Oil Drop Experiment

The oil drop experiment was performed in 1909 by Robert A. Millikan and Harvey Fletcher at the Ryerson Physical Laboratory at the University of Chicago. It is alternatively known as Millikan’s oil drop experiment. 

It was used for measuring an electron’s charge and earned Millikan the Nobel Prize in the year 1923. The experiment itself and understanding the oil drop experiment’s meaning was a breakthrough in physics. It involved using an apparatus that generated an electric field created between parallel metal plates held apart by insulation. Tiny electrically charged oil droplets passed through the metal plates due to the altering electric field. The article further includes all the details regarding the experiment.

Overview of oil drop experiment 

The oil drop experiment helped in measuring the charge of the electron. In the experiment, tiny, charged oil droplets are suspended between two metal electrodes. This process balances the downward gravitational force with upward drag and electric forces. 

As the charged oil droplet was passed between the metal electrodes held parallel to each other at zero electric fields, it helped measure the velocity of the falling droplet. As the electric field was passed between the metal plates, it was adjusted until the drops were suspended in mechanical equilibrium. This indicated that the electrical and gravitational forces were in balance, which helped determine the electric field. The charge of the electric field was calculated to be 1.5924 x 10-19 Coulombs (C).

Principle of oil drop experiment 

The experiment helped calculate the sum of electric forces and the size of the electric field with the help of the charge isolated from the oil droplet. Here are the principles of the experiment:

• The oil drop experiment showed that the oil drop could fall under its weight. Apply an electric field to the oil droplet suspended by using an electric field; the electrostatic force balances its weight. Hence, the electrostatic force is determined by the drop.
• On letting the oil droplet fall quickly into the air, it accelerates at maximum speed and reaches terminal velocity. The viscous drag of the air helps balance the weight of the oil droplet. The viscous drag is calculated with the help of Stoke’s Law.
• The experiment was performed over 150 times, and a high common factor was calculated using 58 experiments. This helped find that the one charge unit could be multiplied to get the total charge across all the oil drips.

Apparatus used in oil drop experiment 

A specially designed apparatus was used for the experiment. The meaning of the oil drop experiment and the right calculation was determined only when the right apparatus was used. 

The apparatus consisted of two metal plates separated by an insulated rod. Each of the plates had four holes, out of which three allowed the passage of light and the fourth was used for viewing. The metal plates are placed in the apparatus in a horizontal pattern, and the application of potential difference creates a uniform electric field between the spaces. The oil droplets are sprayed into the chamber from above the plates, which enter the space between the plates, and it rises or falls by changing the voltage across the plates.

Procedure of the oil drop experiment 

The overview of the oil drop experiment indicates that it helps measure the charge of the electron. Here is the procedure for experimenting:

• The oil droplets are passed through the atomiser, and it further forms tiny droplets that move through the holes located in the upper plate of the apparatus.
• As the droplets move downwards, their movement is observed through a microscope along with the mass of oil droplets, then measuring its terminal velocity.
• The air inside the chamber is ionised by passing an X-ray beam, and due to the ionisation of air, the oil droplets acquire an electrical charge as the droplets collide with the gaseous ions.
• As the oil droplets move through the metal plates, the electric field is set up between the parallel metal plates, and hence the droplets are impacted by the electric field.
• The gravity draws the oil downward, and the charge is pushed upward due to the electric field. As the strength of the electric field is controlled, oil droplets reach a balance in gravity.
• At equilibrium, the charge carried over the oil droplet is calculated, and this depends on the strength of the electric field and droplet mass.

Calculation of Millikan’s oil drop experiment 

The experiment allows the oil drops to fall between the plates when the net electric field is zero. It was found that along with the experimental apparatus, the oil droplets first accelerate due to gravity, but they gradually slow down due to air resistance. 

It can be calculated by the formulae:

Fup – Q.E and Fdown = m

Q.E = m.g

Q = m.gE

Where Q is the charge of the electron, m is the mass of the droplet, E is the electric field, and g stands for gravity.

Conclusion 

The oil drop experiment is an important concept both in chemistry and physics, and Millikan won the Nobel Prize for establishing the charge on the electron using the experiment. The single apparatus helped understand the concept of electric charge, gravitation, and air drag forces. With the oil drop experiment, it was easy to calculate the sum of electric forces and electric field size with the help of the minuscule charge of the isolated oil droplet.