Small effects in the class-room experiments. Ivan Lomachenkov Some physical projects have been...

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

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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).