## fluids

• ### Demo Highlight: Hydraulic Press

Demonstration F1-11: Hydraulic Press is a popular and dramatic way of illustrating Pascal’s Law. See it in action in this new video starring engineering student Sarah Hall:

Pascal’s Law states that, given an incompressible fluid, any change in pressure at one point in the fluid is transmitted throughout that fluid. In a closed container, like this hydraulic cylinder, that pressure exerts a force perpendicular to the walls of the container. The force is dependent on the pressure and on the area. Here, that force is then transmitted up into a wooden board, breaking it.

You can see this illustrated in this EduMedia animation: https://www.edumedia-sciences.com/en/media/442-hydraulic-lift

Experiment with it yourself and see how the force vectors change with area in this simulation by Seng Kwang: https://physicslens.com/hydraulic-press-simulation/

• ### Demonstration Highlight: Buoyancy

Welcome back! For our latest demonstration highlight, we’re exploring the concept of buoyancy, and a few of our demonstrations that let us see buoyancy in action in the classroom.

Technically speaking, buoyancy is the upward force that a fluid exerts on an object that is immersed (partly or wholly) in it. This is the force that determines whether things float or sink, and why some things feel lighter when you hold them in the water rather than in air.

First developed more that 2200 years ago, Archimedes’ Principle (in more modern terms) states that the upward force (or buoyant force) on an object submerged in a fluid is equal to the weight of fluid that it displaces. This means that if the average density (the mass per unit volume) of an object is greater than that of the fluid, it will sing, as the force of gravity on it is still greater than the buoyant force. If its average density is less than that of water, it will float, as the buoyant force is greater than the force of gravity.

We can show the physics behind this in the classroom with demonstration F2-01. Hanging from a spring scale are two cylinders of equal volume – one solid metal cylinder, and one an empty bucket. We lower the solid cylinder into the water, and can measure the buoyant force as the change in weight on the scale. Then, we pour water into the empty cylinder. A volume of water has a weight equal to the buoyant force on an object of that volume, so when we’ve poured in water to equal the volume of the solid cylinder, the weight shown on the scale is back to where it was when both cylinders were hanging in the air!

Conversely, consider demonstration F2-05. A lightweight model boat containing a small, dense weight is floating in a tank water. We can use tape to make the height of the water on the side of the tank. Then the weight is removed from the boat and placed in the bottom of the tank, and we see that the water level on the side of the tank goes down, as the boat floats higher in the water.

The weight is denser than the water. When it is sitting in the bottom of the tank, it displaces an amount of water equal to its fairly small volume. When it is in the boat, though, in order to float the boat-and-weight combination has to displace enough water to match the mass of this boat-rock system, pushing the level in the tank higher. When the weight is removed, the average density of the boat is reduced.

You can experiment with this in the classroom or at home with this Buoyancy Simulation https://ophysics.com/fl1.html by physics teacher Tom Walsh. Try changing the density of the submerged object and the density of the fluid, and see how it floats! You can have the simulation add a free-body diagram to show you the forces in action.

• ### Demonstration Highlight: Coănda Effect

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 Coănda 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!

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.

• ### Demonstration Highlight: Diffusion Distribution Models

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.

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.

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.

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?

And explore more such experiments in our Directory of Simulations!

• ### Demonstration Highlight: Smoke Vortex

A fun an exciting aspect of fluid dynamics is the formation of vortices, such as vortex rings. You can see one way to form them in this demonstration video starring Ruhi Perez:

A vortex ring is formed when fast-moving air is forced into an area of slower-moving air. When you release the membrane, air inside the cylinder is forced outwards through the smaller hole in the end. The air particles come out in a compact mass. The air particles at the outer edge of the mass are slowed as they move past the edge of the hole, and as the mass tries to push past the outside air it experiences friction, they begin to curl back and move in a circle, forming a rotating doughnut-shaped mass of air moving forward together. As the ring moves forward, the air closer to the center of the ring is moving forward faster than the air at the outer edge of the ring.

The higher speed of the air closer to the middle produces an area of lower pressure in the center, and the entire ring moves forward as a coherent mass of air.

You can make a simple vortex generator at home; check out this activity at APS Physics Central: http://www.physicscentral.com/experiment/physicsathome/cannon.cfm

And see them in action with the Lathrop Lab at https://www.youtube.com/watch?v=KIvCgbmls7g

• ### Hot Air Balloon

One of the most popular and visually stunning illustrations of buoyancy and relationship between temperature and pressure is the hot air balloon. Some of you may have had a chance to see one recently at our Maryland STEM Festival event, FLIGHT!

A hot air balloon rises in the air as a result of its buoyancy. As the air is heated, the increased average kinetic energy of the particles in the gas mean its average density is less, and so it rises through the air. In the outdoors, a modern hot air balloon carries its heat source with it, and can keep the air at a constant higher temperature, so the balloon will rise until it reaches equilibrium at an elevation where the density of the outer atmosphere is no longer sufficiently higher than that of the air in the balloon.

Our demonstration balloon, however, more closely resembles the earliest experimental crewed hot air balloons, which heated the air with a heat source located on the ground (a bonfire then, an electric heat gun for us). So these balloons rise only until they have gone too far from the fixed heat source and the air begins to cool down again, reducing buoyancy until they settle back to the ground – or return to the heat source!

The earliest records of the development of uncrewed hot air balloons, like ours, go back over 1,000 years in China, and are recorded some other parts of East and Southeast Asia as well. The were used for entertainment purposes and for signaling between distant points. The earliest known crewed hot air balloon experiments currently known date to the eighteenth century in Europe, though others may have occurred earlier elsewhere.

Simple hot air balloons are easy to make, and are a fun home experiment. Larger demonstration models can be valuable in class to spur discussion of buoyancy and the behaviour of gases, and studying the history of both the technology and the theoretical understanding of their thermodyanmics can be a useful and interesting student project.

• ### Introducing Our Newest Center of Mass Demonstration

This is a simple demonstration, that students can easily try at home. It can also be valuable in class, and is recommended for introductory mechanics lectures. It is perhaps most suited to the second day of discussing center of mass and balance, after students have been introduced to the absic concept of what center of mass is and how it affects the equilibrium position of an object under gravity.

An empty soda can can sit upright on its bottom, or can be laid on its side, but cannot be at rest at any angle between these. However, this can be changed by adding a liquid to the system.

Pour approximately 150ml of water into the can, and then try carefully balancing the can at an angle, as seen in the photo above. (This may require experimenting to find the exact right amount of water for any given can; we recommend doing this in front of the class so they can see the process.)

Ask your students why this should happen? The mass has increased, but why does that change how it balances? The water moves when the can tilts, causing the center of mass to shift – with just the right amount of water, the new center of mass will be above the edge of the can, and so it will balance.

Some cans will tend towards a particular orientation and will roll along the edge to that point; invite students to hypothesize why this is. They may see that the location of the hole and tab in the top of the can affects the equilibrium position – consider how this can be used in class to relate to the concept of symmetry. The can’s behaviour as it reaches equilibrium is a damped harmonic oscillation, and is a good introduction to how fluid action can cause damping from within a system, as well as from an external source.

As you plan for your next class, check it out on our website at B1-18: Center of Mass - Soda Can and Water.

• ### New Demos: Buoyancy and Electromagnetic Forces

We have introduced several new demonstrations recently, in a variety of topic areas. Be sure you take some time to visit our website occasionally and see what's new! Here are brief introductions to two of them.

Electricity and Magnetism: Forces Between Current-Carrying Coils

Check out K1-07 Interacting Coils, developed by student (now alumna) Sarah Monk! This is a new way of illustrating the forces between parallel current-carrying coils. This attractive new tabletop demonstration will be easier to use in many classes. Challenge your students to predict how changing the direction of the current will change the motion of the coil.

This demonstration was itself developed in conjunction with classwork, as part of a final project for PHYS411 last semester. There are many different ways to explore incorporating demonstrations and hands-on learning activities into your classes – not only using demonstrations to illustrate concepts, but advanced students can learn a great deal by developing systems and devices themselves!

Buoyancy, Density, and Pressure

At the suggestion of Mark Eichenlaub, we have also added F2-27 Buoyancy Paradox: Two Spheres. Two identical beakers of water sit on the pans of a pan balance. In one, a ping-pong ball is tethered to the bottom of the beaker so the ball floats submerged in the water. In the other, a steel ball if the same size hangs from an overhead hook, submerged at the same height. Invite your students to predict whether the pans will remain balanced, or will show one as heavier!

This demonstration was first developed for use in a presentation for AAPT's US Physics Team. Every year AAPT trains high school students for the International Physics Olympiad, and UMD Physics hosts their training camp in the early summer, with demonstrations and problem solving exercises.

Physics demonstrations can enhance learning and student engagement in a wide variety of contexts. What experiments could you use to expand student interaction in your classes?

• ### New Portable Ripple Tank

UMD Faculty: Are you interested in showing wave phenomena in class, but unsure if the familiar lecture-hall sized Shive Wave Machine and Ripple Tank will fit in your seminar or discussion room? Here’s another demonstration you may want to check out!

The new portable ripple tank is a small desktop-sized device we use in our physics classes here at the University of Maryland that produces waves in a small tabletop tank. It can be used hands-on for small groups to gather around in the classroom, or displayed on screen with a digital camera for larger audiences.

We fill a small tank with water, and illuminate it with the built-in LED strobe. A small stepper motor drives vibrating needles at the edge of the tank, generating ripple patterns that propagate through the tank. The strobe rate is adjustable, allowing you to seemingly slow down or freeze the movement of the waves to highlight particular interference effects, or you can switch it over to steady illumination to show wave propagation in real time.

Waves themselves are an important physical phenomenon regardless of medium. A demonstration like this shows how waves form in water as a result of a driving force, and are propagated through the water – but, interestingly, while the wave as a structure is moving across the tank, the water is not! At any given point, most of the water is only moving up and down in place, not across the tank. The wave is a physical phenomenon independent of the molecules making up the water. It carries energy across the tank, but mostly not the water itself.

With two sources of wave motion, we can see interference patterns form. Where the peak of one wave meets the peak of another, a larger wave is formed; where the peak of one wave meets the trough of another, they cancel out, forming an oddly calm spot amid the rippling surface. Two waves that line up with their peaks overlapping perfectly are in phase with each other; waves where the peak of one exactly matches the trough of another are out of phase.Where this kind of interference occurs repeatedly across the surface as wavefronts interact, in phase and out of phase at different points, an interference pattern is formed, as the individual waves interfere with each other.

Challenge your students to predict how the interference patterns will change as you vary the vibration rate!

A valuable trait of waves is that many of the properties of wave phenomena are the same for all waves, in any medium. Just like these waves of moving water, other waves light sound waves, radio, and light propagate through space, and can interfere and form interference patterns. The Ripple Tank is a good way to introduce these concepts in class in a familiar form, and analogues can then be drawn to how these effects appear in other situations. The way waves of water diffract around a barrier is much like the way sound waves diffract around a sound-baffling wall by the highway; the calm spots formed by two out of phase sources interfering are much like the quiet zone formed by noise-cancelling headphones as they repeat a soundwave with its phase inverted to remove background noise.

Try out a demo sometime and explore the word of wave physics. But don't get seasick!

• ### Physics Teatime 2: On The Making Of Tea

We talked before about the shape of the teapot; now, another question has come up online: Why does your tea water heat differently in the microwave than in an electric kettle?

Several years ago, Nadia Arumgam mentioned this problem in an article for Slate on making tea. One thing that contributes to the taste of tea is how hot the water is when it is brewing. When you heat water with an electric kettle or on a stove, the heat source is at the bottom. The hot water at the bottom rises, forming convection currents, drawing the cold water downwards to be heated in turn. Over time, the entire pot of water reaches the same temperature. When you see the water boiling at the surface, you know that the entire container is boiling.

In a microwave oven, though, the molecules are heated by electromagnetic agitation at points throughout the container of water, with warm and cool spots. Some of these points may begin to boil while other areas of the water are still cool. When you take the water out and make tea, it may not be as hot as you intend.

Let’s see what this looks like on a larger scale. We have a Dewar flask, an insulated glass container, filled with water. We lower a small heating element into the water so that it heats the top layer of water. We can lower a thermometer probe down to the bottom of the water.

As you can see in the picture, the bottom layer of water is still cold! 23°C, right about at room temperature, and not very appealing for a cup of tea.

The top is boiling; when we move the probe to the top, the temperature is much higher, 97°C. (Not quite 100°C, though, and here we’re only a few centimeters from the heating element.)

This can be a surprising result. We are used to thinking of water as conducting heat well, and it certainly does conduct heat better than air or glass; but not nearly as well as metals or many other materials. In most circumstances, though, most of the heat transfer in water isn’t from conduction, but from convection. In a big container, if your heat source is near the top, there may not be enough convection to make up the difference.

So put on the kettle, curl up with a nice cup of tea, and enjoy the snow!

• ### Physics Teatime 3: Do Not Try This At Home

This week has seen some chilly weather on campus here, and with it a return to warm beverages and the physics behind them. So, for this week, we’re going to look at a bit of physics we heartily recommend you do NOT try at home: superheating water.

In this video, taken by our own Don Lynch, you will see a mug of water that has just been heated in the microwave. Note that the mug is freshly cleaned, smooth, and cylindrical. The water is quite still; but when a teabag is dropped in, suddenly the water bursts into boiling! After a few seconds, the boiling stops, and we are left with what appears to be a cup of tea. (Pity about the mess on the plate beneath.)

Normally, when water reaches its boiling point, it bursts into bubbles as the liquid water begins to turn into a gas. These bubbles usually first form around nucleation sites, tiny (or not so tiny) impurities in the water. The bubbles expand as the force of the vapor pressure of the steam inside the bubble exceeds the external forces of atmospheric pressure and the surface tension of the water. As the bubbles expand, their internal forces exceed the external forces more rapidly, and so once the boiling process has begun it accelerates and the entire liquid boils.

Occasionally, though, if there are no nucleation points and the water is not agitated, no single tiny bubble manages to overcome the atmospheric pressure and surface tension to start the liquid boiling, even though the temperature is at or even slightly above the usual boiling point. When the water is disturbed (in our case, by dropping the teabag in), suddenly there are lots of nucleation points, and the boiling begins in earnest.

We don’t know exactly how hot the water is here; if we put a temperature probe in the container, that itself would trigger the boiling effect before we could make a measurement! But we suspect that it is probably only very slightly above the boiling point. It is certainly not as hot as the more conventional “superheated” water one might find by heating water under pressure – but it is still more than hot enough to cause some very bad burns, so please don’t try to make your tea this way! Take a few more seconds and do things the old fashioned way, and enjoy a relaxing cup of tea and a pleasant winter break.

See you next year!

• ### STEM News Tip: Non-Newtonian Fluids for Cookout Season

As we enter the time of year when many of our minds around here turn to grilling and campouts, it’s time to turn to Scientific Americanfor some valuable and timely knowledge about our favourite non-Newtonian fluid: Ketchup.

Ketchup, or Catsup if you prefer, is a shear thinningfluid – its viscosity decreases when it is under stress. This is why we often have to shake or squeeze the bottle to get it to come out… but is also why it will stay on our sandwich rather than running off immediately. So long as it’s stationary, it’s sticky!

And while we’re on the subject of non-Newtonian fluids, summertime is a good time for messy outdoor expeeriements like making another kind of non-Newtownian fluid. If you mix corn starch and water, you can make a shear thickening fluid – it is stiff under pressure but flows freely when at rest. Read about how to make your own: http://www.scifun.org/homeexpts/lumpyliquids.htm

• ### Teatime in Physics

A question that came in via Twitter recently is one that comes up a lot this time of year, as we tend to want to spend more and more time curled up with a warm beverage. How does my little round teapot fill up so many cups? And why is the tea in the pot still warm when the tea in my cup has gone cold? The answer comes down to geometry!

Here’s a pretty ordinary sized teapot from the cabinet, and an official UMD Physics mug. We’ve tested it twice today and confirmed: This teapot can fill this mug six times. Sure, the pot is bigger than the mug, but it doesn’t look that much bigger, right?

The teapot can even fill this bigger UMD Physics travel mug four times! How?

The answer is related to what biologists call the Square-Cube Law. As an object grows in size, its volume increases faster than its surface area. If you take a cubical container and double its length, width, and height, multiplying by 2 in each direction, then its surface area is multiplied by 4, the square of 2. But its volume is multiplied by 8, the cube of 2. The exact numbers will change, though, depending on the shape of the container. Every shape has its own relationship among liner size, area, and volume. As it turns out, the most efficient shape, with the highest ratio of volume to surface, is a sphere.

This sounds like just abstruse math, but it actually explains a lot about things we deal with every day, from teapots and fuel tanks to giraffes and polar bears. (OK, maybe not all of us deal with polar bears every day, but it’s good to know about them anyway.)

Here’s an example from the demonstration collection. This round flask and this tall cylinder each hold the same volume of water, 500 milliliters. The cylinder is much longer and narrower than the sphere, so it looks bigger, but it has the same volume!

One thing that makes this interesting is that, having a larger surface area, the cylinder is also heavier. It takes more glass to make a 500mL cylinder than to make a 500mL sphere. That might not matter much for our purposes, when we just want one container to sit on the table, but it can make a big difference in large storage containers, or in places where weight is important, like spacecraft.

This is also why fluids in free fall, like raindrops, form into spheres. The surface tension of the liquid is pulling inwards, compressing the surface to the smallest area for that volume of water: a sphere. On a larger scale, this even happens to big accumulations of rock, pulled in by gravity over a long period of time. We call them planets – and luck for us, they do tend to end up round!

That’s all interesting, but isn’t my tea getting cold after all this?

No, and here’s why: The total amount of heat in the container is proportional to its volume. But the radiation of heat away from the container is proportional to its surface area. So my nearly spherical teapot loses heat a lot more slowly than that tall cylinder does. Plus, because there’s less surface area for the same volume, we can make the walls thicker for the same weight, giving it better insulation.

And that’s where the polar bears come in. (Not literally, polar bears should not drink tea.) Ever wonder why so many animals in warm climates evolved long, lanky builds, while arctic animals tend to be rounder? A lot of it comes down to heat. A round polar bear loses heat a lot more slowly, so they can burn fewer calories to stay warm. That can be important in the long winters when there’s not much to eat. In a hot climate where the bigger problem is staying cool, many animals tend to be thinner. Others find other ways to increase their surface area, like the big ears on an elephant, to radiate heat away faster. There are lots of other factors at play in evolution as well, of course, but heat is always an important one.

This relates to why animals only come in certain sizes, too. If you scale up an ant 100 times in each direction, its mass increases by one million - but the surface area of its legs doesn't, so it can't stand up!

So sit back, make a pot of tea, and curl up with a good book about somewhere warmer. And spring will be here before you know it!

(Note: No tea was harmed in the creation of this blog post. But quite a lot of it was consumed.)