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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 l.jpg

Small effects in the class-room experiments.Ivan Lomachenkov

Some physical projects have been realized at University centre of JINR.


Introduction 1 l.jpg

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?


Introduction 2 l.jpg

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.


Physics at the kitchen l.jpg

Physics at the kitchen

Part 1. Surface tension: the intensification of the molecular forces.


The 1 st experiment the swimming sie ve equipment l.jpg

The 1-stexperiment: the swimming sieve.Equipment:

  • metallic sieve;

  • dynamometer;

  • rulers;

  • set of masses (loads);

  • vessel;

  • water.


The surface tension forces l.jpg

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


Set up of experiment the sieve as a booster of the surface tension l.jpg

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


Some estimations l.jpg

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


Some estimations9 l.jpg

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.


The 2 nd experiment the interaction of the smooth glass plates l.jpg

The 2-nd experiment: the interaction of the smooth glass plates

Equipment:

  • two smooth glass plates;

  • ruler;

  • micrometer;

  • medicine dropper;

  • water.


Set up of experiment l.jpg

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


Some estimations12 l.jpg

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!


The 3 rd experiment the life time of the soap bubble l.jpg

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.


Slide14 l.jpg

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


Some analysis l.jpg

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


Some discussion l.jpg

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.


Some discussion17 l.jpg

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”).


Part 2 the intensification of l.jpg

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.


Sounding tube the thermal autogenerator of sound l.jpg

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).


Set up of experiment20 l.jpg

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


Sounding tube the resonance system with positive feed back l.jpg

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


Some results l.jpg

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


Some discussion23 l.jpg

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


One remark l.jpg

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


The water streams l.jpg

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?


Equipment l.jpg

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.


Set up of experiment27 l.jpg

Set-up of experiment:

water

water streams

stroboscope

.

nozzle

clamp

support

sound

loud speaker

generator


Slide28 l.jpg

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


Some estimations29 l.jpg

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.


References l.jpg

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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