Anderson Video - Electromagnetic Waves Intro

Professor Anderson
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<font color="#ffffff">Hi, everybody. How's everybody doing today?</font> <font color="#ffffff">Feeling good? Trying to stay out of the heat?</font> <font color="#ffffff">It's much nicer in the studio here.</font> <font color="#ffffff">It's very cool. Thanks for the air conditioning, Dave, appreciate it.</font> <font color="#ffffff">>> (Dave) We paid our taxes.</font> <font color="#ffffff">>> These guys paid our taxes. Yeah. Um.</font> <font color="#ffffff">How's everybody feeling about this stuff? Are you feeling a little overwhelmed these days?</font> <font color="#ffffff">Okay, good.</font> <font color="#ffffff">That means -- means you're paying attention. You're in the right spot.</font> <font color="#ffffff">Okay. This stuff is very challenging. It should be very time-consuming.</font> <font color="#ffffff">And, just do your best to absorb as much material as you can.</font> <font color="#ffffff">The point of this couple of chapters that we've been dealing with</font> <font color="#ffffff">is the idea that changing one field affects other fields.</font> <font color="#ffffff">Okay? So, changing the magnetic field</font> <font color="#ffffff">led to a changing of some sort of electric field. We call it the EMF.</font> <font color="#ffffff">Right?</font> <font color="#ffffff">The electro-motive force or a voltage, but it's this idea that these things are</font> <font color="#ffffff">actually going to talk to each other</font> <font color="#ffffff">Okay? And so, this is where we're heading.</font> <font color="#ffffff">The next topic, therefore, is just that, electromagnetic waves.</font> <font color="#ffffff">Okay? We are now going to combine both of these things -- electricity and magnetism --</font> <font color="#ffffff">into one concept, electromagnetic waves.</font> <font color="#ffffff">So, let's talk for a second about the following:</font> <font color="#ffffff">Let's say we have a charge -- positive charge.</font> <font color="#ffffff">And, we have a negative charge.</font> <font color="#ffffff">And, let's say we're gonna take these two charters and we're gonna move them up and down.</font> <font color="#ffffff">Okay?</font> <font color="#ffffff">So, let's move them vertically</font> <font color="#ffffff">and let's see what happens as a function of time.</font> <font color="#ffffff">Well, at one instant at time they look like that.</font> <font color="#ffffff">And if I think about the electric field in between these two,</font> <font color="#ffffff">I know what the electric field looks like.</font> <font color="#ffffff">It's pointing from the positive to the negative.</font> <font color="#ffffff">Okay? I'm just gonna draw one line for simplicity.</font> <font color="#ffffff">But now, let's move them vertically. So, the plus is going to come down;</font> <font color="#ffffff">the negative is going to come up.</font> <font color="#ffffff">And so, at some later time</font> <font color="#ffffff">they're nearly right on top of each other.</font> <font color="#ffffff">And at that time,</font> <font color="#ffffff">the E field is essentially 0.</font> <font color="#ffffff">Okay? There's nowhere I can draw an E field very cleanly,</font> <font color="#ffffff">but now let's keep moving them. We're gonna put the positive charge all the way down on the bottom.</font> <font color="#ffffff">and put the negative charge up on the top.</font> <font color="#ffffff">And now, the E field is pointing up.</font> <font color="#ffffff">Okay? So, all we're doing is we're taking these positive charges and negative charges we're putting them</font> <font color="#ffffff">together, and then we're gonna keep moving them so now they're the opposite way.</font> <font color="#ffffff">And then we're gonna come back together again.</font> <font color="#ffffff">So now, the minus is heading back down</font> <font color="#ffffff">and the positive is heading back up.</font> <font color="#ffffff">And eventually, they're back to where they started.</font> <font color="#ffffff">So if I think about what this field looks like,</font> <font color="#ffffff">it is getting smaller and smaller and eventually 0.</font> <font color="#ffffff">And then, it flips direction. It starts getting bigger and bigger</font> <font color="#ffffff">And now, it's a positive.</font> <font color="#ffffff">And then, it gets smaller and smaller until it's 0.</font> <font color="#ffffff">And then, it flips direction and gets bigger and bigger until it's again pointing in the negative direction.</font> <font color="#ffffff">It sort of oscillates up and down.</font> <font color="#ffffff">Okay?</font> <font color="#ffffff">And that, hopefully, looks familiar to you.</font> <font color="#ffffff">Right? We like things that oscillate</font> <font color="#ffffff">And if I think about drawing the electric field as a function of time, now,</font> <font color="#ffffff">what can I say? Well, here it was negative so it's some negative value.</font> <font color="#ffffff">Here, it was 0.</font> <font color="#ffffff">Here, it was positive.</font> <font color="#ffffff">Here, it was 0.</font> <font color="#ffffff">Here, it was negative.</font> <font color="#ffffff">And so, it's gonna look something like this:</font> <font color="#ffffff">Okay, it oscillates up and down as a function of time.</font> <font color="#ffffff">As I move these charges up and down,</font> <font color="#ffffff">the electric field is going to oscillate up and down.</font> <font color="#ffffff">It's pointing down here.</font> <font color="#ffffff">And then, it's pointing up here.</font> <font color="#ffffff">And then, it's pointing down over here, and so forth.</font> <font color="#ffffff">A graph like this is just: what is the size of the electric field as a function of time?</font> <font color="#ffffff">So, here, the electric field is 0.</font> <font color="#ffffff">Here, the electric field is 0,</font> <font color="#ffffff">but at this point, it's a positive number.</font> <font color="#ffffff">At this point, it's a negative number.</font> <font color="#ffffff">All right.</font> <font color="#ffffff">Let's go back to this picture here for a second</font> <font color="#ffffff">and let's think about this charge.</font> <font color="#ffffff">Okay.</font> <font color="#ffffff">Charge moving up and down,</font> <font color="#ffffff">that is a lot like a current.</font> <font color="#ffffff">If I take a wire</font> <font color="#ffffff">and I run current up and down,</font> <font color="#ffffff">then this current I</font> <font color="#ffffff">would look like positive charge heading down,</font> <font color="#ffffff">and negative charge heading up.</font> <font color="#ffffff">These are equivalent pictures.</font> <font color="#ffffff">But,</font> <font color="#ffffff">we know what happens when we have current.</font> <font color="#ffffff">What happens when we have current?</font> <font color="#ffffff">What do we generate,</font> <font color="#ffffff">when we have current in a wire?</font> <font color="#ffffff">It was a title of a couple chapters ago.</font> <font color="#ffffff">Magnetic fields.</font> <font color="#ffffff">Right? If we have current,</font> <font color="#ffffff">we have magnetic fields.</font> <font color="#ffffff">I leads to</font> <font color="#ffffff">magnetic field, B.</font> <font color="#ffffff">And so, there is a magnetic field that develops here.</font> <font color="#ffffff">How does it develop?</font> <font color="#ffffff">Like that.</font> <font color="#ffffff">Okay? And, you can pick the direction based on the right-hand rule.</font> <font color="#ffffff">Remember, you put your thumb in the direction of the current,</font> <font color="#ffffff">so stick your thumb down in the direction of the current.</font> <font color="#ffffff">Your fingers are gonna wrap around in the direction of B.</font> <font color="#ffffff">Okay?</font> <font color="#ffffff">And so, we get a B field that looks like that for this particular picture.</font> <font color="#ffffff">That was this case.</font> <font color="#ffffff">But when those charges come together,</font> <font color="#ffffff">now the current</font> <font color="#ffffff">is equal to 0.</font> <font color="#ffffff">And if the current is equal to 0, then B is equal to 0.</font> <font color="#ffffff">And later on, as we keep going,</font> <font color="#ffffff">the current's going to change direction.</font> <font color="#ffffff">The current is going to be going up later on.</font> <font color="#ffffff">And if current is going up, we again develop a B field.</font> <font color="#ffffff">And, you use your right-hand rule,</font> <font color="#ffffff">and now it's wrapping around this way.</font> <font color="#ffffff">Okay?</font> <font color="#ffffff">Thumb in the direction of the current,</font> <font color="#ffffff">fingers are wrapping around in the direction of the B field.</font> <font color="#ffffff">Okay?</font> <font color="#ffffff">Again, that's gonna be confusing if you look at me in the glass here.</font> <font color="#ffffff">So, look at the computer monitor when I do this</font> <font color="#ffffff">and confirm that your thumb going up, gets you a B field that wraps around like that.</font> <font color="#ffffff">Okay? And when you do it in, the monitor make sure you use your right hand.</font> <font color="#ffffff">Does everybody see that one?</font> <font color="#ffffff">Okay.</font> <font color="#ffffff">So,</font> <font color="#ffffff">the charges went up and down. We developed an e that oscillated in time </font> <font color="#ffffff">But now, we have a current that's going up and down.</font> <font color="#ffffff">And we're gonna develop a B</font> <font color="#ffffff">that oscillates in time.</font> <font color="#ffffff">And so if we draw B</font> <font color="#ffffff">as a function of time,</font> <font color="#ffffff">it is going into and out of the screen.</font> <font color="#ffffff">If I think about just one side of this,</font> <font color="#ffffff">B is now gonna be like this.</font> <font color="#ffffff">Okay, and this is supposed to be coming</font> <font color="#ffffff">out of the screen and into the screen over and over again.</font> <font color="#ffffff">Okay, we're trying to draw it in the third dimension here.</font> <font color="#ffffff">So, the E field was going up and down</font> <font color="#ffffff">but the B field is orthogonal to that. It is 90 degrees to that.</font> <font color="#ffffff">So if the B field's in the plane of the screen,</font> <font color="#ffffff">then if the E field is in the plane of the screen,</font> <font color="#ffffff">then the B field is in and out of the board.</font> <font color="#ffffff">All right.</font> <font color="#ffffff">But, maybe we can put all this stuff together.</font>
<font color="#ffffff">Hi, everybody. How's everybody doing today?</font> <font color="#ffffff">Feeling good? Trying to stay out of the heat?</font> <font color="#ffffff">It's much nicer in the studio here.</font> <font color="#ffffff">It's very cool. Thanks for the air conditioning, Dave, appreciate it.</font> <font color="#ffffff">>> (Dave) We paid our taxes.</font> <font color="#ffffff">>> These guys paid our taxes. Yeah. Um.</font> <font color="#ffffff">How's everybody feeling about this stuff? Are you feeling a little overwhelmed these days?</font> <font color="#ffffff">Okay, good.</font> <font color="#ffffff">That means -- means you're paying attention. You're in the right spot.</font> <font color="#ffffff">Okay. This stuff is very challenging. It should be very time-consuming.</font> <font color="#ffffff">And, just do your best to absorb as much material as you can.</font> <font color="#ffffff">The point of this couple of chapters that we've been dealing with</font> <font color="#ffffff">is the idea that changing one field affects other fields.</font> <font color="#ffffff">Okay? So, changing the magnetic field</font> <font color="#ffffff">led to a changing of some sort of electric field. We call it the EMF.</font> <font color="#ffffff">Right?</font> <font color="#ffffff">The electro-motive force or a voltage, but it's this idea that these things are</font> <font color="#ffffff">actually going to talk to each other</font> <font color="#ffffff">Okay? And so, this is where we're heading.</font> <font color="#ffffff">The next topic, therefore, is just that, electromagnetic waves.</font> <font color="#ffffff">Okay? We are now going to combine both of these things -- electricity and magnetism --</font> <font color="#ffffff">into one concept, electromagnetic waves.</font> <font color="#ffffff">So, let's talk for a second about the following:</font> <font color="#ffffff">Let's say we have a charge -- positive charge.</font> <font color="#ffffff">And, we have a negative charge.</font> <font color="#ffffff">And, let's say we're gonna take these two charters and we're gonna move them up and down.</font> <font color="#ffffff">Okay?</font> <font color="#ffffff">So, let's move them vertically</font> <font color="#ffffff">and let's see what happens as a function of time.</font> <font color="#ffffff">Well, at one instant at time they look like that.</font> <font color="#ffffff">And if I think about the electric field in between these two,</font> <font color="#ffffff">I know what the electric field looks like.</font> <font color="#ffffff">It's pointing from the positive to the negative.</font> <font color="#ffffff">Okay? I'm just gonna draw one line for simplicity.</font> <font color="#ffffff">But now, let's move them vertically. So, the plus is going to come down;</font> <font color="#ffffff">the negative is going to come up.</font> <font color="#ffffff">And so, at some later time</font> <font color="#ffffff">they're nearly right on top of each other.</font> <font color="#ffffff">And at that time,</font> <font color="#ffffff">the E field is essentially 0.</font> <font color="#ffffff">Okay? There's nowhere I can draw an E field very cleanly,</font> <font color="#ffffff">but now let's keep moving them. We're gonna put the positive charge all the way down on the bottom.</font> <font color="#ffffff">and put the negative charge up on the top.</font> <font color="#ffffff">And now, the E field is pointing up.</font> <font color="#ffffff">Okay? So, all we're doing is we're taking these positive charges and negative charges we're putting them</font> <font color="#ffffff">together, and then we're gonna keep moving them so now they're the opposite way.</font> <font color="#ffffff">And then we're gonna come back together again.</font> <font color="#ffffff">So now, the minus is heading back down</font> <font color="#ffffff">and the positive is heading back up.</font> <font color="#ffffff">And eventually, they're back to where they started.</font> <font color="#ffffff">So if I think about what this field looks like,</font> <font color="#ffffff">it is getting smaller and smaller and eventually 0.</font> <font color="#ffffff">And then, it flips direction. It starts getting bigger and bigger</font> <font color="#ffffff">And now, it's a positive.</font> <font color="#ffffff">And then, it gets smaller and smaller until it's 0.</font> <font color="#ffffff">And then, it flips direction and gets bigger and bigger until it's again pointing in the negative direction.</font> <font color="#ffffff">It sort of oscillates up and down.</font> <font color="#ffffff">Okay?</font> <font color="#ffffff">And that, hopefully, looks familiar to you.</font> <font color="#ffffff">Right? We like things that oscillate</font> <font color="#ffffff">And if I think about drawing the electric field as a function of time, now,</font> <font color="#ffffff">what can I say? Well, here it was negative so it's some negative value.</font> <font color="#ffffff">Here, it was 0.</font> <font color="#ffffff">Here, it was positive.</font> <font color="#ffffff">Here, it was 0.</font> <font color="#ffffff">Here, it was negative.</font> <font color="#ffffff">And so, it's gonna look something like this:</font> <font color="#ffffff">Okay, it oscillates up and down as a function of time.</font> <font color="#ffffff">As I move these charges up and down,</font> <font color="#ffffff">the electric field is going to oscillate up and down.</font> <font color="#ffffff">It's pointing down here.</font> <font color="#ffffff">And then, it's pointing up here.</font> <font color="#ffffff">And then, it's pointing down over here, and so forth.</font> <font color="#ffffff">A graph like this is just: what is the size of the electric field as a function of time?</font> <font color="#ffffff">So, here, the electric field is 0.</font> <font color="#ffffff">Here, the electric field is 0,</font> <font color="#ffffff">but at this point, it's a positive number.</font> <font color="#ffffff">At this point, it's a negative number.</font> <font color="#ffffff">All right.</font> <font color="#ffffff">Let's go back to this picture here for a second</font> <font color="#ffffff">and let's think about this charge.</font> <font color="#ffffff">Okay.</font> <font color="#ffffff">Charge moving up and down,</font> <font color="#ffffff">that is a lot like a current.</font> <font color="#ffffff">If I take a wire</font> <font color="#ffffff">and I run current up and down,</font> <font color="#ffffff">then this current I</font> <font color="#ffffff">would look like positive charge heading down,</font> <font color="#ffffff">and negative charge heading up.</font> <font color="#ffffff">These are equivalent pictures.</font> <font color="#ffffff">But,</font> <font color="#ffffff">we know what happens when we have current.</font> <font color="#ffffff">What happens when we have current?</font> <font color="#ffffff">What do we generate,</font> <font color="#ffffff">when we have current in a wire?</font> <font color="#ffffff">It was a title of a couple chapters ago.</font> <font color="#ffffff">Magnetic fields.</font> <font color="#ffffff">Right? If we have current,</font> <font color="#ffffff">we have magnetic fields.</font> <font color="#ffffff">I leads to</font> <font color="#ffffff">magnetic field, B.</font> <font color="#ffffff">And so, there is a magnetic field that develops here.</font> <font color="#ffffff">How does it develop?</font> <font color="#ffffff">Like that.</font> <font color="#ffffff">Okay? And, you can pick the direction based on the right-hand rule.</font> <font color="#ffffff">Remember, you put your thumb in the direction of the current,</font> <font color="#ffffff">so stick your thumb down in the direction of the current.</font> <font color="#ffffff">Your fingers are gonna wrap around in the direction of B.</font> <font color="#ffffff">Okay?</font> <font color="#ffffff">And so, we get a B field that looks like that for this particular picture.</font> <font color="#ffffff">That was this case.</font> <font color="#ffffff">But when those charges come together,</font> <font color="#ffffff">now the current</font> <font color="#ffffff">is equal to 0.</font> <font color="#ffffff">And if the current is equal to 0, then B is equal to 0.</font> <font color="#ffffff">And later on, as we keep going,</font> <font color="#ffffff">the current's going to change direction.</font> <font color="#ffffff">The current is going to be going up later on.</font> <font color="#ffffff">And if current is going up, we again develop a B field.</font> <font color="#ffffff">And, you use your right-hand rule,</font> <font color="#ffffff">and now it's wrapping around this way.</font> <font color="#ffffff">Okay?</font> <font color="#ffffff">Thumb in the direction of the current,</font> <font color="#ffffff">fingers are wrapping around in the direction of the B field.</font> <font color="#ffffff">Okay?</font> <font color="#ffffff">Again, that's gonna be confusing if you look at me in the glass here.</font> <font color="#ffffff">So, look at the computer monitor when I do this</font> <font color="#ffffff">and confirm that your thumb going up, gets you a B field that wraps around like that.</font> <font color="#ffffff">Okay? And when you do it in, the monitor make sure you use your right hand.</font> <font color="#ffffff">Does everybody see that one?</font> <font color="#ffffff">Okay.</font> <font color="#ffffff">So,</font> <font color="#ffffff">the charges went up and down. We developed an e that oscillated in time </font> <font color="#ffffff">But now, we have a current that's going up and down.</font> <font color="#ffffff">And we're gonna develop a B</font> <font color="#ffffff">that oscillates in time.</font> <font color="#ffffff">And so if we draw B</font> <font color="#ffffff">as a function of time,</font> <font color="#ffffff">it is going into and out of the screen.</font> <font color="#ffffff">If I think about just one side of this,</font> <font color="#ffffff">B is now gonna be like this.</font> <font color="#ffffff">Okay, and this is supposed to be coming</font> <font color="#ffffff">out of the screen and into the screen over and over again.</font> <font color="#ffffff">Okay, we're trying to draw it in the third dimension here.</font> <font color="#ffffff">So, the E field was going up and down</font> <font color="#ffffff">but the B field is orthogonal to that. It is 90 degrees to that.</font> <font color="#ffffff">So if the B field's in the plane of the screen,</font> <font color="#ffffff">then if the E field is in the plane of the screen,</font> <font color="#ffffff">then the B field is in and out of the board.</font> <font color="#ffffff">All right.</font> <font color="#ffffff">But, maybe we can put all this stuff together.</font>