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Small effects in the class-room experiments. Ivan Lomachenkov

Small effects in the class-room experiments. Ivan Lomachenkov. Some physical projects have been realized at University centre of JINR. Introduction 1. The main idea: let’s contrast the ’serious’ physics (the physics of microcosm) and the ordinary physics (”the physics at the kitchen”).

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Small effects in the class-room experiments. Ivan Lomachenkov

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  1. Small effects in the class-room experiments.Ivan Lomachenkov Some physical projects have been realized at University centre of JINR.

  2. Introduction 1 • The main idea: let’s contrast the ’serious’ physics (the physics of microcosm) and the ordinary physics (”the physics at the kitchen”). • The physics of microcosm: searching very small effects (for example parity nonconservation experiments) - there is need to intensify these effects: resonance mechanism, suppression of the background, a large detectors at al. • Can we indicate the small effects in the frame of ’’ordinary” physics? Can we intensify these effects?

  3. Introduction 2 The answer is YES. Some criterions: a) the available and simple equipment; b) not complicated physical model of phenomenon; c) the opportunity to repeat phenomenon many times; d) class-room experiments in addition to basic course of physics; e) not only computer animation of the phenomenon.

  4. Physics at the kitchen Part 1. Surface tension: the intensification of the molecular forces.

  5. The 1-stexperiment: the swimming sieve.Equipment: • metallic sieve; • dynamometer; • rulers; • set of masses (loads); • vessel; • water.

  6. The surface tension forces • The surface tension is very small: F=sL, s- coefficient of the surface tension, • s=73 mN/m (water). • For L=100 m F=7.3 N – very small in comparison with gravitation. water L

  7. Set-up of experiment: the sieve as a boosterof the surface tension. m • mass of the sieve: M=170 g; • diameter of the sieve: D=14.3 cm; • mass of each load: m=35g; • mass of each ruler: m0=14 g; • dimensions of elementary cell: s0=l´l, l=1 mm m0 M M wire netting l l elementarycell D

  8. Some estimations The surface tension forces support an elementary cell: F0= 4s× l. Summary surface tension forces support a sieve: F=F0×N, N=S/s0, N – the number of the elementary cells; s0=1 mm2, S=pD2/4, S»160 cm2. N»16000 ! – the intensification factor of the surface tension forces. sl elementary cell

  9. Some estimations Equilibrium condition of the sieve: 4sl×N = G, G = (M+4m+3m0)g – the weight of all bodies, g – acceleration of gravity.G » 3.4 N. We can extract the estimation for s from this experiment: sext »53 mN/m. The precise value is s=73 mN/m. The reason of discrepancy: there is the partial wetting between water and wire netting.

  10. The 2-nd experiment: the interaction of the smooth glass plates Equipment: • two smooth glass plates; • ruler; • micrometer; • medicine dropper; • water.

  11. Set-up of experiment: Strong pressure between plates is induced by pressure fall under curved surface of water. There is almost absolute wetting between water and plates. atmospheric pressure P0 P0 d P water plate on a large scale P- pressure inside of water; P =P0 - 4s/d (Laplace’s pressure); d – thickness of water

  12. Some estimations P0=105 Pa, P=P0 – DP, DP=4s/d; d»0.02 – 0.08 mm; F=DP×S, S=0.13×0.18 m2; dmin»0.02 mm F 0.18 m 0.13 m F Fmax »336 N! d 34 kg ! We can hang up!

  13. The 3-rd experiment: the “life-time” of the soap-bubble. There’re two questions: a) can we increase the life- time of the soap-bubble?; b) what’s the main reason which restricts this time? Equipment: • transparent pellicle pipe; • hygrometer; • cylindrical vessel with water; • soap-bubble or wire ring with soap pellicle; • stop-watch.

  14. Set-up of experiment: the humidity of air – the main reason that restricts the life-time of the soap-bubble. threads j0=70%, t0»1 min transparent pipe j=80%, t»1.5min . j=85%, t»2 min L»2 m Stop-watch j=90%, t»2.5 min j=95%, t»3.5 min water soap-ring D»0.3-0.4 m hygrometer

  15. Some analysis There’re in the class-room: j0»70%, t0» 1min. In the frame of the simple model we can obtain the formula: t= t0×(1 – j0)/(1 – j), j – the humidity of air alongthe pipe. t, min 10 exp 9 8 theor 7 6 5 4 3 2 1 j, % 75 80 85 90 95 100

  16. Some discussion Let’s suppose: we’ve created the ideal conditions for the soap-bubble (there aren’t air flows and speck of dusts, j=100% at al.). Can the soap-bubble ”lives” for ever? The answer is NO. stopper cover × × × × j=100% × × × glass vessel · × × × × soap-bubble × × water drop P P0 process of diffusion P=P0 + 4s/r, r – radius of the bubble P0 - atmospheric pressure According to observations the “life-time” of soap-bubble in closed vessel may be more than 10 hours!This time drastically depends on soap solution.

  17. Some discussion There’re two main reasons why the soap-bubble can’t “live” for ever: a) the molecules of water slide down on the surface of soap-bubble and the thickness of the wall of bubble is decreasing drastically; b) the pressure inside of the soap-bubble is greater then atmospheric pressure by a factor 4s/r ( r-radius of the bubble). Therefore there’s the process of diffusion molecules of air outside of the soap-bubble (“diffusion wasting away process”).

  18. Part 2: the intensification of undulatory movement The objects of investigations are the air and water streams. There are some opportunities to intensify the oscillations of air stream inside glass tube (Rieke’s effect) and to display the structure of water stream. In addition to we can discuss the influence of sound field on the water stream.

  19. Sounding tube – the thermal autogenerator of sound Equipment: • glass tube about 80 – 100 cm; • small heater about P~100 – 200 W; • transformer for AC (voltage about 30 – 40 V); • laboratory support; • oscilloscope (not obligatory); • microphone (not obligatory).

  20. Set-up of experiment microphone air flow (draught) oscilloscope glass tube ( L» 80 cm, Æ»35 mm) heater ~220 V ~30-40 V ~127 V transformer

  21. Sounding tube – the resonance system with positive feed-back. There’s air flow through the tube forming of the standing wave inside the tube. The heater provides the positive feed-back. x Dp=0 (node of pressure) · · · · · · · · · · ·· · · Dx=0 (displacement of air) · · · · · · · · · Dpmax(antinode) Dpmin · draught draught · · · · · · · · · · · · Dx · · · Dx · · · · · · · Dx, Dp ·· · · Dp Dx stage of pressure stage of rarefaction

  22. Some results The positive feed-back extremaly depends on location of the heater. There’s an effect (sound) only in case when the heater is located in lower part of the tube. In accordance with the experiments h=L/4. Dp l=2L – the wave-length of standing wave; c – the velocity of sound in the air; L Dx f0 = c/l = c/2L – the frequency of main harmonic; h

  23. Some discussion displacement of air The directions are opposite: there’s the negative feed-back the oscillations of air will be suppressed. The directions are the same: there’s the positive feed-back the oscillations of air won’t be suppressed. draught displacement of air stage of pressure

  24. One remark In this case the effect of the sounding tube can’t be found. This experiment demonstrates that really there’s the pressure antinode in the centre of the tube.The positive feed-back is absent. small hole Dp=0 L/2

  25. The water streams Introduction: There are some questions: a) can we observe the process of disintegration of water stream? b) can we influence on this process? C) can we extract some physical quantities from these observations?

  26. Equipment: • volume about 5 litres (vessel for water); • rubber or plastic hose about 2 m, Æ=10-15 mm; • medicine dropper (nozzle); • clamp; • loupe; • stroboscope; • sound generator; • loud speaker; • support.

  27. Set-up of experiment: water water streams stroboscope . nozzle clamp support sound loud speaker generator

  28. Some discussion.It’s necessary to have a stroboscope to observe thedropping structure of water stream. There’s the capillary wave on the surface of water stream. The direction of motion of the capillary wave is opposite the water stream one. But the velocity of the capillary wave always equals the water stream one: c = v. So we can observe the capillary wave like the standing wave. The reason of the existence of the capillary waves is the surface tension. droppings structure of stream loupe v c capillary wave l stroboscope nozzle

  29. Some estimations: There’s the simple estimation for l: l>9/2×r, r »0.5 mm – radius of the nozzle. Hence l>2.25 mm. It’s easy to determine the velocity of the stream: v » 2 m/s, therefore c » 2 m/s. According to the observations the resonance frequency of the dropping process is about 300 Hz: fres » 300 Hz. Therefore we can calculate the wave-length of the capillary wave: l = c/fres, lobs » 6.6 mm.

  30. References • I. Lomachenkov. The International School of Young investigators “Dialogue”, Dubna, 1999 (in russian). • I. Lomachenkov. Quantum, №2,56 (1999). • V. Mayer. Simple experiments with streams and sound. M., 1985 (in russian).

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