• Demo Highlight of the Week: Coanda Effect
  • Highlight of the Week: Modeling Diffusion
  • Demonstrations
  • How many demonstrations?
  • New Resource: Directory of Simulations
  • Spotlight: COVID-19 Animation and UMD Research
  • New Resource: Demonstration Video Channel
  • Spotlight: CU2MIP
  • Visit the UMD COVID-19 Dashboard

Welcome back to the Demo Highlight of the Week! This week, we’re exploring the motion of air with physics student Kathleen Hamilton-Campos and demonstration F5-09, the Coanda Effect with a hair dryer and a ping-pong ball.

Named for Romanian scientist Henri Coandă, the Coandă Effect describes the phenomenon where a stream of moving fluid will tend to stay in contact with a curved surface, and conversely an object in a stream of moving fluid will tend to remain within that stream. While superficially similar to the Bernoulli Effect, which describes the changes in speed and pressure in a constrained fluid, the differences between the two can be important when analyzing things like the movement of aircraft, and in this demonstration! 

 coanda effect diagram, as described in text

A ping-pong ball is placed in the stream of air coming out of a hair dryer. The moving stream of high-speed air entrains the slower air around it, pulling it along. Around the surface of the ball, though, this becomes asymmetric, creating a low-pressure region in the center of the stream with a high pressure region around it. The ball is effectively trapped in this low-pressure core.



 Today we’re taking a look at some models of diffusion: Demonstrations I6-21 and I6-25. These both use the behaviour of ping-pong balls to model the behaviour of molecules in a gas.

I6-25: An array of wooden pegs, and lines of white and orange balls ready to drop through them

Each of these models uses ping-pong balls of different colors to represent different molecules in a gas. In I6-21, we have a mechanically shaken chamber divided by a plastic barrier. We can put balls of one color on one side and balls of another color on the other side. When the chamber vibrates, the balls bounce around like the molecules in a gas. When the barrier is removed, the balls begin to drift onto each other’s sides, and soon there is no distinction between the two.

 I6-21 GAS DIFFUSION - MODEL - pingpong balls of two colors in a large transparent box

This is also a good example of the principle of entropy – while it is very easy and probable to disorder this system, as the two sets of balls mix together, it is highly improbable (though not impossible, given a small enough number of balls) that all of the balls of each color will suddenly sort themselves out again! Thus, the system tends towards the more disordered state.

In I6-25, we start with columns of balls at the top of an array of pegs. The balls are held in place by a small plastic baffle. When the baffle is removed, the balls fall down through the array, scattering as they go. By the time they reach the bottom, they have spread out into a curve, roughly approximating a proability graph. The columns at the bottom with more balls are the areas more probable for balls to scatter into, and those with few or no balls are less probable. As with I6-21, we can use different colors of balls to show how gases diffuse together over time.

 I6-25 pegboard with stacked balls, and then afterwards with the balls scattered at the bottom

 Now, you can try this in class or at home with this simulation from the PhET Collection at the University of Colorado. You can let a small or large number of particles of two different gases diffuse through each other, and watch their behaviour. How do the simulated particles here resemble the model “particles” of our demonstrations? What’s different? How can we explore the differences when talking about the behaviour or real gases?

  screenshot of PhET diffusion simulator. Top, particles separated; bottom, particles diffusing together.



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!


The Conference for Undergraduate Underrepresented Minorities in Physics returns January 8-10, 2021, and we've gone virtual!

cu2mip 2020 ug flyer

Download the flyer today, and visit the website for registration and more information!

CU2MiP is co-sponsored by UMD Physics and NIST.

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