• Demo Highlight of the Week: Tesla Coil, Part 1
  • Demo Highlight of the Week: Tesla Coil, Part 2
  • Past Highlight: Parallel Plate Capacitor
  • Demonstrations
  • How many demonstrations?
  • New Resource: Directory of Simulations
  • Spotlight: COVID-19 Animation and UMD Research
  • New Resource: Demonstration Video Channel
  • Visit the UMD COVID-19 Dashboard
  • Welcome to Fall 2021!

Welcome back! This week and next, we’re spending some time with the ever-popular Tesla Coil, demonstration K7-61 in our demonstration index. You can see it in action in this video with graduate student Haotian Wang

A Tesla coil is a particular type of transformer circuit, a tuned or resonant transformer, that can produce very high voltages. While transformers are used in many applications in technology to transfer energy and convert to different voltages, a resonant transformer incorporates an inductor-capacitor combination, or LC circuit (from the traditional notation on circuit diagrams for inductors and capacitors), to both step up the voltage and to temporarily store energy, releasing it in dramatic bursts.

 Circuit diagram of a Tesla coil

This kind of induction device was historically used as part of early spark-gap radio systems. Today they are primarily used for research, education, and entertainment; although small Tesla coils also have applications for detecting minute changes in the conductivity of their surroundings, making them useful for testing vacuum seals in equipment.

We’re paying a second visit to the Tesla Coil today, exploring more about how it works. 

tesla coil

Broadly speaking, we can wave our hands at the Tesla Coil and talk about inductance and resonance, but what does that really mean, and how does it lead to those lovely purple sparks?

Sparks on Tesla Coil

 Electromagnetic induction is the process by which a voltage is produced across an inductor in a changing magnetic field. In this case, we’re taking advantage of the studies of Maxwell and Faraday that showed the relationship between electricity and magnetism. An electrical current generates its own magnetic field; a changing electrical current thus produces a changing magnetic field, and so a changing electrical current in one conductor can induce a current in a nearby conductor. We can carefully choose these to create higher or lower induced voltages.

Electrical resonance occurs when a circuit is built to have a particular resonant frequency, at which the impedance (the way a circuit element resists an alternating current) of different components cancels out to let the circuit build up higher voltages or currents.

tesla coil circuit diagram

Our Tesla coil, circuit above, uses a 5000 volt transformer to charge a large oil capacitor. When the potential across the capacitor reaches the breakdown potential of the spark gap, breakdown across the gap occurs. The spark gap then becomes a conducting part of the RLC circuit, which resonates at a frequency of about 200 kilohertz. The large coil in the resonant circuit is the primary coil of the final transformer and the long coil of very fine wire is the secondary, producing about 200,000 volts at 200 kilohertz.

You can see what’s happening by examining a simulation of a similar circuit’s behaviour, like . The initial transformer creates a high voltage, which eventually builds up enough to exceed the breakdown voltage of the air and make a spark across the spark gap. This then feeds into resonant circuits which build up very high electrical potential, which can create the discharge we see.

This uses Paul Falstad’s Circuit Simulator Applet, which you can explore further at

To learn more about Tesla Coils, check out:

Nikola Tesla’s patent:

Richie Burnett’s Operation of the Tesla Coil:


Kelley and Dunbar, “The Tesla Coil,” American Journal of Physics 20(32).


Today we’re visiting the ever-popular Parallel Plate Capacitor, in its simplest form found in our demonstration index as J4-01, or with a dielectric plate at J4-22. A capacitor stores energy in the electric field between its plates. The capacitance of a capacitor is technically the amount of charge stored per volt – in a sense, how capable it is of storing charge at a given potential. In a parallel plate capacitor, the capacitance goes up with greater surface area, and goes down with greater separation between the plates.

 Parallel Plate Capacitor: Two metal plates, a meter, and a power supply

The parallel plate capacitor consists of two large aluminum plates with an air gap. The capacitor is charged with a potential of around 1000 Volts using a low-current DC power supply. The plates may then be separated and the voltage observed, demonstrating that for a fixed amount of charge, the voltage is proportional to the plate separation.

But if you insert a dielectric sheet into a charged capacitor, the voltage goes down, which means the total capacitance of the system has gone up! The capacitance of a system depends on the dielectric constant of the medium – for air, this is very nearly the same as pure vacuum, but some materials have a much greater dielectric constant. This plastic plate has a dielectric constant nearly 5 times that of air.

Now, try out these simulations to see if you observe the same behaviour!

The first, from the PhET collection at the University of Colorado, places a capacitor in a simple DC circuit. In the first simulation tab, you can adjust the input voltage and plate size, and measure the electric field, capacitance, and energy stored. The additional tabs show variations: you can add a dielectric to the capacitor, or place multiple capacitors in series and parallel.

The second simulation, at oPhysics, additionally lets you control many characteristics within an idealized circuit. Compare the results you get for different combinations of capacitor size and input voltage between the two simulations.

Our capacitor has 22cm diameter circular plates, rather than the square plates used in the simulations. In the simulations, try setting the plate area to be the same as ours and see how it responds to other voltages.

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?

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.

Sample subsection titles: Electricity & Magnetism Simulations, Mathematics Simulations, Optics Simulations, Oscillations & Waves Simulations, Quantum Simulations, Thermodynamics & Statistical Mechanics Simulations

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 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!


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 videos have their own YouTube channel, linked both here and on the Tools & Resources Menu above; check them out 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.

Keep Terps Safe - UMD COVID Public Dashboard


Welcome to Fall 2021 at UMD Physics!

The Lecture-Demonstration Facility is off to a running start, and we look forward to working with you for your demonstration needs this semester.

Be sure to explore our new online resources as well, including the demonstration videos channel, LecDem blog, and the directory of simulations.

If you have any questions about finding the right demonstrations or other resources for your class, be sure to call or email.

Please remember to order your demonstrations before the cutoff deadline for the order form system: For morning classes, before 1PM the previous working day; for afternoon classes, before 4AM the day of the class. Where possible, we appreciate having the orders at least one full working day ahead, to ensure plenty of time to make sure everything is ready for you.