Diffraction and Interference

Diffraction is the phenomenon where waves can be bent around obstacles. When coherent light passes through a fine slit some of the rays are diffracted. The varying levels of diffraction cause part of the beam to interfere, thus producing an interference pattern.

Light from most sources is incoherent. This means that the many waves coming from the source do not line up with each other, or are not in phase. Common sources of light such as the sun or a light bulb emit photons at random intervals, but as there are so many overall we see it as just a constant source.

Lasers provide a source of coherent light due to the way that they work. All the wavefront’s emitted from a laser line up with each other. We can use this source of coherent light to demonstrate interference from diffraction in by a single slit.

If the beam from a laser is shone through a fine slit, such as that between the edges of two razor blades, we can easily see how the waves are diffracted and produce interference. If the light from the slit is projected onto a paper screen we can observe and measure the patterns produced.

This phenomenon is related to Huygen’s Principle. This says that every point on a wave front acts as a source of tiny wavelets that move forward with the same speed as the wave. The wave front at a later instant is the surface that is tangent to the wavelets.

Refraction and the Spectrum

When light enters a denser medium is effectively slowed down. This is due to the light being repeatedly absorbed and re-emited by the atoms in the material. This slowing of the wavefront’s causes the beam to be bent at an angle dependent on the material and the wavelength (colour) of the light.

This image shows how white light is split into its component parts using a prism.

Homemade Solar Panels

This shows a simple demonstration of how a homemade solar panel can be used to show the photoelectric effect making electricity from light. Making a solar panel this way would be no where near good enough to generate useful free energy for powering a home, but it great for demonstrating scientific principles.

One sheet of copper is given a coating of cupric oxide as this semiconductor will convert sunlight into electricity. To do this a copper sheet is heated over a hob for around 30 minuets. When the sheet has turned black it is allowed to cool. As it cools down the black copper oxide will start to crack up and come off. this is because the materials underneath are contracting at a different rate. it is usually necessary to finish removing some of the black copper oxide by hand.

The remaining layer is covered with cupric oxide which is the important material for producing the photoelectric effect.

If the copper sheets are placed in a strong salt solution in the arrangement shown above, a small voltage will appear between the two sheets. The salt water is really just meant as an electrical conductor, but it also has the effect of making the device operate like a battery. this means it still generates small amounts of electricity even in the dark.

Free Energy from the Environment

This is a simple setup that uses the thermoelectric / seebeck effect for converting heat directly into electricity with no moving parts. There are devices available called Peltier Heat Pumps which are used to keep electronic components cool.

They are commonly used on processors as they can move heat away from a source under electrical power. When connected to a DC power supply a peltier element will heat up on one side whilst becoming cold on the opposite side. A large heatsink is necessary to dissipate the excess heat so that the other side can remain cold.

These peltier elements can also work in reverse. If it is heated on one side whilst being cooled on the other side a voltage will appear across the terminals. As long as there is a temperature difference between the two sides of the peltier device, then there will be a voltage between the electrical terminals.

Peltier heat pumps are not a very efficient method of generating electricity, but at least the supply of energy is free.

In a practical application a large heatsink could be buried in the ground to remove heat. If the peltier element is placed onto this is can be arranged to be heated on one side by the sun. A small black heatsink on the top of the peltier device could be used to collect heat from the sun that has been focused by a large mirror. Another method might be to use a larger heatsink and to place it inside a greenhouse. With this method it would be important to insulate between the hot and cold heatsinks so that the ground would not be heated directly.

Liquid Metal Experiments

Most metals are a tough solid material at room temperature, but some metals can be liquids. The most obvious one is the element Mercury, which has a melting temperature of -38.83 °C. When certain metals are mixed together they can form what is known as an alloy. The atoms of the various metals will bond together to form the new metal which may have unique properties. Below you can learn how an alloy can be made at home which will melt in hot water. The alloys melting temperature is less than that of any of the original metals.

Using three common metals you can make a metal alloy which will melt in hot water. The metals are Bismuth, Lead, and Tin. Bismuth can be found in the form of ‘lead free’ fishing weights, and the other metals can be found together in the form of solder. Most solder is a mix of 60% Tin and 40% Lead, but for this experiment you will need the type that is 40% tin and 60% Lead.

A eutectic alloy is one where the ratio of the materials is made to give the lowest melting temperature possible. The eutectic alloy of Bismuth, Lead, and Tin would have the following ratios. 52.53% Bismuth, 32.55% Lead, and 14.92% Tin, by weight. The resulting alloy from this mixture can be expressed with the formula, Bi₈Pb₅Sn₄. When mixed in the correct proportions this metal will melt at 95°C

Another liquid metal can be made that will melt at -20°C. This metal is made from gallium, indium, and tin.

As you can see from this image, This type of metal will easily stick to other materials. This is why mercury is still used in many devices.

Hydrogen Cannon

This experiment demonstrates the splitting of water into Hydrogen and Oxygen by electrolysis, and the re combining of these elements in an energetic reaction.

You can never recover the full energy from the Hydrogen fuel due to the lack of efficiency in the process used to make the hydrogen gas. During the electrolysis a current is flowing in the water to seperate the Hydrogen from the Oxygen, it is this current that produces wasted energy in the form of heat.

A plastic tube with one closed end has two electrodes inserted in the base so that they can be connected to wires on the outside. The electrodes should be made from carbon, but any conductor such as thick wire will do for this experiment. The space between the electrodes should be as small as possible but without them touching each other.

When the device is complete the tube should be filled with water until it just covers the electrodes. When the switch is pressed, the battery is connected to the electrodes and the flowing current begins to split the water into Hydrogen and Oxygen.

At some distance above the electrodes there are two more wires that are used for creating a spark to detonate the Hydrogen and Oxygen gas inside. The Piezo Sparker comes from a cheap lighter with click type ignition. When this is pressed it creates a high voltage that is used to create a spark. The two wires inside should be separated so that the spark will jump between them, and should be far enough from the water so that they don’t get drenched when the water is bubbling.

The projectile should fit snugly inside the end of the tube so that it doesn’t fall down to the water. It is important that this object is able to move freely enough to to leave the tube without a lot of resistance or else the whole thing could explode. The best type of projectile is a tube like the main one but slightly smaller so that it fits inside. If it is inserted so that the open end is down inside the larger tube the Hydrogen/Oxygen gas can build up behind it. As the tube is forced out when the gas is detonated it is forced to move in a straight line and can be more accurate than other shaped projectiles.

To fire the cannon the switch should be pressed for around 20 seconds depending on the electrodes and battery used. When enough gas has built up inside, the piezo sparker can be pressed detonating the H2 and O2 gas mixture. The gas expands rapidly forcing out the projectile and the product of the energetic reaction is just water again. The energy released in the explosion is almost the same as the energy supplied by the battery but it is released in a fraction of a second.

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.