Demos

A Guide to Physics Demonstrations and Tours offered by The Society of Physics Students

Original Document: Joshua Kaggie, Alexis Olsen, Joseph Ulmer July 28, 2010

Revision: Teddy Anderson May, 2017

Introduction:

Our club, the Society of Physics Students (SPS), offers tours of the University of Utah physics facilities, as well as physics-related demonstrations, to groups of scouts, elementary and middle-school classrooms, or any other interested groups. This document offers information to the public and to SPS club members on the details of how to arrange and carry out these tours, as well as in-depth descriptions of some popular demonstrations we can present to groups. These demonstrations are described below in the following categories: liquid nitrogen, electromagnetism, energy, and waves and light.

1- Visiting groups:

The SPS club has established relationships with different organizations, including scout troops and elementary school classrooms, who have come for tours in the past.  These groups usually contact us via our SPS email account, through the physics department main office, or through any private contact they may have in the department.  In addition to these established groups, we can reach out to any other classrooms, clubs or organizations whose members may benefit from a tour or a demonstration.  Groups usually range in the size of 15-20, but larger or smaller groups can also be accommodated.  

Once we are in contact with an interested group, we first determine whether their visit will involve research lab tours, demonstrations, or both.  We establish a time and day based on the availability of researchers and their labs, the availability of SPS club members, and the availability of the touring group.  *Please note: for telescope-related tours, there is already well-established outreach through the South Physics Observatory.  We can help by contacting the astronomy outreach coordinator to arrange these tours, and volunteering our assistance if needed.

2- Lab Tours:

If a group wishes to tour our research labs, the first thing to do is contact different researchers about their availability.  While some labs and researchers are natural favorites for young groups, all labs should be considered.  When we have confirmation from one, two, or three labs, depending on the group’s preference, we can confirm with the visiting group.  Labs can typically accommodate about ten people at a time, including teachers and club members.  With more than ten people, we can rotate through different labs by breaking into smaller groups.   

When a group arrives for a visit, the SPS officers and volunteers first meet them in a central location, then break them up into smaller groups if necessary, each with a teacher or representative from their own organization.  Each group will also have an SPS representative to guide them from one place to the next, and walk with them to each of the labs.  Once in a lab, the researcher will give a presentation, showing the young students their equipment, and describing to them their research.  We spend about 15 minutes in each lab, then move on to the next.  We can also show groups around any publicly-accessible places in the physics department.  

3- Liquid Nitrogen Ice Cream:

Whether also giving demonstrations, or not, we usually end a group’s visit with liquid nitrogen ice cream.  When a group has booked with us, we must order liquid nitrogen for our own dewar through the front physics office.  This should be done several days in advance to ensure its prompt arrival.  Be sure we are stocked with the other necessary equipment as well: disposable cups and spoons, big metal bowl, metal stirring spoons, leather gloves, milk, vanilla, and sugar.  For the recipe and instructions, please see “Making liquid nitrogen ice cream,” section 4.1.6, below.  

4- Demonstrations:

If a group also wishes to be given a demonstration by one of the SPS club members, we can arrange this without relying on any of the research labs for assistance.  What follows is several pages describing the many demonstrations that we can give to student groups.  At the time of this document, the SPS club is not in possession of all of the equipment necessary for these demonstrations, but with advance notice, most of it should be available from the physics demonstration specialist in the department.

4.1 Liquid Nitrogen

Nitrogen makes up 78% of the Earth’s atmosphere. Pure nitrogen is an unreactive colorless diatomic gas at room temperature. Liquid nitrogen exists between 63K and 77K. Because it is so cold, liquid nitrogen is used as a coolant for many things. It is also used to preserve biological samples and to freeze off warts.

Do not touch metals that are in contact with liquid nitrogen. Do not touch glass or most solids that are currently submerged in liquid nitrogen. Do not spill large amounts on your shoes or clothes. Do not leave your skin in contact with liquid nitrogen for over 5 seconds.

If there is too much nitrogen in the air, it can displace oxygen and cause asphyxiation. Make sure a door or window is open if over 3 liters of nitrogen are used over a period of 15 minutes.

4.1.1 Shrinking balloons demonstration:

In this demonstration, a balloon is held in liquid nitrogen until it shrinks. While submerged in liquid nitrogen, the air inside it gets very cold, and the balloon shrinks down to appear almost empty, as if there were no air in it at all. This is because cold air molecules slow down and stop hitting the sides of the balloon as frequently, thus creating a lower pressure.  

4.1.2 Smashing roses:

To do this demonstration, hold a rose or other flower in a cup of liquid nitrogen for a minute or so until it is frozen solid. Then hit the flower against the edge of a desk or other hard material, and watch it shatter into many pieces.  This is because the water in the flower freezes solid, turning it into a flower-shaped ice cube, which is very brittle.  

4.1.3 Smashing racquetballs:

This demonstration is essentially the same thing as the flower demonstration. You put a racquetball in the liquid nitrogen long enough that is loses its elasticity and freezes solid. It will feel as if it is made of stiff plastic. Then throw the ball against the wall and watch it break into many pieces.  

A racquetball is made of rubber, which at normal temperatures can move and bend.  When the ball gets cold the molecules aren’t free to flex and move around each other, so when it is subjected to the force of the wall, it breaks.   

4.1.4 Exposing the liquid nitrogen to your hand:

To show kids the difference in temperature between liquid nitrogen and a person, you can dip your hand into a small cup of liquid nitrogen. You can also pour it on your hand. As long as you do it quickly it will not hurt you, but will boil the nitrogen instead.

This demonstration can be explained to the kids with a comparison. Remind them how cold liquid nitrogen is, and then how warm a person is, and then tell them that when the nitrogen touches you, it feels as you would feel if you stuck your hand on the stove. A person is so much hotter than liquid nitrogen that the liquid nitrogen immediately boils away, and doesn’t freeze your hand. However, if you left your hand in the cup for too long, it would cool your hand down (similar to the racquetball) and then could cause tissue damage.

4.1.5 Levitating a magnetic cube:

For this demonstration you will need a Styrofoam cup, a small, black superconducting disk, and a small, brass-colored cube magnet. Place the super conducting disk in the bottom of the Styrofoam cup, then fill the cup with the liquid nitrogen. When the disk has been covered in nitrogen for about thirty seconds, the magnetic cube can be placed in the cup and it will levitate. Once the magnet is floating, try to make it spin so the kids can see that it is really floating above the super conducting disk.

The simple explanation for this is that magnets push on electrons, but the electrons don’t like being pushed so they push back.  In this demonstration, the magnet above the disk pushes on the electrons in the disk, and so the electrons in the disk push back on the magnet. We tell the scouts that normally the electrons wouldn’t be able to push the magnet hard enough to make it float, but because of the super conducting disk they can. This is because the cold of the liquid nitrogen allows the electrons in the disk to push a lot harder than they normally could and so they make the magnet float.

4.1.6 Making liquid nitrogen ice cream:

4.2 Electricity and Magnetism:

Electromagnetism is the study of electricity and magnetism and their relationship to each other. When explaining these experiments to kids, use simple terms, and avoid mathematical equations. Start with the fundamentals, such as: like charges repel, and unlike charges attract.  Some groups will have already learned about atoms, while others have not. You can explain protons, neutrons and electrons very briefly, if appropriate.

4.2.1 Dropping a magnet down a copper tube:

Place a copper tube upright and then drop a magnet into it. The magnet will move slower down the copper pipe than it would normally would under the downward force of gravity alone.

This slower downward acceleration is due to the magnetic field created between the magnet and the copper pipe. The magnetic field causes the electrons to push against the magnet, effectively creating a drag force. A moving magnet creates eddy currents which oppose the downward force of gravity.

4.2.2 Rolling a nonmagnetic and a magnetic ball down an aluminum ramp:

Place both a nonmagnetic and a magnetic ball on an aluminum ramp and let them roll down at the same time. The nonmagnetic ball will reach the bottom of the ramp before the magnetic ball.

This is because a magnetic field is created between the aluminum ramp and the magnetic ball, the same as above, which causes the ball to roll slower under the force of gravity.

4.2.3 Using ferro-fluid to show magnetic field lines:

The ferro-fluid can be identified as a brownish liquid inside of a clear container with a white cap. Do not place a magnet over the fluid, or it will stick to the magnet.

Ferro-fluids respond to external magnetic fields because they contain tiny, suspended magnetic particles. By moving a magnet over the ferro-fluid, magnetic field lines will appear within it.

4.2.4 Moving a magnet across a copper plate:

Try to move a magnet across a copper plate.  This will be difficult because they are oppositely charged, and the negative and positive charges want to stay together.  This demonstrates magnetism as an attractive force.  

4.2.5 Shocking each other with the Van de Graff generator:

The Van de Graff generator looks like a large silver ball with a clear plastic tube holding it up. Before turning it on, make sure that neither you nor the generator are near any outlets. Do not touch (or stand on or near) any outlets, plugs, or even insulated cords while in contact with the generator. Do not put any metals near the generator. Keep the generator away from any electronics. Keep it away from other metals as this can create extreme capacitance and be unsafe. Generally, credit cards in your pocket will be far enough away and not in any danger of damage.

Turn on the Van de Graff generator and touch the silver ball. If you turn off the lights you can see sparks when you put your hand near the generator. You can ask for volunteers to touch it, then to make a chain. They may notice that they will get shocked when someone new is added to the chain.

If the generator isn’t creating enough charge to show this effect, the aluminum plates can be added to the top. However, test it at a lower charge first, before trying it with the added plates. If the chain is long enough and the charge is low enough, you could see what happens when someone touches the door handle.

The Van de Graff generator creates an electric charge in the air around it, which will seek to discharge by creating a current through the easiest path. It is easier for current to move through the water in a person’s skin than through the air, so it will move onto a person, and from person to person, rather than through the air or the soles of shoes. However, touching a metal door handle allows the current to discharge into an even better conductor.  

4.2.6 Flying fur and pie tins with the Van de Graff generator:

Place a piece of fur, or a stack of pie tins, upside down on the Van de Graff generator. The fur or pie tins will start to fly off the generator as the electric charge builds up on the sphere. You can also move your hand close to the fur, and watch as it moves away from your hand.

This works because like charges repel each other, and the sphere will collect the same charge as the tins and the fur. The top pie pan will hold the other ones down until it builds enough charge to repel it from the generator. The rest will then fly off, one at a time.

4.3 Energy

There are many types of energy, such as heat, electromagnetic energy and kinetic energy. One type of energy can be converted into another type of energy, but the overall energy of a system is always conserved.

4.3.1 The big sphere floats, the little one doesn’t:

This demonstration uses a large sphere and a small sphere, and a container filled with water.

Ask the group if they think the balls will sink or float. Place the little one in the water, and watch it sink. Then place the large one in the water, and watch it float.

Because the large ball has a lower density than water, it doesn’t sink, but the small ball has a higher density than water, so it sinks.

4.3.2 Two balls colliding can create heat and burn paper:

Hit two steel balls together with a piece of paper in between them. It helps to have someone else hold the piece of paper. (Just watch out for any fingers!) Hit the steel balls together a few times, and watch as you see burn spots appear in the paper, and smell the paper burning.  This shows that kinetic energy was converted to heat, the law of conservation of energy. Inelastic collisions often lose their energy to heat.

4.3.3 A Stirling engine powered by the heat of your hand:

Place the Stirling engine on a warm computer, warm water, or your hand, and watch the rotors rotate.

Here’s an explanation taken from the web:

Every Stirling engine has a sealed cylinder with one part hot and the other cold. The working gas inside the engine (which is often air, helium, or hydrogen) is moved by a mechanism from the hot side to the cold side. When the gas is on the hot side it expands and pushes up on a piston. When it moves back to the cold side it contracts.

Similar processes make the radiometer turn. In fact, every engine runs on similar processes. This is an example of heat being converted into kinetic energy.

4.3.4 A gyroscope is difficult to rotate:

Have someone hold the gyroscope (which looks like a bicycle wheel) and then spin it for them. Make sure they hold it tight, and not let anything get caught in it. Now try to rotate the gyroscope left or right and ask them how difficult it is. The faster it is spinning, the more difficult it will be to rotate left or right.

Gyroscopes are used in tops, bicycles, ships, airplanes, space shuttles, and even satellites. An effect called precession counters your force at right angles. When the force is applied to the axle, the section at the top of the gyroscope will try to move to the left, and the section at the bottom of the gyroscope will try to move to the right. If a gyroscope is not spinning, it will flop over on its side. If the gyroscope is spinning, think about what happens to these two sections of the gyroscope: Newton’s first law of motion states that a body in motion continues to move at a constant speed along a straight line unless acted upon by an unbalanced force. So the top point on the gyroscope is acted on by the force applied to the axle and begins to move toward the left. It continues trying to move leftward because of Newton’s first law of motion, but the gyro’s spinning rotates it.

4.3.5 Energy is conserved when two airpucks collide:

The airpucks are green saucers that must be charged and then turned on in order to work. There is a black switch underneath each airpuck. It’s best to place them on the floor, turn them on, and then let them collide with each other.

The colliding airpucks show conservation of energy. The energy from one is given to the other, or split between the two. Some of the energy is lost to heat and friction, but it is close to frictionless because of the air lift.

4.3.6 Friction causes a top to invert

This experiment uses some small wooden tops about 3 inches long. They are semicircular spheres with a prong pointing out of the middle end. Spin the wooden tops. If you get it right, the tops will invert so that the heavy end is on top of the small end. This almost defies logic!

The top wants to remove as much surface friction as possible, so it inverts. If you could spin it perfectly up and down, or on a frictionless surface, then it would not invert. There is less friction on the sides than on the bottom, so it wants to move toward its side. There is even less friction on the small, light end than on the sides, so the top inverts.

4.4 Waves, Sound, and Light:

Light is made up of photons, which are both massless particles, and packets of energy. Light acts like a wave and like a particle. You can shine one photon through a slit and detect that single photon. If you shine multiple photons through a slit, you will see a diffraction pattern that is a result of the wavelike nature of light.

4.4.1 Different types of waves:

Take the large spring and have someone grab the other end. Separate the two ends of the spring to different sides of the room. It will not contract fast enough to cause harm, but still make sure that it is held tight. You can show two different types of waves, longitudinal (compressed waves) and transverse.

Transverse waves are easy to show. Just move the spring up and down or left and right. To show longitudinal waves, have a helper grab part of the spring in front of you and push it toward or away from you, compressing the spring between its two ends. Then release the part that was grabbed, and you’ll see longitudinal waves, or, the compression of the spring at various points.

4.4.2 Singing rod:

Grab the 60cm long, silver colored metal rod.  Slide your fingers repeatedly along it. You can control the volume and the pitch by how tightly you grip, where you hold it, and how quickly you move your hands over it.

The explanation for the changing sound is that sound is generated by vibrating molecules. A vibrating object causes the surrounding air to vibrate. Our ears detect the vibrating air so we can hear the sound.

4.4.3 Singing bowl:

Fill the bronze bowl half way with water. Wet the palms of your hand and rub the two handles of the bronze bowl back and forth. At first the water will ripple only slightly, but then the waves will begin to stack on top of each other, and the water will jump up in streams.

Your hands cause the handles to vibrate, which in turn causes the bowl to vibrate at about 300 Hz, which creates standing waves in the water. The water spouts up for the same reason light diffraction patterns happen, because the waves reinforce each other where they overlap. This is the same principle that allows a vibrating crystal glasses to make sound.

4.4.4 Light is absorbed into black paint causing a windmill in a partial vacuum to rotate:

The windmill encased in a glass bulb is called a radiometer. It was first created by William Crookes in 1875 when he was experimenting with vacuums.  The radiometer can be used with either a visible light source, or a near-infrared source (such as the heat from your hands). When light hits the radiometer, it is absorbed by the dark parts of the radiometer and reflected by its bright parts. The dark parts heat up, and then cool down again when the radiometer is turned away from the light source.

There are two incorrect assumptions sometimes made in explaining why the vane moves: that it’s due to the pressure of light, or that the dark side releases a gas when light is present.  In reality, heat is the movement of molecules, and the movement is caused when heat from light moves from one side of the vane to the other. The radiometer will therefore not work in a full vacuum, where heat transfer can’t occur. The radiometer turns different directions when it is heating up versus when it is cooling down, as the heat is moving in different directions.

4.4.5 Light polarization:

The polarizing filter looks like a black plastic sheet, or a small square sheet framed in card- board. There are other filters available as well. Shine laser light through the filter, then watch as you rotate the filter to make the light dimmer or brighter.

The explanation for this is that light works like waves. Imagine a wave that moves up and down and a wave that moves left and right. Most light that we experience is moving in all sorts of directions. When you sort out the light that is moving just up and down (or in any one-dimensional direction,) you create what’s called polarized light.   

4.4.6 Polarized laser light is rotated in Karo syrup, showing different intensities:

Shine the laser light through the tube filled with Karo syrup. You will notice that there are different intensities of light throughout the Karo syrup. It is easier to see with the light off.

The Karo syrup acts like a polarizing filter. It also rotates the polarization of the light (the direction of the wave) as the light moves through it, showing different intensities at different spots.

4.4.7 Different colors can be seen from polarized white light in Karo syrup:

Shine a white light source, such as a flashlight, through the polarizing filter, and then through the Karo syrup. You should see different colors throughout the Karo syrup. It may help if you turn off any other lights.

White light is made up of all the different colors of the rainbow. Each color has a different wavelength (different size of wave). Each wavelength gets rotated slightly more or less frequently than other wavelengths as this experiment is carried out. When one color becomes dim, you will see other colors brighter in that spot.

4.4.8 A diffraction pattern occurs when light goes through something small:

Shine the laser light on some hair, or through a diffraction grating. If you are far enough from the wall or screen that it’s shown on, you will see a diffraction pattern – a pattern wider than the initial laser light with bright and dark spots.

Light works like a wave. Picture two water waves hitting each other. When crests from both waves hit each other, the crests will be extra large. When troughs from both waves hit each other, they will be deeper.  When a crest and a trough hit the same spot, they will cancel each other out. The diffraction grating splits the light up into tiny packets, which show light’s wavelike nature.  

4.4.9 A mirage that you can’t touch:

This demonstration uses the two black hemispheres. Place something small like a penny or a figurine inside, at the very center. The object will now look like it’s someplace that it’s not, and will still look very real. You can shine a light on it and look at it from different angles, and it will still look real.

Light rays can be bent. The most common bending of light rays is done by glasses or a magnifying glass. The light rays coming from the object at the center are bent by the glass to look like they are originating at the top instead of the center.  

5- Questions for kids:

Whether for fun, or in order for scout groups to receive a physics patch, here are a few questions we can ask.

What is Astrophysics?

Astrophysics is the branch of astronomy that deals with the physics of the universe, including the physical properties (density, temperature and chemical composition) of astronomical objects such as stars and galaxies.

What is Cosmology?

Cosmology is the study of the Universe and humanity’s place in it. In recent times, physics has come to play a central role in shaping the understanding of the Universe through scientific observation, experiments, and theorizing.

What is condensed matter physics?

Condensed matter physics is the study of the macroscopic physical properties of matter.  In particular, this deals with situations where the number of particles is large, and interactions between particles are strong.

What is nuclear physics?

Nuclear physics is the study of the nucleus of the atom. It has three main aspects: studying protons and neutrons and their interactions, classifying the properties of nuclei, and advancing technology.

What is atomic physics?

Atomic physics is the study of atoms as a system comprised of electrons and the nucleus. It is primarily concerned with the arrangement of electrons around the nucleus and the processes by which these arrangements change.

What is the difference between theoretical and experimental physics?

The difference between theoretical and experimental physics is that theoretical physics involves understanding nature by using mathematical models and experimental physics deals with observing natural phenomena through experimentation.

How cold is Liquid Nitrogen?

Nitrogen is a liquid between 63 K and 77.2 K (-326F and -320F).  Any colder and it will become a solid, and warmer than that it becomes a gas.

What is a laser?

A LASER (Light Amplification by Stimulated Emission of Radiation) is an optical source that emits photons in a coherent beam. A coherent beam means that the wavelength and frequency of the light emitted is the same for all the photons.

What is a nanometer?

A nanometer is really small. It is one-billionth of a meter.

5- Conclusion:

Thank you for reading and using this manual. We hope it makes it easier for you to plan and conduct tours and demonstrations.  Remember, have fun with your groups! This a great opportunity to get all kinds of young people interested in science, and maybe even spark a lifelong interest that you or a visitor didn’t even know was there.  

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