Millikan Oil Drop Experiment

Introduction

The Millikan Oil Drop Experiment was a groundbreaking experiment conducted by Robert A. Millikan and Harvey Fletcher in 1909 to measure the elementary electric charge (the charge of the electron). This experiment provided the first precise measurement of this fundamental physical constant.

The Setup

The experiment involved observing tiny oil droplets between two horizontal metal plates. The droplets were sprayed into a chamber above the plates, and the ones that were small enough to remain suspended in the air due to air resistance were used for the experiment.

Key Concept: Electrostatic Force

The key principle underlying the experiment is that the electric force on a charged object is given by , where is the charge on the object and is the electric field. In the experiment, the electric field between the plates was controlled by adjusting the voltage difference between the plates.

Key Equations: Balancing Forces

When the electric field was adjusted just right, the electrostatic force on a droplet could balance the gravitational force, causing the droplet to remain stationary. This balance of forces is expressed by the equation

or

,

where is the mass of the droplet, is the acceleration due to gravity, and is the charge on the droplet.

The charge on the droplet is an integer multiple of the elementary charge , so for some integer . Therefore, the previous equation becomes

.

Calculation of Charge

By carefully measuring the rate of fall of a droplet under gravity alone, Millikan was able to determine the mass of the droplet. By observing the droplet as it was suspended stationary in the electric field, he was able to calculate the charge on the droplet. By repeating the experiment with many droplets, he found that the charges were always a multiple of a certain minimum charge—the charge of a single electron.

Results and Implications

From his measurements, Millikan determined the value of the elementary charge to be approximately coulombs, which is very close to the currently accepted value. This experiment confirmed the quantization of electric charge and the existence of a fundamental unit of charge, providing key support for the atomic theory of matter.

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