Millikan's Oil Drop Experiment

Introduction

The oil drop experiment was performed in 1909 by Robert A. Millikan and Harvey Fletcher to measure the elementary electric charge (it means the electron's charge). This experiment took place in the Ryerson Physical Laboratory, which is present at the University of Chicago. Also, this experiment has proved to be very crucial in physics.

Before this experiment, the existence of subatomic particles was not accepted universally. Millikan's apparatus has an electric field created between a parallel pair of metal plates held apart by an insulating material. The oil droplets, which are electrically charged, enter the electrical field and are balanced between two plates by altering the field. When the charged drops fell at a constant rate, the gravitational forces and electric forces on it were equal.


Oil Drop Experiment

Millikan allowed charged small oil droplets to travel through a hole into an electric field in the experiment. With the electric field's varying strength, the charge over an oil droplet is calculated, and it always comes as a fundamental value of 'e.'

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Apparatus

Millikan and Fletcher designed the experiment apparatus. It included two metal plates held at a distance by an insulated rod. There were four holes in the plate, three of which were there to allow light to pass through, and one was there to allow viewing through the microscope.

They did not use ordinary oil for this experiment, as it would evaporate by the heat of the light, and could cause an error in the Millikan Oil Drop Experiment. The oil, which is usually used in a vacuum apparatus with low vapor pressure, was also used.


Procedure

  • Oil passes through the atomizer, from where it came in tiny droplets form. The same droplets pass through the holes in the upper plate of the apparatus.

  • The droplet's downward movements are observed through the microscope and the mass of the oil droplets, and then their terminal velocity is measured.

  • The air present inside the chamber is ionized by passing through the X-ray beam. Collisions obtain the electrical charge on these oil droplets with gaseous ions produced by the ionization of air.

  • Then, the electric field is set up between the two plates so that the motion of the charged oil droplets can be affected by the same electric field.

  • Now, gravity attracts the oil in a downward direction, and the electric field pushes the charge upwards. Also, the electric field strength is regulated so that all the oil droplets reach an equilibrium position with gravity.

  • The charge on the droplet is calculated at equilibrium, which depends on the mass of the droplet and strength of the electric field.


Millikan Oil Drop Experiment Calculations

The experiment initially allows the oil drops to fall between the plates in the absence of the electric field. They accelerate first due to gravity, but gradually the oil droplets slow down because of air resistance.

The Millikan oil drop experiment formula can be given as below.

Fup = Q ⋅ E   Fdown = m

Where Q is an electron’s charge, m is the droplet’s mass, E is the electric field, and g is gravity.

Q ⋅ E = m ⋅ g

Q = \[\frac{m.g}{E}\]

By this, one can identify how an electron charge is measured by Millikan. Millikan also found that all the drops had charges, which were 1.6x 10-19 C multiples.


Conclusion

The charge for any oil droplet is always an integral value of e (1.6 x 10-19). Thus, Millikan's Oil Drop Experiment concludes that the charge is said to be quantized, which means that the charge on any particle will be an integral multiple of e always.

Millikan discovered the charge on a single electron using a uniform electric field between the oil drops and two parallel charged plates.


Importance of Millikan's Oil Drop Experiment

Millikan's experiment is quite essential because it establishes the charge on an electron.

Millikan used a simple apparatus in which he balanced the actions of electric, gravitational, and air drag forces.

Using the apparatus, he was able to calculate the charge on an electron as 1.60 × 10⁻¹⁹ C.


Principles of Millikan's Experiment

The Millikan experiment is complicated and fiddly while performing in school. It is more likely that we will use a simulation or a film clip of the experiment to show its principles to the students. Few of such principles are,

  • An oil drop can fall under its own weight. If a charge is given to the drop, it can be suspended by using an electric field. At this point, the electrostatic force balances the weight of every drop. Then the size of the electrostatic force depends entirely on the drop. So Millikan should have figured out the charge as soon as he knew the weight.

  • Millikan allowed the drop to fall through the air to find the weight of the drop. It reaches its terminal velocity quickly. At this point, the weight is balanced by the viscous drag of the air. Drag can be calculated from the Stokes' Law, which allowed Millikan to determine the weight.

  • Millikan repeated the same experiment thoroughly for over 150 oil drops and selected 58 of Millikan oil drop experiment results and got to find the highest common factor. It means the single unit of charge that could be multiplied up to give the charge he measured on all of his oil drops.

FAQ (Frequently Asked Questions)

1. Why Was the Negative Plate Earthed in Millikan's Oil Drop Experiment?

There are three possible reasonable ways to clear it.

The first reason is safety. Grounding ("earthing," in this context), the equipment is so important, particularly the time when you are working with high voltages. The same would be applied to protecting the equipment and for personal safety as well.

The second reason would be to establish a good stable reference point for the voltage measurement. A massive and solidly connected grounding cable would perform that job in a better way.

Finally, from an electrical standpoint, the two plates used in Millikan's experiment form a capacitor. On the other side, this capacitor is being charged to a very high voltage. In such cases, it is suggested to have a discharge path on one of the terminals or plates in order to avoid damage to either humans or equipment as well. Therefore, the negative plate is earthed.

2. Why Do We Use Oil Instead of Other Liquids in the Millikan Oil-drop Experiment?

Oil is one of the best liquids for Millikan's oil drop experiment. It retains its mass over a while and exposes to higher temperatures. Also, we employ an atomizer for ultra-fine droplets. So less dense liquids like water and oils are preferred over water because water cannot survive at such higher temperatures.

The atomizer employment is also an important reason behind using oil for this experiment. Moreover, it should be noted that oil would retain the exact volume/mass/weight. This would enable an exact measurement of the charge. Other liquids would separate or dissipate or even evaporate.