Anderson Video - How to Detect Electromagnetic Waves

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