Informe Practica Milikan

8/10/2019 Informe Practica Milikan

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Informe de Laboratorio / Inform of Laboratory

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EXPERIMENTO DE MILLIKAN

Jose Cedillo1, Klever Ordoez2, Juan Marin3

Resumen Abstract

En este informe de laboratorio se determina la

carga del electrn, usando para ello una gota deaceite y un campo elctrico homogneo,

Para esto se usara el equipo Millikan, el cual

permite medir la intensidad de la fuerza elctrica

contra la fuerza de atraccin gravitatoria, usandominsculas gotas de aceite suspendidas entre dos

placas de un condensador.

En la experimentacin se realiz la medicin de lavelocidad de descenso de la gota sin aplicacin de

campo elctrico, para posteriormente obtener un

equilibrio en la misma por medio de una corrienteelctrica, la cual carga la gota y la hace descender

cargada elctricamente.

Palabras Clave: Carga Elctrica, Experimento deMillikan, Electrn.

In this laboratory report the load of the electron is

determined, using for it a drop of oil and ahomogeneous electric field,

For this the team Millikan was used, which allows to

measure the intensity of the electric force against the

force of gravitational attraction, using minusculedrops of oil suspended badges of a condenser

between two.

In the experimentation he/she was carried out themensuration of the speed of descent of the drop

without application of electric field, he/she stops later

on to obtain a balance in the same one by means ofan electric current, which loads the drop and he/she

makes it to descend loaded electrically.

Keywords: Electrical charge, Millikan's experimentElectron.

12 3Estudiante de la Carrera de Ingeniera Mecnica, Universidad Politcnica Salesiana sede Cuenca.


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Informe de Laboratorio / Inform of Laboratory

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1.Theoretical Framework

1.1 The millikan oil drop experiment

The oil drop experiment was an experiment

performed by Robert A. Millikan and Harvey

Fletcher in 1909 to measure the elementaryelectric charge.

The experiment entailed balancing the

downward gravitational force with the upwarddrag and electric forces on tiny charged droplets

of oil suspended between two metal electrodes.

1.2 Background

Starting in 1908, while a professor at the

University of Chicago, Millikan, with the

significant input of Fletcher,[1] and after

improving his setup, published his seminal studyin 1913.[2] This remains controversial since

papers found after Fletcher's death describe

events in which Millikan coerced Fletcher into

relinquishing authorship as a condition for

receiving his PhD.[3]

1.3 Experimental procedure

Millikans and Fletcher's apparatus incorporateda parallel pair of horizontal metal plates. By

applying a potential difference across the plates,

a uniform electric field was created in the spacebetween them.

Simplified scheme of Millikans oil drop experiment

A fine mist of oil droplets was sprayed into a

chamber above the plates. The oil was of a type

usually used in vacuum apparatus and waschosen because it had an extremely low vapourpressure. Ordinary oil would evaporate under the

heat of the light source causing the mass of the

oil drop to change over the course of the

experiment.[4]

1.4 Method

Initially the oil drops are allowed to fall between

the plates with the electric field turned off. They

very quickly reach a terminal velocity because of

friction with the air in the chamber.A likely looking drop is selected and kept in the

middle of the field of view by alternatelyswitching off the voltage until all the other dropshave fallen. The drop is allowed to fall and its

terminal velocity v1 in the absence of an electric

field is calculated.The drag force acting on the drop can then be

worked out using Stokes' law:[4]

(1)

Where v1 is the terminal velocity of the falling

drop, is the viscosity of the air, and r is theradius of the drop.The weight w is the volume D multiplied by the

density and the acceleration due to gravity g.However, what is needed is the apparent weight.

The apparent weight in air is the true weight

minus the up thrust (which equals the weight ofair displaced by the oil drop). For a perfectly

spherical droplet the apparent weight can be

written as:[5]

(2)At terminal velocity the oil drop is not

accelerating. Therefore the total force acting on

it must be zero and the two forces F and w must

cancel one another out. This implies:

(3)

Once r is calculated, w can easily be worked out.

Now the field is turned back on, and the electric

force on the drop is

(4)

where q is the charge on the oil drop and E is the

electric field between the plates. For parallelplates

(5)


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Apellido Autor et al / Titulo del Articulo

3

where V is the potential difference and d is the

distance between the plates.

One conceivable way to work out q would be toadjust V until the oil drop remained steady. Then

we could equate FE with w. Also, determiningFE proves difficult because the mass of the oildrop is difficult to determine without reverting to

the use of Stokes' Law.

A more practical approach is to turn V upslightly so that the oil drop rises with a new

terminal velocity v2. Then:[5]

(6)

2.

Materials and methods

For this demonstration of they used the teams of

Laboratory of the Polytechnic UniversitySalesiana, which consist of:

-1 apparatus of Millikan-1 service unit for the team

-Feeding cables

For the obtaining of results in the experimentation,he/she is carried out an observation of the physical

phenomenon, obtaining the necessary data for the

later one calculates and prosecution of the same

ones.

3. Results and discussionNext we will show the experimentation of thepractice, this experiment you the hiso for parts

one for the taking of measures of the drops of oil

in suspension and another for the mensuration of

drops in ascent and descent, for the one whichfirst we will show the well-known data of the

dimensions of the teams and of the estates of the

fluids.

Table 1 Datos

Voltaje Aplicado entre placas

(V)425

Distancia entre Placas (m) 0.02

Espacio para determinar la

Velocidad (m)0.001

Densidad del Aceite (Kg/m3

)a 20 C

871

Coeficiente de la viscosidad

del aire (Kg.m/s) a 20 C

1.81x10-5

Densidad del Aire (kg/m3) a

20 C1.21

For the results to obtain carry out the following

procedure.

1. I dewed oil with the rubber pear among theboards of the condenser in such a way that the

drops of oil are seen in the whole observationfield.

2. Displacing the reading microscope forms a

plane in which is seen sharply, as a luminous

point, the drop of oil selects.3. Then put the switches OR and t down.

4. Light the switch of the condenser with the

switch OR and adjust a tension of (400 to 500 V)

with the revolvable potentiometer in such a way

that the selected drop of oil ascends 1 or 2 scaledivisions per second (in the ocular you leave like

he/she falls). Then to reduce the tension until thedrop of oil exactly is suspended.

5. Disconnect the tension of the condenser with

the switch U.

Method of Suspension


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6. So soon the drop of oil is beside a mark of the

scale selected by you, begin the mensuration of

the time with the switch t.

7. So soon the drop of oil has descended other 20marks of the scale (it corresponds to 1 mm) (in

the ocular you leave as that it ascends) it stopsthe mensuration of the time with the switch t andconnect the tension of the condenser again with

the switch U. If the drop of oil moves along a

journey s' of x graduations of the scale of themicrometer scale (= x.10-14 m), keeping then in

mind to the increase of the objective of 1.875

times, the road s really journey is: s=(x/1.875)

.10-4 m.8. Repeat the mensuration for other drops of oil,

and score in the chart 2.

b) Method of the descent and ascent

9. So soon the drop of oil is beside a mark of the

scale selected by you, begin the mensuration ofthe time with the switch t.

10. So soon the drop of oil has descended other

20 marks of the scale (it corresponds to 1 mm)

(in the ocular you leave as that it ascends),connect the tension of the condenser again with

the switch U. This begins the mensuration of the

time automatically t2.11. So soon the drop of oil is beside a mark of

the scale selected by you, begin the mensuration

of the time with the switch t.

12. Repeat the mensuration for other drops of oil.

Table 2 Mtodo de Suspensin

T (seg)

desce

nso

S' (m)

desce

nso

T

(seg)

asce

nso

S'

(m)

asce

nso

20.50 0.001

22.6

8

0.00

1

20.19 0.00117.7

5

0.00

1

17.74 0.00120.0

8

0.00

1

In a same way those that are obtained with the

literal b.

Table 3 Mtodo de Ascenso y Descenso

T (seg)

desce

nso

S' (m)

desce

nso

T

(seg)

asce

nso

S'

(m)

asce

nso

16.43 0.00115.8

4

0.00

1

21.23 0.00118.9

5

0.00

1

19.87 0.00122.4

41

As it was indicated in the procedure the value of

s' it is a value of increase of the 1.875 times for

this the real journey serious s for 1 mm the

following one.

Calculations

To carry out the calculations we help ourselves

of you formulate them proposals in the report of

Millikan of the laboratory of Physics, next thefollowing ones literal.

1) to calculate the radius average of a drop of oil.

Table 4 Velocidades Promedio

Velocidades 1

Suspension Desc m/s

1 48.78x10-6

2 49.52x10-6

3 56.36x10-6

Descenso

1 60.86x10-6

2 47.11x10-6

3 50.32x10-6


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Apellido Autor et al / Titulo del Articulo

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Velocidad 1

Promedio51.99x10-6

Velocidades 2

Suspension Asc m/s

1 43.98x10-6

2 56.33x10-6

3 49.81x10-6

Ascenso

1 63.13x10-6

2 52.77x10-6

3 44.56x10-6

Velocidad 2

Promedio52.76x10-6

The speed average 1 should be used since it is

the one that is in free fall.

Replacing the data in the radio equation, we havethe following result.

2) it calculates the practical value of and.

To calculate the practical value of and we will

use the following expression up to now using ina same way the data of the chart 1 and the

calculated ones.

C

Consequently with the help of the mass of eachdrop you can calculate the electric load of each

drop.

Taking into account that the radius average ofthe drop is then of 0.704e-6 m we take out the

volume of the same one considering it a sphere.

4. Conclusions

The proposed objectives were completed beingthis the one of being able to measure the electricload that possesses a particle in this case a drop of

oil, thanks to the mensuration of the experienced

force with the help of an electric field of well-known magnitude.

The utilized formulas are previously already in the

report shown with those that depending thatnecessity one had the one it was used that was


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required as example the radius of the drop, their

carrying capacity, etc.

With regard to the experiment the difficulty of

seeing the drops since its size is microscopic inspite of the help of the lens, in a same way the

difficulty of measuring its displacement due todesenfoque, giving final results to a speed ascentaverage and descent of 21 seg.

According to the results the necessary

expectations were obtained since to consider thatthe experiment was realized in a successful way

the securities they are certainly similar to results

found in examples and previous experiments.

5. Annexes

1.

As it would calculate the fundamentalelectric load, acquired by the drops in the

experience. That it formulates it would use.

According to that experienced it would be helped

with the own instruments of he/she practices it

since for such a purpose the formula of the exactmultiple of the electron it would be used being this

the equation 2 of the report of Millikan, since to

measure the radius of the alone drop it would be

enough to measure the radius of the drop with the

help of the own scale of opposing mensuration inthe finder and for ende to know the data of the

estates of the fluid and of the utilized voltage.

2. As it would demonstrate the cuantizacin ofthe load, in function of the experimental

data and realized calculations.

Making a certain number of test-error experiments

to know if the data of previous experiments are

always the same ones in any part, taking into

account the external factors as temperature, in asame way you can use the softwares that you/they

go of the hand with the simulations to credit thatwhat passes in the real life also happens in a

simulation.

References

[1] Michael F.Perry (May 2007).

Remembering The Oil Drop Ex-

periment, Physics Today: vol 60 # 5, p..

56.

[2] Millikan, R. A. (1913) . On theElementary Electric charge and the AvogadroConstant. Phys. Rev. (2): 109143.

[3] Serway, Raymond A.; Jewett, ,,John W...(2004). Physics for Scientists and

Engineers (6th ed.)

[4] Thornton, Stephen T.; Rex, Andrew(2006). Modern Physics for Scientists and

Engineers (3rd ed.)

[5] http:// webphysics. Davidson. edu/

applets/pqp_preview/contents/pqp_errata/cd_errat

a_fixes/section4_5.html

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Informe Practica Milikan