Resonance and Simple Harmonic Motion

This experiments is a simple way to demonstrate the principles of resonance and simple harmonic motion.

This diagram show four identical springs attached to a support. Each spring has weight with a different mass attached to the end. If a weight is pulled and released, it will set it in motion bouncing up and down until all its energy is dissipated through friction. As each spring holds a different mass, they will each have a different fundamental or resonant frequency. The first spring with the most mass will bounce slowest. We say that this has the lowest resonant frequency out of the four springs. The frequency is simply the number of times the mass bounces up and down each second and is measured in Hertz (Hz).

If the support holding the springs is moved up and down at a set speed, we will see the weights begin to move up and down buy different amounts. The amount of displacement of each mass will be dependant upon the frequency that the support is oscillating at.

We will see the maximum displacement when the frequency of the moving support is the same as the resonant frequency of one of the weights. For example; If the resonant frequency of the first mass and spring is 10Hz then it will move the most when the support is moved up and down at 10Hz. This is known as resonance. If the support is moving at 9Hz or 11Hz then there will be significantly less movement of the mass. This is because resonance usually occurs when the frequencies match almost exactly.

The exact same principles are used in radios and televisions in order to tune into a specific station. Instead of oscillating springs, it is the electrical currents that are oscillating. By using different components it is possible to make a circuit that has a resonant frequency that is the same as the radio wave that we wish to detect. At resonance the amplitude of the signal in the circuit will be much higher than any of the other signals which allow us to use just one channel at a time.

Thermal Updrafts

Thermal updrafts are a natural phenomenon produced by localised heating of air. This causes air to rise and fall in relatively small localised areas.

We can make a simple experiment to demonstrate this effect. The diagram on the left shows how the top half of a bottle can be used to trap heat from the sunlight. As the air warms up inside it rises and exits the top of the bottle. If a simple propeller is places above, it will turn due to the air passing through it. The bottle must be placed on some small blocks so that the air can flow in at the bottom.

The propeller can be made by cutting out a shape like shown from some stiff card. If the blades are all twisted the same way this will spin when air passes through it. It can be easily mounted by attaching a stiff wire to the bottle neck and inserting it into a hole in the propeller.

Water Vortex

There are several ways to demonstrate the motion of a vortex. The most simple method would be the popular vortex in a bottle type. This can be made by sticking two plastic bottles end to end by their lids. With a hole through the adjoining lids water can be made to pass from one bottle to the other, under the influence of gravity. A simple swirl of the bottle is enough to make the water spin and accelerate to form a vortex.

Vortex Cooling

A vortex tube is a method commonly used for cooling. By forcing compressed air into the device vortices will form which allow hot air to exit from one end whilst the cold dense air exits the other end.

Info from Wikipedia
The vortex tube, also known as the Ranque-Hilsch vortex tube, is a heat pump with no moving parts. Pressurized gas is injected into a specially designed chamber. The chamber’s internal shape, combined with the pressure, accelerates the gas to a high rate of rotation (over 1,000,000 rpm). The gas is split into two streams, one giving kinetic energy to the other, and resulting in separate flows of hot and cold gases.

The vortex tube was invented in 1930 by French physicist Georges J. Ranque. German physicist Rudolf Hilsch improved the design and published a widely read paper in 1945 on the device, which he called a Wirbelröhre (literally, vortex tube).

Vortex tubes have lower efficiency than traditional air conditioning equipment. They are commonly used for inexpensive spot cooling, when compressed air is available. Commercial models are designed for industrial applications to produce a temperature drop of about 80 °F (45 °C).

Another application is for uranium enrichment. South Africa used vortex tubes in their Helikon vortex separation process.

Dave Williams, of Engineers Without Borders, has proposed using vortex tubes to make ice in third-world countries. Although the technique is inefficient, Williams hopes it could yield helpful results in areas where using electricity to create ice is really not an option.

Plasma Vortex

A plasma vortex can be made through the interaction of electric and magnetic fields. A simple setup in a homemade vacuum chamber allows a plasma column to form within the magnetic field of a pair of rare earth magnets. The plasma column will rotate around the magnets in a direction which depends upon the polarity of the fields.

Exploding Water

This experiment demonstrates how a substance can change from a liquid to a gas in an instant! A cup of hot water will immediately explode when a small amount of a very cold alcohol based substance is added. Great care must be taken when performing this experiment, and the cold liquid should be added remotely or from behind a safety screen.

As soon as the liquid is added the entire cup of water will explode launching hot water and probably the cup into the air, so it is important to wear safety goggles and to be a good distance away when it happens.

The liquid is an alcohol based aerosol called ‘Tetrafluorethane Dimethyl ether’. The ‘Dimethyl ether‘ part is an organic compound used as propellant in other aerosols and is used with propane for cryogenic freezing.

This substance is found in aerosol cans in DIY or electronics shops but its expensive at around £10. It is commonly used for rapidly cooling electronic parts, removing chewing gum, or for freezing water in pipes that need to be cut. In the shops it can usually be found under the name “Freezer Spray”.

How it’s Done

The freezer spray is designed to evaporate rapidly as it is sprayed onto a surface as this draws heat away from it very quickly. For this experiment the freezer spray must be collected in liquid form before it can be used.

This is done by holding the nozzle in the plastic lid or similar container like shown on the left. The button is then pressed lightly so that the spray starts to come out slowly and collect in the lid. The first bit of liquid will probably evaporate quickly, but as the lid cools down it becomes easier to collect. The amount to collect will depend upon how big the cup of water is and how long it takes to go from collection to adding it to the water.

Once the liquid is collected it is tipped into a cup of hot water at a distance. It is important that the collected freezer fluid is tipped in at once and not dripped in. If it is added too slowly it will just evaporate before any violent reaction can occur.

Sometimes only fizzing and sputtering occurs, but when done correctly the reaction is immediate and quite powerful.

Important Safety Notes

– The water needs to be hot but not boiling or hot enough to scald.
– The cup holding the hot water should be a light weight plastic type like those from a vending machine.
– The freezer spray should be added to the water away from people, animals, or anything else that would be damaged by hot water.
– The spray should be used outdoors in a well ventilated area away from sources of ignition.
– Safety goggles, gloves and other protective gear are recommended to avoid injury from spilling or

Kelvin’s Thunderstorm

This is a device is a simple method for generating a high voltage charge or static electricity. It works by dripping water through an arrangement of metal containers and loops of wire.

Water, like many molecules is in the form of a dipole. This means that it has a difference in electric charge from one end to the other. The chemical formula for water is H₂O, meaning there are two Hydrogen atoms and one Oxygen atom in each molecule.

You can see from the image on the right the way that the H2+ is more on one side therefore making an uneven distribution of charge i.e. a dipole. This is what gives water all its interesting properties such as its powerful solvent abilities and its ability to stick to other materials.

This experiment works best with pure or deionised water as it is a very good electrical insulator. Tap water contains dissolved ions which become mobile allowing the water to conduct electricity. 

As the streams of water droplets pass through the rings, charges are induced making the drips and ring oppositely charged. If a water droplet is made more negative it will help to make the drops on the other side more positive because of the cross connection between the collector cans and the metal loops. This process continues to cause the voltage to rise until the water drops start being deflected by the strong fields or a spark is produced.

The top containers can be made from plastic but it may work better if they are made from metal and connected to ground.

Static Electricity Motor

A CD with one side covered in foil can be made to spin at high speed by using static electricity, or high voltage DC. With the CD on a spindle with the foil side up, areas under the CD can be charged by nearby electrodes. If the electrodes are placed either side of the CD and just underneath it, charge will be deposited on the plastic surface of the disk.

The plastic disc is non conductive, so in order for the charge to get to the other electrode (of opposite polarity), the disk its self must turn. This physical transportation of charge still constitutes a current flow, even though the current does not flow within the material itself. The flowing current is directly proportional to the speed of the rotating disc.