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Physics32. Electromagnetic WavesWhat is an Electromagnetic Wave?

Let's ask this question: how are we going to detect these electromagnetic waves? Well, let's draw the electric field coming along. Okay? It looks like that. It's pointing up. Then, it's pointing down. Then, it's pointing up, and then it's pointing down. So, let's take a wire, metal wire. And, let's just hold it up, right there in space. Okay? When the electric field comes along and it is pointing up, what happens to the charges in the wire? They, of course, feel a force going up. The positive charges feel a force going up. The negative charges feel force going down. And so, there is a current, I, which develops in the wire. Okay? And later when the electric field is pointing down, the current is gonna flip and come back down. Okay, so the current will, in fact, oscillate up and down. All right, let's take this whole wire and let's connect it to a circuit. Let's connect it to a circuit that you are familiar with now. Okay, let's put in an L and a C in our circuit. And, let's connect another inductor right there. This looks like a transformer. And, we're going to send this signal to an amplifier. And now, we're going to adjust the capacitor. How do you adjust the capacitor? Well you can move the plates relative to one another and that will adjust the value of the capacitance. And if I do that, I can find a resonant frequency of this circuit that just matches the resonant frequency of the wave coming in. And, this is a device that you're all familiar with. What device have we just drawn? I can almost guarantee you you've used it today. Anybody know what we just drew? >> (student speaking) Your phone? >> Okay. Nope, not your phone. It is your car radio. Okay? This is your car radio. You guys are like: "I don't have a fancy car with one of these radios in it." I did for a while but then it got stolen them haven't replaced it. So now, I just listen to my iPod while I drive, which is illegal if you have two headphones in So I don't do it. All right. This is your car radio. What is this thing? What is this big metal wire? That's your antenna. Right? You go outside your car. You reach in and you pull out this big telescoping wire. That's your antenna. What is it there to do? It's there to collect the radio waves that are coming in. This is from the radio station. Okay, those radio waves come in, they hit your big metal bar. It excites current up and down. It transfers to your car stereo system. And when you pick just the right value of the capacitor, you tune in to a particular frequency Every different station is coming in on a different frequency and so there will be different oscillations here. And by adjusting C, you move that frequency up or down the dial.