Millikan Oil Drop Experiment: Videos & Practice Problems

The Millikan oil drop experiment, conducted by Robert Millikan and Harvey Fletcher in 1913, was pivotal in determining the fundamental charge of an electron. Utilizing oil droplets, charged plates, gravity, and X-rays, they were able to measure the electric force and suspend a charged oil droplet between two plates. By adjusting the voltage to counteract gravity, the droplet's suspension allowed them to calculate the electron's charge as:

$-1.60\times {10}^{-19}\text{coulombs}$

a critical value for understanding atomic structure. This experiment built upon J.J. Thomson's earlier work, providing a quantitative measure of the electron's negative charge and advancing the field of atomic physics.

Millikan Oil Drop

In 1897, J.J. Thomson's cathode ray experiments led to the groundbreaking discovery of the electron and its negative charge. However, the precise measurement of the electron's charge was not achieved until 1913, when Robert Millikan and Harvey Fletcher conducted the famous oil drop experiment. This experiment utilized charged oil droplets, gravity, and X-rays to determine the fundamental charge of an electron.

The apparatus included an atomizer, which dispersed individual oil droplets into the experimental setup. When the atomizer was activated, it sprayed a mist of oil droplets into the chamber. The droplets then fell through a positively charged plate, while an X-ray source ionized them, imparting a negative charge. The setup also featured a microscope to observe the charged droplets and a voltmeter to measure the voltage across the plates.

As the power to the charged plates was turned on, an electric field was created. The relationship between the electric force and gravitational force was crucial: if the voltage was increased to a level greater than the gravitational force acting on the droplets, the droplets would rise. Conversely, if the voltage was insufficient, the droplets would fall. The goal was to find the precise voltage that would allow the droplets to be suspended in mid-air between the positively and negatively charged plates.

Through careful manipulation of the electric field, Millikan and Fletcher successfully suspended the oil droplets, allowing them to calculate the charge of an electron. Their findings revealed that the charge of an electron is approximately -1.60 \times 10^{-19} \text{ coulombs}, a fundamental constant in physics representing the charge of a single electron. This experiment not only confirmed Thomson's earlier work but also provided a quantitative measure of the electron's charge, significantly advancing our understanding of atomic structure and charge interactions.

Electron Mass Calculation Example

The fundamental charge of an electron is approximately \(-1.60 \times 10^{-19}\) coulombs, and the charge-to-mass ratio of an electron is \(-1.76 \times 10^{8}\) coulombs per gram. To find the total mass of an electron, we need to use these values in a conversion process.

First, we identify the given amount, which is the charge of the electron, and the conversion factor, which is the charge-to-mass ratio. The goal is to convert the charge of the electron into mass using the charge-to-mass ratio as a conversion factor. The conversion factor can be expressed as:

\[\text{Charge-to-mass ratio} = \frac{-1.76 \times 10^{8} \text{ coulombs}}{1 \text{ gram}}\]

To perform the conversion, we set up the equation to cancel out the units of coulombs:

\[\text{Mass} = \frac{-1.60 \times 10^{-19} \text{ coulombs}}{-1.76 \times 10^{8} \text{ coulombs/gram}}\]

By canceling the units of coulombs, we are left with grams. This calculation simplifies to:

\[\text{Mass} \approx 9.10 \times 10^{-28} \text{ grams}\]

This result indicates that the total mass of an electron is approximately \(9.10 \times 10^{-28}\) grams. Understanding this relationship between charge and mass is crucial in fields such as particle physics and quantum mechanics, where the properties of subatomic particles are fundamental to the study of matter and energy.

Here’s what students ask on this topic:

What was the oil drop experiment?

The oil drop experiment, conducted by Robert A. Millikan and Harvey Fletcher in 1909, was a pivotal experiment designed to measure the electric charge of a single electron. The setup involved spraying tiny oil droplets into a chamber and allowing them to pass through a hole into an electrically charged area between two metal plates. By adjusting the voltage across the plates, Millikan and Fletcher were able to balance the gravitational force on an oil droplet with the electric force, causing the droplet to suspend in mid-air.

By carefully observing the behavior of the oil droplets and knowing the density of the oil, they could calculate the mass of a droplet. Combining this with the electric force allowed them to determine the charge on the droplet. Through repeated experiments with different droplets, they noticed that the charges were always multiples of a certain smallest value, which they determined to be the charge of a single electron. This experiment provided the first accurate value of the electron's charge, which is a fundamental constant in physics.

What scientist proposed the oil drop experiment?

The oil drop experiment was proposed and conducted by American physicist Robert A. Millikan, with the help of his graduate student Harvey Fletcher. This groundbreaking experiment took place in 1909 at the University of Chicago. Millikan's objective was to measure the electric charge of a single electron, which he achieved by carefully observing the behavior of tiny oil droplets suspended between two electrically charged plates. By adjusting the voltage across the plates and analyzing the rate of the droplets' rise or fall, Millikan was able to determine the charge on the droplets, which in turn led to the calculation of the fundamental charge of the electron. This experiment played a crucial role in the development of atomic physics by providing the first accurate value of the electron's charge, a fundamental constant in physics.

What did Robert Millikan's oil drop experiment prove?

Robert Millikan's oil drop experiment, conducted in 1909, was pivotal in proving the existence of electrically charged particles, which we now know as electrons. The experiment allowed Millikan to calculate the charge of a single electron with high precision. By carefully observing the behavior of tiny oil droplets between two electrically charged plates, Millikan was able to determine how the droplets were suspended in the electric field. By adjusting the strength of the electric field, he could measure the amount of charge needed to counteract the gravitational force on the droplets. This groundbreaking work provided the first accurate value of the electron's charge, which is approximately:

$-1.602\times {10}^{-19}\text{coulombs}$

Millikan's findings supported and quantified the atomic theory of matter, confirming that electric charge is quantized, meaning it exists in discrete packets rather than being continuous. This experiment was a significant step forward in the field of atomic physics.