32. Electromagnetic Waves

What is an Electromagnetic Wave?

Physics32. Electromagnetic WavesWhat is an Electromagnetic Wave?

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