Abstract "UNUSUAL EXPERIMENTS FOR PHYSICS LESSONS" János Kosztyu dr., Phd, Zsigmond Móricz High School, Kisújszállás, HUNGARY The autor teaches physics in secondary grammar school in Hungary, where physics is a compulsory subject for three years. In order to arouse students interest in physics in secondary schools, help them study this subject and have success in it we make experiments in physics lessons as well. Our experiments include few measurements and the calculations and conclusions about the data are made together with the students of the entire class. In this presentation I will talk about the classroom measurements which students do in their physics lessons. As there is in Hungary in the secondary grammar schools physics is a compulsory subject for three year in the measurements take part the entire class usually of about 25 30 students of nine, ten or eleven grade. I am going to present: 1) Experiment with mechanical standing waves, measuring the wave-length and wave velocity of the mechanical waves in the thread; 2) Experiment with acoustical waves, measuring the wave-length and frequency of the sound; 3) Wave velocity measurements of the ripples induced on water surface, using the waveinterference for measuring the wavelenght and the sound resonance for tuning-fork s frenquency measurements; 4) Experiment with ultra short waves (microwaves), measuring the wave-length in air, the wave-length in pitch and the refraction coefficient of the pitch. The more we use demonstrations and experiments, the more students enjoy physics. The more interested they will be in this subject. In order to arouse their interest in physics, help them study this subject and have success in it; we make experiments in physics lessons as well. Experiments in high school physics lessons are most valuable if the measurements are tightly planned, the experiments include few measurements and the calculations and conclusions about the data are made together with the students of the entire class. In this presentation I would like to tell you about some experiments from different areas of physics with simple measurements. These experiments last only a few minutes. The teachers of secondary schools with their students may carry out these experimental measurements in physics lessons without any great effort using simple instruments, except perhaps in one case. I have chosen these experiments so you may see the possibilities. All of these experiments were developed by me as I worked with my students, and the participants of the scientific circle in our school. All of them had been performed many times during lessons either by me or by my colleagues. Our experience has always been that the students enjoyed the experimental measurements, participiated with enthusiasm and often improved these experiments and measurements with their own ideas. In addition, all the students very much enjoyed the calculations of the data that was their own measurements and conclusions from these experiments. Our results should be students who enjoy the study of physics, and realize it's great beauty.
Experiments 1. Experiment with mechanical standing waves Physical quantities to measure: Formulas: wave-length - l (m); wave velocity - u (m/s). where l - the lenght of the thread (m); n - the number of the antinodes; f - the bell's frequency (Hz). l = 2 l / n (1); u = l f (2); Instruments: An electrical bell; thread of 3 m length one end of which we bind to the bell's hammer and the other one we bind to the tripod; a measuring tape; a tripod at a distance of about 2,5 m from the bell, and 2-3 paper-clips. l l/2 Fig. 1. Standing waves with 2 antinodes.
Measurements: When we ring the bell we can see standing waves which form one or more antinodes on the thread. The number of antinodes are changed by varying the tension on the thread, by changing the number of paper-clips we hang on it. We can calculate the wavelength of the waves which appear on the thread by measuring the length l of thread (distance between the bell and the tripod) Form. (1): l 1 for n = 1; l 2 for n = 2; l 3 for n = 3 etc. and the velocities of the waves (for f = 100 Hz ) form. (2): u 1 for l 1 ; u 2 for l 2 ; u 3 for l 3 ; etc.
2. Experiments with acoustical waves Physical quantities to measure: wave-length - l (m); frequency - f (Hz). Formulas: see above - (1) and (2). Requirements: 3-4 pieces of cylindrical stemmed glass vessels each 30-40 cm in height; a 1l glass vessel from which water is easily poured in a thin jet. Experiment: At the begining of our experiment we pour the water into one of the stemmed glass vessels. Thus permitting us to hear sounds of higher and higher frequency. Then we increase the volume of water in the stemmed glass vessels. We begin to blow air over the vessels one after the other. Sounds of different frequency are produced. Measurements and calculations: The level of the air in the stemmed glass vessel contacts the water causing a shuttered whistle effect. A shuttered whistle pitch-note of a standing wave has a half-node form (h = l / 4, see fig.2.) Using Form. (1) and (2) for the wavelength and the frequency we will have the h 1 following expressions: h 2 l = 4 h (3) and f = u / l (4). We have to measure the level of the air for all stemmed glass vessels and calculate the wavelengths and the frequencies. Fig. 2. Shuttered whistle effect in two stemmed glass vessels. Explanation for the students: When we are blowing over the vessels we make the air vibrate, and as it has different thicknesses the originated sound has various wavelenghts and frequencies (tone). Pouring of a thin jet water into the stemmed vessel and permanently changes the height of the air column as it becomes less and less, the originated sound becomes higher and higher in frequency.
3. Wave velocity measurement of the ripples induced on water surface. Physical quantities to measure: wave velocity - u (m/s); wave-length - l (m); frequency - f (Hz). Formulas: see above Form. (1) and (2). Instruments and requirements: A large, shallow, rectangular glass vessel with water; overhead projector and a screen; tuningfork; ruler; a cylindrical stemmed glass vessel about 30 cm in height and a glass pipe nearly the same length as the stemmed glass vessel but with a smaller base diameter. Experiment: Firstly, we show how fast the wave lines move on the water's surface by projecting it on the screen. Then we induce circular waves (ripples) on the water's surface by using a tuning-fork. In the screen between the tuning-fork's stems we see dark and light strips pointed rotating. This is due to the interference of the waves induced by the tuning-fork's pointed stems.
Measurements and calculations: In order to measure the wavelength l we must to measure the distance d between the stems of the tuning-fork, then count the number N of the dark or light strips: l = 2 d / N (5). Fig.3. Dark and light strips between the pointed stems of the tuning-fork in the screen. The tuning-fork's frequency f we measure by using the sound resonance. For this we use a the stemmed glass vessel with water in it and the glass pipe. We hold the tuning-fork above stemmed glass vessel which has the pipe in it. Fig.4. The sound resonance measurements. We strike the tuning-fork to produce sound. We slowly move the pipe up and down changing it's position in he stemmed glass vessel in order to observe the sound resonance. Then we have only to measure the air level in the case of sound resonance and to calculate the frequency f = u / l (4), using the value 340 m/s for the sound velocity in the air. The circular wave velocity u we calculate: u = l f (6).
4. Experiments with ultra short waves (microwaves) Physical quantities to measure: wave-length in air - l o (m); frequency - f (Hz); angle of incidence - a; angle of refraction - b; wave-travelling velocity - u (m/s); refraction coefficient of the pitch for microwaves - n 21 ; wave-length in pitch - l (m). Instruments and requirements: A transmitting set of ultra short waves, 10 GHz frequency, a receiver, a prism made of pitch, an iron sheet approximately 2 mm thick and with a surface of 30x30cm 2, a ruler and a protractor.
Experiment: Firstly, we place the transmitter and the receiver at a distance of approximately 1,5 m from one an other. We set the aerials so the midle-point is equal to the width of the iron sheet which we must move between the transmitter and the receiver. As the iron sheet slowly moves between the transmitter Fig.5. Measurements of the refraction and receiver it cuts down the lower half of coefficient n 21 of the pitch for microwaves the antinodes. We shall hear pulsating sounds on the loud-speaker. When we place the prism made of pitch in front of the transmitter the electromagnetic waves fall into the prism perpendicularly. Then we must change the position and direction of the receiver to hear the sounds of the loud-speaker again. Measurements and calculations: The students are to listen to the loud-speaker's pulsating sounds and count the number N of pulsations (antinodes). When the iron sheet is moved a distance from the transmitter to the receiver for example d = 30 cm, we may calculate the wavelength l o of the microwaves: l o = 2 d / N (7). The frequency f of the transmitter may be calculated formula (4) and the microwaves velocity value c = 3.10 8 m/s: f = c / l o (8). The students must also measure the angle of incidence (a) when the wave goes out from the prism into the air as well as the angle of refraction (b). The wave-length l, the velocity of the traveling wave u in the pitch and the refraction coefficient n 21 of the pitch for microwaves can be calculated by the equations: l = c / f (9); u = c Sina / Sinb (10); and n 21 = Sinb / Sina (11). In conclusions, I hope that the experiments about which we have spoken will make you consider increasing such experiments in your physics lessons also.