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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”).
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) class-room 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.
The surface tension forces
The surface tension is very small: F=L, - coefficient of the surface tension,
=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 boosterof the surface tension.
mass of the sieve: M=170 g;
diameter of the sieve: D=14.3 cm;
mass of each load: m=35 g;
mass of each ruler: m0=14 g;
dimensions of elementary cell: s0=ll, l=1 mm
M
mm0
D
elementary cell
l
l
M
wire netting
Some estimations
The surface tension forces support an elementary cell: F0= 4l. Summary surface tension forces support a sieve: F=F0N, N=S/s0, N – the number of the elementary cells;
s0=1 mm2, S=D2/4, S160 cm2. N16000 ! – the intensification
factor of the surface tension forces.
l
elementary cell
Some estimations
Equilibrium condition of the sieve:
4lN = 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 from this
experiment: ext 53 mN/m.
The precise value is =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.
on a large scale
water
plate
atmospheric pressure
P0 P0
P
P- pressure inside of water;PP0 - 4d
d
(Laplace’s pressure);
d – thickness of water
Some estimations
P0=105 Pa, P=P0 – P,
P=4/d;
d0.02 – 0.08 mm;
F=PS, S=0.13m2;
dmin0.02 mm Fmax 336 N!
F
0.13 m
0.18
m
34 kg !
F
We can hang up!
d
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.
Set-up of experiment: the humidity of air – the main reason that restricts the life-time of the soap-bubble.
water
hygrometer
soap-ring
Lm
threads
tran
spar
ent p
ipe
70%, t0min
tmin
=85%, t2 min
=90%, tmin
=95%, tmin
Dm
.Stop-watch
Some analysis
There’re in the class-room: 70%, t0 1min.
In the frame of the simple model we can obtain the formula:
t= t0(1 – 0)/(1 – ), – the humidity of air along the pipe.t, min
, %75 80 85 90 95 100
123456789
10exptheor
Some discussion
Let’s suppose: we’ve created the ideal
conditions for the soap-bubble (there
aren’t air flows and speck of dusts,
=100% at al.). Can the soap-bubble
”lives” for ever?
The answer is NO.
cover
glass vessel
water
soap-bubble
stopper
=100%
dropprocess of diffusionP0P
P=P0 + 4/r, r – radius of the bubble
P0 - atmospheric pressure
According to observations the “life-time” of soap-bubble in closedvessel may be more than 10 hours! This time drastically depends onsoap 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 4r ( r-radius
of the bubble). Therefore there’s the process of diffusion
molecules of air outside of the soap-bubble
(“diffusion wasting away process”).
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.
undulatory movementPart 2: the intensification of
Sounding tube – the thermal autogenerator of sound
Equipment: glass tube about 80 – 100 cm; small heater about P100 – 200 W; transformer for AC (voltage about 30 – 40 V); laboratory support; oscilloscope (not obligatory); microphone (not obligatory).
Set-up of experiment
V127 V
~30-40 V
heater
air
flow
(dr
aug h
t)
oscilloscope
microphone
glass tube
transformer
( L 80 cm, 35 mm)
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
x, p
stage of pressure
p=0 (node of pressure)
pmax(antinode)
x=0 (displacement of air)
x
stage of rarefaction
pmin
px
x
drau
ght
drau
ght
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.
h
L
p
x
In accordance with the experiments h=L/4.L – the wave-length of standing wave; c – the velocity of sound in the air;f0 = c/ = c/2L – the frequency of main
harmonic;
Some discussion
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.
stage of pressure
displacement of air
displacement of air
drau
ght
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.
L/2
small holep=0
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:
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:
soundgenerator
support
water
clamp
nozzle
loud speaker
water streamsstroboscope
.
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.
v
c
loup
e
capillary wave
droppings structure of stream
nozzlestroboscope
Some estimations:
There’s the simple estimation for : 9/2r, r 0.5 mm –radius of the nozzle. Hence 2.25 mm.It’s easy to determine the velocity of the stream: v 2 m/s,therefore c ms.According to the observations the resonance frequency of the
dropping process is about 300 Hz: fres z. Therefore we
can calculate the wave-length of the capillary wave: c/fres,
obs 6.6 mm.
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).