Demonstration J4-51: The Theremin is a fun and exciting way to illustrate electrical capacitance. You can see it in action in this new video with Angel Torres.
The theremin is an electronic musical instrument invented in the early 20th century by Russian scientist, engineer, and cellist Leon Theremin. As well as his musical work, Leon Theremin developed many other electronic devices in his career as an engineer, including an early motion detector and listening devices for espionage.
The two metal “antennas” on the sides of the theremin are not antennas in the usual sense. Each one functions as one plate of a capacitor. When you move your hand near the antenna, your hand serves as the other plate of that capacitor. Thus, each functions as a variable capacitor, where the capacitance, the ability of this air-filled capacitor to store electrical charge, varies as you move your hand and body near the antenna.
Each of these capacitors is part of a variable RLC (resistor-inductor-capacitor) oscillator circuit. One of these variable oscillators controls a second internal oscillator circuit; these together create the output frequency (or pitch) of the sound from the theremin.
The other variable oscillator, meanwhile controls the output amplitude. The resulting signal is fed through an amplifier circuit to a speaker. Together they form an electronic system that can create music, controlled by the motions of your body – without the player ever actually touching the device.
The theremin has been used in a wide range of music. Much of the early technique of playing it was developed by classical violinist and thereminist Clara Rockmore. The theremin can be heard in the work of orchestral composers like Dmitri Shostakovich and Percy Grainger, and in rock bands including the Rolling Stones and Led Zeppelin. And you can hear it on the sound tracks of movies ranging from Cecil B. deMille’s The Ten Commandments, to the science fiction classic The Day the Earth Stood Still, to the 2006 animated film Monster House. It’s a beautiful way to see and hear the fusion of art and science.
Our theremin seen here was built by the Moog Corporation, best known for their electronic synthesizers.
Continuing the theme of Center of Mass, this week we’re taking a look at two more popular demonstrations. Demo B1-01 shows us that how an object hangs when suspended is dependent on its center of mass. We can use this to locate the center of mass of an irregularly shaped object, as you see in the picture here.
Demo C1-02, in turn, looks at the center of mass of an irregular object in motion. As this very familiar irregularly shaped object tumbles through the air, its ends seem to be moving every which way. But the center of mass follows a parabolic path, just like a simple thrown ball.
The UNSW School of Physics has some videos analyzing this. You can see how the ends of an object or the limbs of a running athlete move through the air, while the center of mass traces out that same parabolic path. And they found the center of mass of a runner the same way we did with the plunger: just see where it balances!
The Lecture Demonstration Facility at the University of Maryland is designed to help faculty spark student interest, identify misconceptions, help students make predictions, facilitate discussion, and reinforce curricular concepts.
We’re often asked how many demonstrations we have in the collection. That’s a more complex question than it might at first seem.
At last count, we have just over 1,500 demonstrations published to the website – that is, that’s how many demonstration pages exist in the collection. But some pages describe a single setup than can be used in several different ways. Take a look at K2-61: Thomson’s Coil, for example. This single page actually describes four different, related demonstrations that can be performed with this device. They don’t require very different equipment to be delivered, just slight changes in preparation, though, and they’re usually all relevant at approximately the same point in a syllabus, so it’s simpler to list them all in one place. Conversely, there are many demonstrations that use the Optical Board – browse through section L and you will see many of them! Since ray optics is divided into several sections in the demonstrations catalog, each of the configurations of the Optical Board is listed separately, to make it easier to find the one you need; and if you’re only doing one demonstration with it, we can configure it for you in advance to save you time in class.
On the other hand, consider M1-12 and H2-22. These are both listings for Interference Transparencies, a popular way to illustrate the interaction of wavefronts. Here, we made the unusual decision to list the same demonstration twice in two different sections, since otherwise someone planning a course on sound might not think to look for relevant demonstrations in the optics section, and vice-versa. These occasional cross-references make it easier to find the demonstrations you need for your class.
And even aside from the demonstration listings as they stand, we’re often called on to combine equipment in unique ways to demonstrate something new! If it’s a combination that’s likely to be repeated or that proves useful to others, it will be added to the website, but we’re generally open to creatively reinterpreting demonstrations to fit a new class context.
Every year we add more demonstrations to the collection; and occasionally a demonstration is retired, if it no longer meets an instructional need or has been superseded by others. So defining just how many demonstrations we have might not be the right question to ask. Ask, rather, what can we demonstrate for you today?
Science is all about data, and our current pandemic is no different.
Be sure to check the UMD COVID-19 Dashboard for the latest campus data and links to reopening plans and proper safety procedures.
In support of most classes moving to an online model this year, the Lecture-Demonstration staff are doing our part to help connect you to resources you need for teaching remotely. As one part of this project, we have begun compiling a Directory of Simulations from around the internet, organized by general area of physics. Find it under the Tools and Resources menu above, or click the image below.
There are a tremendous number of simulations out there, that folks have been creating for years. We’re testing them out, choosing ones that we can confirm currently work (always a question as internet technology marches on) and that seem useful for our department’s classes. As of this posting, we have just over fifty simulations collected. Our initial focus has been on physics that is hard to demonstrate in the classroom, or experiments that are difficult to present as static pictures or live video.
This project is ongoing! As we continue to explore we will be adding more subjects and more demonstrations per subject. We also invite recommendations! If you have a favourite simulation, let us know (email lecdemhelp at physics.umd.edu) so we can check it out and add it to the directory.
We’ll have more new projects posted soon; watch the site for news!
As the COVID-19 pandemic continues, researchers at UMD and around the globe continue to try to better understand the virus and how to treat it.
In our ongoing work to support remote teaching, we are pleased to announce a new resource. Over the summer of 2020, a Teaching Innovation Grant helped to create our new Demonstration Videos. These can be used for remote, hybrid, and in-person classes to present demonstrations in conjunction with class engagement questions.
The Conference for Undergraduate Underrepresented Minorities in Physics returns January 8-10, 2021, and we've gone virtual!
CU2MiP is co-sponsored by UMD Physics and NIST.
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