is carrying energy away from a charge to infinity we call this a radiating field so bringing things back to our LW rule when a charge receives one of these status updates it does different things with the position velocity information than it does with the acceleration information one forms the position velocity field which is non radiating and one forms the acceleration field which is radiating in fancy language we say that the energy of the position velocity field falls off like an inverse fourth power law where the energy of the acceleration field falls off like an inverse second power or inverse square law and so the energy in the field at any distance is a constant even at infinite distances now that's a bit confusing they say that a picture's worth a thousand words so gifts got to be worth like a million don't place down an electric charge and look at the strength of its electric and magnetic field now its magnetic field is zero because it's not moving and it's electric field falls off with distance like this this is a case where there's only the position velocity fields now let me give it a quick acceleration a quick jostle you see that that was the radiating field that right there the blob that was light compare this to our legit sinusoidal snowflake it doesn't look sinusoidal at all in fact let me accelerate the charge again but just a little differently you know a different flick of the wrist you see the shape changed the shape of electromagnetic radiation is just totally arbitrary it just depends on the specific way a specific charge happened to accelerate that one time this isn't light in the wild this is and it's just a time-delayed status update of an acceleration so let's just recap our misconceptions 1 electric fields are caused by electric charges magnetic fields are caused by electric currents no the electromagnetic field I detect as a point is just an out of date status updates of the past position and velocity acceleration of a charge to electric and magnetic fields created by these sources then interact with each other they don't interact with each other their form was set the second that particle did a thing in the past 3 there is a perfect solution to these interactions a very special arrangement that has the very special property of reinforcing and creating itself yeah again they don't interact and for these factory assembled field these cheat codes in the way fields interacts are called electromagnetic radiation or lights and they travel the universe in this beautiful elegant sinusoidal electromagnetic wave shape ya know they kinda just look like they what depends what the acceleration was so pleased that makes things about electromagnetism a little clearer have a good this video and all atoms and sparks videos are sort of a condensed discussion on a set of blog posts from the atoms and sporks blog if you want to find out more detail but the stuff that was discussed here the corresponding blog post is linked below also check the blog to vote on new episode topics and see some discussion on other topics

Channel: Atoms and Sporks

Views: 85,661

Keywords: physics, electromagnetism, science, light, electromagnetic waves, Maxwell's equations, Lienard-Wiechert, electrodynamics

Id: uZnXhRgztEg

Length: 18min 36sec (1116 seconds)

Published: Wed Oct 03 2018