Small effects in the class-room
experiments.
Ivan Lomachenkov
Some physical projects have been realized
at University centre of JINR.
Introduction 1
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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
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) classroom experiments in addition to basic
course of physics; e) not only computer
animation of the phenomenon.
Physics at the kitchen
Part 1. Surface tension: the intensification of
the molecular forces.
The 1-st experiment: the swimming sieve.
Equipment:
metallic sieve;
 dynamometer;
 rulers;
 set of masses (loads);
 vessel;
 water.
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The surface tension forces
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The surface tension is
very small: F=sL, scoefficient 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.
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m
mass of the sieve:
M=170 g;
diameter of the sieve:
D=14.3 cm;
wire netting
mass of each load:
m=35 g;
mass of each ruler:
m0=14 g;
dimensions of elementary
cell: s0=ll, l=1 mm
m0
M
M
l
l
elementary cell
D
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 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
Equipment:
 two smooth glass plates;
 ruler;
 micrometer;
 medicine dropper;
 water.
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
d P0
P
water
plate
on a large scale
P- pressure inside of water;
P =P0 - 4s/d (Laplace’s pressure);
d – thickness of water
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
Fmax  336 N!
F
0.13 m
F
d
We can hang up!
34 kg !
The 3-rd experiment: the “life-time” of the
soap-bubble.
There’re two questions: a) can we increase the lifetime 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.
Set-up of experiment: the humidity of air – the main
reason that restricts the life-time of the soap-bubble.
transparent pipe
threads
j0=70%, t01min
j=80%,t1.5 min
L2m
.
j=85%, t2 min
Stop-watch j=90%, t2.5min
j=95%, t3.5min
water
D0.3-0.4m
soap-ring
hygrometer
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 along the pipe.
t, min
10
exp
theor
9
8
7
6
5
4
3
2
1
75
80
85
90
95 100
j, %
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.
P
P=P0 + 4s/r, r – radius of the bubble
P0 - atmospheric pressure
 
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 
stopper
cover
j=100%
glass vessel

 soap-bubble
  


water
drop
P0
process of diffusion
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 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
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
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).
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Set-up of experiment
microphone
oscilloscope
glass tube ( L 80 cm, 35 mm)
heater
~220V
~127 V
~30-40 V
transformer
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)
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Dx=0 (displacement of air)
Dpmax(antinode)
Dx
Dx, Dp
stage of pressure
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Dpmin
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Dx
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draught
draught
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Dp
Dx
stage of rarefaction
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.
Dp
L
Dx
h
In accordance with the experiments h=L/4.
l=2L – the wave-length of standing wave;
c – the velocity of sound in the air;
f0 = c/l = c/2L – the frequency of main
harmonic;
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.
displacement of air
stage of pressure
One remark
Dp=0
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
small hole the tube.The positive feed-back
is absent.
L/2
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:
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volume about 5 litres (vessel for water);
rubber or plastic hose about 2 m, =10-15 mm;
medicine dropper (nozzle);
clamp;
loupe;
stroboscope;
sound generator;
loud speaker;
support.
Set-up of experiment:
water
water streams
nozzle
stroboscope
.
clamp
support
sound
generator
loud speaker
Some discussion.
It’s necessary to have a stroboscope to observe the
dropping 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
droppings structure of stream
is the surface tension.
v
capillary wave
nozzle
stroboscope
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z. Therefore we
can calculate the wave-length of the capillary wave: l=c/fres,
lobs  6.6 mm.
References
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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).