# Five Demonstrations for the First Day of Class!

On the first day of school, it is a good idea to show a demonstration that gets students thinking and sets the stage for what the class is going to look like over the course of the year. In this video, I'll tell you about five demonstrations that you might want to show to your students on the first day of class. The idea is to give a preview of what is coming in the school year. Some of these demos are puzzles. The idea is that the students have to guess not only the outcome but develop a model that explains the phenomena. The topics covered are waves, thermodynamics, electromagnetism, and the nature of science. This video should be helpful for anyone who teaches any physical science. Although I personally am a physics teacher, I would do some of these demos with my 8th-grade physical science class.

Salt vs. Fresh Water Melting

Which will melt ice faster, salt water or fresh water? Ice will be placed into cups of water, as often is done, and begin to melt. That the water will not be stirred is an important consideration. The result is surprising both for the outcome and for the mechanism. The result is that the freshwater melts the ice faster. As the ice is melted it becomes cold fresh water. Cold fresh water has a higher density than regular water and so the cold water sinks – but this will not happen in the salt water. Salt water has a higher density than cold fresh water and that causes the cold water to stay on top. This acts as a refrigerator for the ice, only being bathed in cold water and prevents further melting. I owe this demo to Bernard Cleyet a fellow Physics technician from UC Santa Cruz, although the food dye illustration is an original contribution.

The set-up cups of water-one fresh and one salt-will result in a surprising conclusion.

Make it visible by dribbling the ice with blue food dye, or any color really. it appears that the cold water does not leave the ice in the case of salt water.

Two Candles

Find two candles, a tall one and a short one. Although a candle on a stand would work fine. Light them both and place them in a glass jar. Now, they are both going to go out eventually…but which one will go out first. And why?

The set-up for the two-candles demonstration. A great puzzle for the first day of class.

CO2 fills the top of the jar first since it is a hot gas and has a very low density compared to the surrounding oxygen and nitrogen in the normal air, thus smothering the taller candle first.

An incorrect explanation might be that CO2 is denser than ordinary air, which is true, and that would cause the lower candle to go out first. But the flame is also hot and that causes the CO2 to rise. This is the dominant effect. It causes a buildup of CO2 which does not allow the candle to get oxygen and the top candle snuffs out.

This demonstration speaks to the nature of science, we have multiple variables happening at the same time and we cannot use a single fact to explain a phenomenon. It is necessary to parse out the variables and decide which one is making the main effect. I owe this demonstration to my own physics teacher, Scott Dukes.

It is actually possible to cause the lower candle to go out first. If there is a very large chamber like an aquarium, then the CO2 has time to cool down and it will sink. This will cause the lower candle to go out first. It is hard to be sure this is what we are seeing, however. It might be due to a convection effect (swirling).

The Magic of Electricity

In this demonstration, we show that electricity can communicate over vast distances causing us to power objects that are far away. A magnet on a spring is dipped into an Air Core Solenoid and a similar coil is set up on the other side of a table. When a magnet is dipped into one of the coils it generates a current which is supposed to have an effect in the other coil, only that doesn't happen! You have to connect the wire. When the wire is connected, the electric current generated in the first coil can travel to the second coil and enable its motion at a similar frequency. You will notice in the video that I am moving my magnet at close to the normal frequency of the bouncing spring. These wires that connect the two magnets can be as long as you like and they will still communicate the effect.

The bouncing magnet demo is easily illustrated with two Air Core Solenoid coils. The most important part is to not connect the wire. This highlights that it is not the shaking of the table that is causing the effect and also that there really is electricity traveling through the wires. For the sake of the camera, I kept these wires closer than I normally would in class.

This demonstration provides a tantalizing preview of all the things that electricity can do. It is the first step toward understanding that having a masterful control over nature is not done through longer levers or bigger hammers, but through the control of electrons, whose motion is easily amplified. Although Michael Faraday did a similar demonstration (he only had a compass on the receiving coil), I owe this demonstration to Bill Layton of UCLA.

When I think of all the amazing things that electricity has empowered us to do, I think of how it is the modern version of magic made real, although we do not always find it impressive anymore. We have light at night, the ability to see through people without harming them, and self-applied electrical messages. But, to belabor the point, perhaps the ability to see each other over any distance at any time is the greatest accomplishment. Facetime is probably the most magical achievement of electricity. Even more so than the laser light show.

The demonstration can also be performed with a hand-cranked generator. Which I show in the video. The generator does a much better job of generating electricity than the "dip the magnet" approach. In the generator is a small DC motor which is cranked backward to produce electricity. If electricity is sent into the generator it will act like a motor and the crank will turn. In the video, you will again notice that I use the generator with the same frequency as the bouncing magnet.

You can read more about Electromagnetism by checking our "Three Right-Hand Rules of Electromagnetism" blog.

Hot and Cold Mixing Cups

In this demonstration, I say how even though it is a very simple demonstration, it gives us a common language that we can share as a class. By this I mean, we need a window into the definition of temperature. When we say something is hot or cold, what do we mean?

To perform this demonstration, take some cold and hot water in equal amounts and add a drop of food dye to each. In my case, I used blue and red food dye for dramatic effect. However, two dyes of the same type will work fine. What difference do we expect to see between the hot cup and the cold cup?

Two cups, one hot and one cold will show very different results when food dye is added.

The result is that the hot water mixes the dye much more quickly. Hot water is hot because its water is moving around faster. While this does demonstrate the point very well, there is a bit of a cheat in the explanation. What we are really witnessing is convection: the movement of a hot substance. However, we still need energy to cause that movement and that is powered by the molecular motion of the hot substance.

In the common language, temperature is a measure of molecular motion. This gives us an experiment to call back to as a class, one that we can all relate to, one that we can use to help us imagine molecules moving about in future lessons.

In a deleted scene, I take the temperature of each of the cups. It is important to use temperatures below 65 degrees Celsius. Above this, the water becomes scalding hot. I recommend 60 and below. The cold water is easily made from pouring off ice water. Usually, you get between 1 and 4 degrees. But what would happen if I mixed water at 2 degrees with water at 60 degrees in equal amounts? The result is an average. 31 degrees is half of 2+60 degrees, and that is very close to the result. Some chemists out there will want me to work in Kelvin, but the result is the same through the averaging process whichever scale you use.

Pendulum Wave

A very exciting demonstration is the Pendulum Wave apparatus. Challenge your students to explain how it works. Different length pendula have different periods, this causes a traveling phase change which gives the appearance of a sine curve. The different phases result in different apparent wavelengths as the pendula go in and out of phase.

The pendulum wave apparatus works by having several pendula of different lengths. Each of these will vibrate with a slightly different period than its neighbor. As the pendula swing, they seem to create sine curves of different lengths. This effect grows and shrinks over time because of the phase relation between each pendulum. The result is that we get the appearance of a wave from oscillatory behavior. The meaning behind this demonstration is that waves and simple harmonic motion (such as pendula) are closely related phenomena.

The pendulum wave apparatus does not really show wave behavior but a phase relationship between neighboring pendula. This is a good way to have students visualizing what waves look like however and a very fun science toy to investigate.

At the end of the video, I perform the "Snake Pendulum" demonstration. This is achieved by giving a much larger amplitude to the far end of the Pendulum Wave Apparatus than the close end. However, since period does not depend on length in a pendulum, we get to see all the same effects as before. Different here is that the size of the swings is not all the same. This illustrates that the effect is not amplitude dependent.

The snake pendulum effect. The same wave patterns result but with different amplitudes. This effect is made possible by the fact that the pendula do not change their periods of oscillation with time.

BONUS: Hand Boiler

In the video, I also demonstrate using the Hand Boiler. The instructions on the box are incorrect (explaining about Charles' Law). The correct explanation is that the phase change from liquid to gas causes an enormous expansion. In a liquid, the molecules of the substance are touching, but in a gas, they are separated by a vast empty space. The phase change is made possible by the very low pressure inside of the glass allowing for rapid/easy boiling with just a little input heat (liquids boil more easily at low pressures). As the liquid is evaporated by the heat of a person's hand, it changes into the gas phase, causing an expansion into the chamber which pushes the rest of the liquid out and up the neck. There is not much boiling going on, mostly evaporating. The appearance of boiling is more about air seeping in and jumping up the glass neck. As shown in the video, there are several colors and shapes of hand boilers, however, they all use the same liquid. The gas phase is clear, the dye is left behind in the remaining liquid.

The hand boiler can demonstrate a simple heat engine, but what it does not do is demonstrate Charles’ Law. Rather it is an illustration of phase changes causing volume changes. The added volume of the gas increases the pressure in the base and pushes the rest of the fluid up the stem.

The hand boiler can be controlled with warm water and not just the heat from a hand. Do not, however, use hot water which might cause it to crack.

The Hand Boiler can also be used to demonstrate distillation. Since the dissolved dye will not vaporize, a clever experimenter can manage to warm one end of the hand boiler and cool the other. This will collect clean clear liquid on one side while concentrating the dye on the other. The liquid inside is not water, but ethyl alcohol, which is the same type of alcohol that we drink. I do not recommend drinking this alcohol however, it is likely to be taken from petroleum rather than plants. They also tend to "denature" the alcohol by adding a bad flavor, best to avoid this. Sometimes these hand boilers have been used as a love meter, or a "pulse glass," meaning that it checks that you are alive or at least that you have warm hands. I am not sure how having warm hands is a measure of love, but it certainly can help identify blood flow and blood pressure. Supposedly this device was used by Benjamin Franklin and may have been of his own design.

Do not forget to recognize that the process by which this works is similar to how the drinking bird works. But do NOT use the explanation on the box. Charles Law (a gas' volume increases linearly with temperature) has almost nothing to do with the effect. In fact, it is almost a non-effect when you consider that we are using the kelvin scale. For example, imagine we start at 24 celsius then increase to 30 celsius from our hand's heat (human body is 37C). That is only a change of 6 degrees out of 300 degrees on the kelvin scale. This amounts to a 2 percent volume change!! That is not enough to fill up that glass bubble. Therefore, we are mathematically demonstrating the failure of Charles' Law to explain the hand boiler's behavior and we correctly recognize phase changes causing volume changes as the main factor. Just like the two candles demo, the hand boiler can illustrate that the wrong theory can be used to explain the right effect. This is best avoided, please put a sticker or paint over the explanation on the box.

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August 10, 2018 James Lincoln