Intro to Electromagnetic Waves (how EM waves are created, Poynting vector)

waves remember waves from physics one back when physics was easy the vectors only had two dimensions right so we did mechanical waves last year so if i shake my hand over here the the spring is acting as the medium and the spring is what the wave is traveling through to transfer the energy across the table right so we're going to look at another type of wave an electromagnetic wave so an electromagnetic wave is different in that it doesn't need a medium so here if there was no spring right and i do this obviously nothing happens right that's stupid this needs a medium to travel through but an electromagnetic wave is different let's look at how electromagnetic waves are created so i'm going to start with the simplest case i have an ac source and it's connected to these two wires right and this ac source even though it's not connected to a circuit right it's still creating a potential difference in the wire right it doesn't have to be connected to do that because this creates a voltage that gets bigger and then gets smaller and bigger in the other direction and smaller it keeps doing this right so let's look at what happens when this thing starts pushing current up right so it's going to start with a small voltage and then it starts pushing the current up and then it's going to go down and go the other way and then flip right so let's look at just the up part so when it goes up i know that positive charge is going to get pushed up here right because of conventional current and negative will go down here right because thanks ben franklin you had one job and you screwed it up so if that current goes up and pushes positive up here that creates an electric field right that electric field is going to look like that so that electric field points down from positive to negative so as that current gets bigger that field that electric field gets larger and that electric field around in the wire propagates out in all directions imagine throwing a rock and a pond and the wave spreads out same thing happens right so this electric field is going to get bigger and bigger i'm just going to look at going this way so it's going to start small and then it's going to get larger and larger and larger until it finally hits a maximum value and then when the current i'm sorry when the voltage starts decreasing to go the other way then it's going to get smaller and smaller and smaller right and so it kind of looks like the top part of a sine curve the bottom part i should say let's look at the magnetic field that creates that gets created now so current pointing up right magnetic field i can use my right hand rule the curl one to figure out the direction of the magnetic field so from your perspective right if i point my thumb up for the current then the magnetic field should be pointing into the page from your perspective so if you can't see this pause the video and try this until you can so you should be able to take your right hand and thumb pointing up and your fingers on this side of the wire will point into the page from your perspective right so at me so pause it if you can't see it make sure you can see that so that magnetic field is going to change because of ampere's law right if current increases magnetic field is going to increase also and also because of maxwell's equations right changing fields create each other right so this magnetic field is going to kind of look like this but it's going to be pointing into the page from your perspective right so it'll look kind of like this okay i know that's terrible drawing but imagine from your perspective the electric field is pointing down and the magnetic field is pointing into the page so it kind of looks like that but it's head on right so this is what it looks like when the current goes up but remember that current is going to go up and it's going to create appeal that gets bigger then it's going to turn around and go backwards so what happens when the current starts going the other way right well let's look at that so same thing follow me here current points down right so the current was going up now it's going to come back down and go down to a maximum negative value right so that current going down creates an electric field that electric field as the current gets bigger the electric field also gets bigger and bigger and bigger right and then it turns around and gets smaller and smaller smaller goes back to zero so that electric field is the blue one the magnetic field now if i take your right hand right thumb pointing down you should see from your perspective the field on this side is pointing out of the page at you right so on this side to be in the page so again this this is happening in all directions going out like dropping a rock and a pawn in a circle right i'm just looking at this way so my field is pointing out of the page from your perspective right and you see how it gets bigger and bigger and bigger and it gets smaller and smaller and smaller so that's basically what's happening as the current goes up and gets smaller it does this and as the current goes down and goes back it gets like this right so i could do this even with one charge if i took one charge and just did this and moved it up and down made it accelerate this would happen so let's throw all this together and kind of look at what it looks like okay so it kind of looks like this so what's happening is my electric field is in the plane of the page and my magnetic field is from your perspective right it's going out of the page and then into the page out of the page into the page out of the page into the page right and so these two things are at 90 degrees to each other and they're self-propagating because electric fields create magnetic fields and magnetic fields create electric fields right so all that happened from charges moving up and down that creates these two fields that interact and like make each other propagate right so the way to figure out the direction just by looking at the feeling so that's a question you might see sometimes it's like here's these electromagnetic fields which way is the way of traveling is to use something called the pointing vector we're not going to calculate this right because it's a cross product but you can use this to figure out the direction because the velocity of the electromagnetic wave is going to be the cross product of the electric and the magnetic fields right so again when you do cross products you cross the first vector is your thumb your second vector is your fingers and then when you cross them that gives you the vector right that's the direction so if i take my electric field and like let's look it over here this one's easier no this one's easier to do so my electric field pointing down right i would take my thumb point it down and my magnetic field is from your perspective pointing into the page right so if you line up your thumb pointing down and point your fingers into the page when you cross them boom there's your velocity right okay let's go look over here if i take my thumb which is my electric field and point it up and i put my magnetic field out of the page from your perspective right it looks like this boom cross on it looks like that so you can see what's happening these vectors are just doing this and the velocity is always pointing in the direction of motion right and so all it's doing is going like that and the velocity is always formed and so electromagnetic waves what creates them is basically any charge moving right here i used an ac source right this is like the example from your book but it could be just taking like you can't grab a proton but if you take a proton and you shake it like that that'll happen so i'm going to leave you with a simulation because this is not the greatest drawing i'm going to leave you with the simulation to that shows a better example of what i'm talking about instead of me trying to draw okay so here's the simulation i got an antenna and i can move an electron up and down like i was showing you and let me put this on oscillate so it looks better right and so when it moves it creates this electric field in the green and that electric field is what travels out in space and it can interact with other things and that's what carries the energy from like this electron over here to this electron in this antenna right and that's how low energy electromagnetic waves work right so this wouldn't work for like x-rays or gamma rays or uh high energy stuff like that right because the size of the antenna is going to matter so you can't make an antenna that small to make one this is how like radio waves and microwaves stuff like that work and so again i was talking about how we only look at it going in one dimension right but in reality this is what the wave looks like so that electron moving creates a field that propagates in all directions so you can see some going there and you can watch it going here right and then you can see another one coming down this way right so it looks like you dropped a rock in a pond and it ripples out and it goes out in all dimensions but we only ever really look at it as going one way just to make it easier right so this is how this is the basic way electromagnetic waves are produced right there's a bunch of other ways we'll eventually get to but this is the simple way low energy electromagnetic fields are created

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Length: 8min 19sec (499 seconds)

Published: Tue Jan 05 2021