MIT OpenCourseWare http://ocw.mit.edu 8.02 Electricity and Magnetism, Spring 2002 Please use the following citation format: Lewin, Walter, 8.02 Electricity and Magnetism, Spring 2002 (Massachusetts Institute of Technology: MIT OpenCourseWare). http://ocw.mit.edu (accessed MM DD, YYYY). License: Creative Commons Attribution-Noncommercial-Share Alike. Note: Please use the actual date you accessed this material in your citation. For more information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/termsMIT OpenCourseWare http://ocw.mit.edu 8.02 Electricity and Magnetism, Spring 2002 Transcript – Lecture 27 Before we're going to dive into electromagnetic waves, I would like to discuss a few more mechanical resonances with you. Last Friday, we discussed the resonances of string instruments and wind instruments. But there are several that you see around you quite often -- without realizing it, perhaps -- that you're looking at a resonance frequency. You may have noticed that traffic signs have the tendency, sometimes, to do this, and at certain wind speeds, they go like this. Enormously strong amplitude, that's a form of resonance. Undoubtedly, you have been motels or at homes where you open a faucet, and then all of a sudden, when the water's running in a certain way, you hear an incredible noise, a terrible noise. You close the faucet a little, or you open it a little further, and that noise goes away. That's clearly an example of resonance. You drive your car, or you're in someone else's car, and at a certain speed, something begins to rattle. Very annoying. You go a little faster, it stops. You go a little slower, it stops. Or, if you go a little faster, something else begins to rattle, there's some other resonance of something else in the car. And of course, there are cars whereby something rattles at any speed.But in any case, there's this idea, then, of resonance, which is all around us. I remember when I was in a student, and when we had an after-dinner speaker which we didn't like, we would very quickly empty our wine glasses -- in those days, we were still allowed to drink, by the way --and what we would do is the following, something extremely annoying. We would generate the fundamental of our wine glasses. You take your finger, you make it wet, and you rub it like this. Listen. [Rubs glass] Believe me, if 100 students do that, it's very annoying. But it's also extremely effective. Speaker -- speaker gets the message very quickly. [Rubs glass]. What the glass is doing, it's the fundamental of the glass, it's the lowest frequency, the glass is actually doing this. And there are rumors that people can break glasses by singing. And we'll talk about that in a minute. Um, I remember a, um, commercial, Memorex. Memorex is an audio tape. And they bragged about breaking glasses -- some of you may actually have seen that commercial. There was a, uh, a picture that I can show you that goes with the commercial, and then a very dramatic story. The story is that someone goes to a concert.And there is a woman singer, puts a glass on the table, raises her voice, hits the resonance frequency of the glass, [pshew!], and there goes the glass. And this gentleman was recording it, of course, on his Memorex tape. So let's, um, see this, uh, this slide. So if we get the slide -- yes! You see this, um, this glass, maybe you can focus a little better John, thank you. Memorex. So the story then goes that the guy goes home and tells his wife about this. Well, she is smart enough not to believe this story. But he plays back his tape. And at the moment that this glass breaks at the concert, he has some wineglasses himself at home, and lo and behold, they also break. And so then the idea is, that is the commercial -- that's the great pitch of Memorex -- that the reason why they break at home is because of the enormous quality of this tape which is made of very special material. And the material, as you could have read on the box, is a very special chemical compound, it is MRX2. Two atoms of X, one of R, and one of M, and then you make it oxide, and then you have the best tape that you can imagine in the world. Well, they overlook a small detail, and that is that, um, for one thing, a tape recorder would never generate enough volume to break a glass in the first place. But in the second place, the glasses that this guy had at home, obviously didn't have exactly the same resonance frequency as the glass at the concert. So this could never have happened.But like with all commercials, you know that you're being swindled, and this, of course, no exception. I've always questioned whether it is actually possible that a person, without the aid of strong amplification, and without the aid of huge sound volumes which you can generate with loudspeakers, whether you can actually break a glass. I've always wondered about that. People say it can be done. Caruso, famous singer, was known for being able to do that. He put the glass there, he would rub it with his finger so that he knew the resonance frequency [kllk], and there he would go, and [poit] bingo. Frankly speaking, I don't believe it. I don't believe it can be done by a human being without the aid of amplifiers and speakers. And when I lectured 8.01 several years ago, together with Professor Feld here at MIT, we discussed the -- the possibility of designing something that actually would be able to break a glass. And -- and he actually deserved a lot of credit for that, he worked with a graduate student, and he managed to design a setup that works most of the time. But don't put your hopes too high, it doesn't work all the time. So here is a wineglass, the same series as that one. By the time -- when -- when he got it to work, we bought 500 of those glasses -- we got a good discount, by the way, because we wanted to be sure that we can do it for years to come. So here's the wine glass, and here is the loudspeaker, and we are going to generate sound very close to the resonance frequency of this glass, which we have already determined before you came in, 488 Hertz.You're going to see the glass there, and to make you see, actually, this wonderful motion of the glass, we will strobe it with light at a frequency slightly different from the frequency of the sound so you see the glass move very slowly. And then we will increase the volume of the speaker, and then with some luck, if we are right on resonance, [poit], the glass may
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