About paper
Czech originalA Bit of Heuristics from Malá Hraštice
Malá Hraštice is a small village near Dobříš, famous for its exquisite bakery and a pastry shop - and for insiders it is also known for its tourist base, which is perfect for holding all kinds of events in "field conditions". For a few days in May this year it was revived by physics.
Since the year of 1997, a group of students of the Department of Physics Education, some of their teachers and other enthusiasts, who all create a vaguely defined Society for Renovation of Hraštice Tables, has been using the opportunity to experiment truly unboundedly.
(Note: The term Hraštice tables has already penetrated the International Conference [1]. Can you imagine what happens, when they will really exist...)
What is going on at Hraštice
This above mentioned assembly of students of physics education is a perfect way to deepen the teachers’ contact with the students, to play psychological games or games for pure fun and so on. Physics is not ignored, on the contrary, there are many attractive activities for the participants. They have the opportunity to conduct experiments using simple tools, to develop their own alternative of the experiments, to apply uncommon materials etc. The physics learned from textbooks, lectures and labs now gets a new view. And not only a view. The motto of our camps is with your own hands and mind.
The participants themselves discover a lot of things form physics, sometimes again and sometimes – considering for example the connection of “college” and “elementary” physics – maybe even for the first time. At the same time they are spontaneously getting familiar with the heuristic method of teaching. The knowledge gained by experimenting and discovering is more valuable than the one we are indoctrinated with. We believe that the students will apply this experience in their future profession.
This years’ main theme was creating simple physical instruments and equipment. The given suggestions were very general: Construct an apparatus for measuring any quantity of your choice (length, time, speed, mass, coefficient of friction etc) or any technical device that uses physical principles (telephone, power plant, balloon, submarine etc.). Ingenuity knows no limits. And it all had to be made by using primitive tools and resources; wire, nails, boards, wooden sticks. The most advanced instrument was an electric hand drill.
A medley of ideas
The built instruments and tools worth mentioning are time measuring devices (sundial, candle clock and hourglass) or dynamometers, that can be used to measure (or estimate) the viscosity of honey. It is really an unforgettable view looking at somebody towing a slice of bread by the dynamometer along a pan covered with honey. The most challenging construction was a power station built on a nearby stream. The water wheel powered the generator via a gear. The generator was formed by coils wound on a plastic cocoa bottle, in which a magnet revolved. The rubber band drive only lasted a short moment, yet the power plant was still able to light up a LED. Now let us talk about other ideas in detail.
String phone with echo
Most of us constructed a telephone made from cups and a string when we were kids. Someone maybe also created a phone which lead around the corner or a phone with more participants with the strings leading radially from the centre.
The first years, in order to have more participants, constructed the “central line”, a 30 meters long string which was strung between two trees. To this string were almost perpendicularly connected strings attached to the cups. It is seems as not a very good idea to connect the strings perpendicularly, for it obviously reduces the efficiency of the device. The sound in the string is carried as longitudinal waves. However, thanks to its layout, the structure showed a surprising effect.
The wave in the "central line" reflected at the end of the string. If you shouted in the phone loudly and shortly, you could hear 3-4 echoes!
You can find something new even on a notoriously known string phone. In addition, it offers a lot of physical phenomena to study: How fast does the sound propagate in the string? How does the wave reflect at the ends, how does it reflect at the joining of two strings? How does the wave reflect and damps at the membrane which is the bottom of the cup? How stiff should the membrane be, if we wanted to have the best efficiency of the phone? Now we would have to consider the terms as impedance, impedance matching and real electroacoustic devices. There are lot of levels on which you can “let loose”, from the most playful up to the university level.
Archimedes’ Balance
This is also an original model constructed by first-year students. Its function is obvious when we look at the picture: A plastic cocoa bottle is weighted with sand, so that it floats in a bucket filled with water. A rope is led from the bottle over a pulley. The other end of the rope is fixed to the object we want to weigh. There is a scale on the surface of the bottle (the bottle is yellow and it is easy to write on it with a marker). The index mark is the water level.
It is a simple and practical device that can be modified for weighing 10 decagrams of salami (for considerably lighter items the capillary forces would apply) up to even the teacher himself (this would require only a bigger bucket, a bottle and a stronger pulley).
It is good to remember that in this arrangement the amount of water in the bucket does not matter; neither does the shape of the bucket. Well, it is Archimedes' law in practice.
Measuring of Young’s modulus of jam
The Young’s modulus of substances is taught in high school and in the introductory physics course in college. However, it is often used to describe the elasticity of only some materials, like steel. To this apply some experiments: stretching thin wires. But what about measuring the Young’s modulus of “less traditional” materials? For example jam, that we spread on the bread or Christmas cake for breakfast.
Jam can be compressed more easily than steel. Thus we can compress a small block of jam with the sides of 1 or 2 centimeters long, and even with small forces applied we can achieve a noticeable deformation.
The apparatus for measuring the Young’s modulus of jam and similar materials can be very simple. The base is a lever of the first class. A suitable material proved to be a common hexagonal wooden pencil (it is stiff enough so that it does not bend). We drill a small hole with the diameter of 1 mm in the middle of the pencil with auger gimlet. (In our case, it was the PCB drill, probably even an ordinary drill would do the trick.) The axis of rotation leads through this hole, the axis is represented by a simple safety pin. We bend the wire of the safety pin behind the hole; we cut the clasp off (this can be easily done with regular pliers), thus we obtain a second sharp end. Using pliers, we insert both ends into a wooden board which forms a base of the apparatus. The lever is finished. We glue a thin plate, e.g. Formica plate, at the end of the pencil on the side facing the board. The plate will compress the block of jam. Moreover we need to attach two wooden skewers – the one by the Formica plate will serve as a weight holder and will compress the block of jam (we will use bolt nuts as weight); the other and longer one will be used as a pointer. And we are done.
It is easy to calculate the given deformation and the tension inside the block of jam caused by the proportions of the apparatus and the mass of the bolt nuts (we can also construct some special scales to weigh the nuts. We can also measure the Young’s modulus of elasticity of other materials found in the kitchen. At Hraštice I also tried the dumplings, sausages and marshmallow. Jam, however, has one advantage. It sticks on the smooth plate, so we can also measure its tensile elasticity.
However, it is rather difficult to check our results with the tabulated value. Our High School Handbook of Chemistry and physics does not include the Young’s modulus of such important materials like jam or dumpling. (Now you know what will be the Hraštice tables for.) For the curious ones, here are some of the results: For all the above mentioned materials, the values of the Young’s modulus of elasticity E are very similar - million times lower than the tabulated value of the modulus of elasticity of wood.
Amplifier of magnet swings detector
The following structure was constructed to make time standard, a device "ticking" in periodic intervals. Such standard can be pendulum. But what is the easiest way to register its passing the balance position?
If the pendulum is a magnet suspended on a thread, we can detect by induction when it passes the balance position. We place a coil under the balance position. The moving magnet causes varying magnetic flux, which induces voltage in the coil. If you connect some measurement device to the coil, it will deflect with every swing.
However, a sensitive pointer instrument is not what we want to use with these simple experiments. Small digital multimeters usually cannot respond to the voltage pulse, due to its short duration. We would like to “visualize” every time the magnet passes through the balance position – for example by flashing of LED.
The light-emitting diode (LED) however, requires a voltage of at least about 1.6 V. The coil though induces voltage substantially lower. Therefore we need to amplify it.
One way to do this is shown on the diagram below. The exact values of components do not matter; almost all resistors have a protective function (so that when the potentiometer slider is in the upper end, the semiconductor devices would not get destroyed by high current). We can use the cheapest small types of NPN and PNP transistors, the potentiometer with value of about 5-50 kΩ that can be bought in sale will suffice. The device can be powered from a flat battery, which is the most expensive component of this circuit
We connect the above mentioned coil between the terminals marked in the diagram; the coil has let’s say 50 turns of wire on the coil frame of about 5 x 5 cm. (For initial attempts to "revive" the amplifier we can connect these two terminals.)
If the slider of the potentiometer is at the end connected to the negative terminal of the battery, the NPN transistor is closed; therefore the current does not flow in the second transistor. Hence, it is also closed and the LED is not lit. If we slide the slider until the voltage on the base of the first potentiometer is increased to about 0.7 V, the transistor begins to "open", it lets the current to flow to the second transistor, which amplifies it and the diode shines.
For our purposes, we set the potentiometer so that the diode is just beginning to shine. Even small changes in voltage induced in the coil will be amplified and result in a significant increase in the brightness of the LED. In the opposite polarity of the induced voltage, the brightness decreases.
If the magnet swings above the coil, every time it passes the balance position, the diode flashes. A more careful look reveals that in addition there is also a point, when the brightness dims - of course, since the flux in the coil first increases and then decreases, so that the voltage of both polarities is gradually induced.
The entire device can of course also serve other purposes than just to capture swings of a magnet. With a sufficiently strong magnet it can be used to demonstrate the induction of voltage in a coil with a few turns. As already mentioned, it is also suitable in situations where the resulting voltage pulses are too short to be reliably measured by a classical measuring device.
Literature
[1] L. Dvořák: On the Road to Hrastice Tables (Non-Traditional Elements in Pre-Service Training of Physics Teachers). In: Sborník z konference Science and Technology Education in the New Millennium, Prague, June 15 to 18 2000 Ed. R. Šulcová. ISBN 80-86360-14-8. p 238-242