1 THE MILLIKAN OIL-DROP EXPERIMENT INTRODUCTION: This experiment is one of the most important of the experiments in the second year undergraduate laboratory. The experimental apparatus is patterened after the original apparatus, made and used by R.A. Millikan to show that, in nature, electric charge exists as integral multiples of "e" the charge on a single electron. Historically this experiment ranks as one of the great experiments of modern physics. The Leybold-heraeus apparatus, which you will use, should be capable of giving a measurement of "e" which is within the experimental error of that accepted today. We suggest that the student refer to Millikan's book entitled: 'The Electron', which gives an account of the original oil-drop experiment in considerable detail. The instruction sheets for the apparatus in the laboratory can be made available to you through your demonstrator. GETTING STARTED: The student should first derive the equations for an oil drop falling in air under gravity, then derive the equations for an oil-drop moving upward between the parallel plates of the cell, when an electric field is applied to the plates. You may have to discuss what is needed with your demonstrator. The viscous force always opposes the motion and it is of course responsible for the steady terminal velocities observed, with and without the electric field. The telescope eyepiece should be adjusted to bring the scale into sharp focus; check for parallax. Play with the equipment and become familiar with it before you take any serious measurements. If in doubt, ask you demonstrator. Once you have decided how you will do this experiment, check with your demonstrator before proceeding. SOME USEFULL INFORMATION The density of the oil used is 875.3 Kg/m 3 The density of air at is 1.29 Kg/m 3 The acceleration due to gravity is 9.80 m/s 2 The viscosity of air at room temp and latm is 1.81 10 -5 Ns/m 2 The separation of the parallel plates is 6.0 mm The scale viewed through the eyepiece has a range 10 cm Some of the above values are approximate; you should confirm your choice of physical constants. QUESTIONS TO CONSIDER

THE MILLIKAN OIL - U of T Physicsphy225h/millikan-oil/...THE MILLIKAN OIL-DROP EXPERIMENT INTRODUCTION: This experiment is one of the most important of the experiments in the second

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THE MILLIKAN OIL-DROP EXPERIMENT

INTRODUCTION:

This experiment is one of the most important of the experiments in thesecond year undergraduate laboratory. The experimental apparatus ispatterened after the original apparatus, made and used by R.A. Millikan to showthat, in nature, electric charge exists as integral multiples of "e" the charge on asingle electron. Historically this experiment ranks as one of the greatexperiments of modern physics. The Leybold-heraeus apparatus, which you willuse, should be capable of giving a measurement of "e" which is within theexperimental error of that accepted today.

We suggest that the student refer to Millikan's book entitled: 'TheElectron', which gives an account of the original oil-drop experiment inconsiderable detail. The instruction sheets for the apparatus in the laboratorycan be made available to you through your demonstrator.

GETTING STARTED:

The student should first derive the equations for an oil drop falling in airunder gravity, then derive the equations for an oil-drop moving upward betweenthe parallel plates of the cell, when an electric field is applied to the plates. Youmay have to discuss what is needed with your demonstrator. The viscous forcealways opposes the motion and it is of course responsible for the steady terminalvelocities observed, with and without the electric field.

The telescope eyepiece should be adjusted to bring the scale into sharpfocus; check for parallax. Play with the equipment and become familiar with itbefore you take any serious measurements. If in doubt, ask you demonstrator.Once you have decided how you will do this experiment, check with yourdemonstrator before proceeding.

SOME USEFULL INFORMATION

The density of the oil used is 875.3 Kg/m3

The density of air at is 1.29 Kg/m3

The acceleration due to gravity is 9.80 m/s2

The viscosity of air at room temp and latm is 1.81 � 10-5 Ns/m2

The separation of the parallel plates is 6.0 mmThe scale viewed through the eyepiece has a range � 10 cm

Some of the above values are approximate; you should confirm yourchoice of physical constants.

QUESTIONS TO CONSIDER

Page 2: THE MILLIKAN OIL - U of T Physicsphy225h/millikan-oil/...THE MILLIKAN OIL-DROP EXPERIMENT INTRODUCTION: This experiment is one of the most important of the experiments in the second

1. How will you measure the radii of the spherical oil drops?

2. How will you correct for the buoyancy of air?

3. Stokes law for a sphere falling in a viscous medium depends on thedrop radius and the air pressure, how?

4. How will you correct for the R and P dependence in your measurementof the charge Q on each drop (Millikan's book will help here.)

5. What about the very smallest of the oil drops, is their motion unusual?Why?

6. Give some indication as to the limits of your measurements whencompared to those of Millikan. Would you expect your measurementto be as good as his? Why?