---
title: "Millikan's Oil Drop Experiment | College Physics I"
description: "Millikan's oil drop experiment showed that electric charge comes in discrete multiples and let physicists measure the electron's charge in College Physics I."
canonical: "https://fiveable.me/intro-college-physics/key-terms/millikans-oil-drop-experiment"
type: "key-term"
subject: "College Physics I – Introduction"
unit: "Unit 30"
---

# Millikan's Oil Drop Experiment | College Physics I

## Definition

Millikan's oil drop experiment is a physics experiment that measured the charge of tiny oil droplets and showed that electric charge is quantized. In College Physics I, it is a classic example of how scientists identified the electron and measured a fundamental constant.

## What It Is

Millikan's oil drop experiment is a physics experiment that measured the charge on tiny suspended oil droplets and showed that charge comes in fixed units, not any random value. In College Physics I, you see it as one of the classic experiments that helped prove the electron exists and that electric charge is quantized.

The setup is simple in idea but careful in practice. Tiny oil drops are sprayed into a chamber between two metal plates. Some drops pick up extra electrons, so they become charged. By adjusting the electric field between the plates, Millikan could make a drop hover, rise, or fall, then use the balance of electric force and gravity to figure out the drop's charge.

The key move is comparing forces. If the upward electric force equals the downward weight of the drop, the drop stays suspended. Since the weight can be estimated from the drop's size and density, and the electric field is known from the plate voltage and spacing, the charge can be solved for from F = qE. Millikan repeated this for many drops and found that the measured charges were always multiples of one smallest value.

That pattern mattered more than any single measurement. It showed that charge is quantized, meaning you do not get half an electron's charge or 2.7 times some charge by accident. Instead, the charges on the drops clustered around integer multiples of the elementary charge, e.

For physics, this experiment sits right next to Thomson's cathode ray work. Thomson gave evidence that electrons exist and have a negative charge-to-mass ratio. Millikan then pinned down the size of the electron's charge, which made the electron a much more concrete particle in the modern atomic model.

## Why It Matters

Millikan's oil drop experiment matters because it gives you a direct example of how physics turns a hidden particle into a measurable quantity. The electron is far too small to see directly, so the experiment uses force balance, electric fields, and careful measurement to infer its charge.

It also shows one of the big patterns in modern physics: some properties are continuous, but others come in discrete steps. In this case, charge is not just any value you want. Every charged object's net charge can be written as an integer multiple of the electron's charge, which is a big clue about how matter is built.

In College Physics I, this term often shows up when you are connecting electricity to atomic structure. It helps explain why the elementary charge is treated as a constant, why forces on charged particles can be calculated from qE, and why tiny measurements can reveal subatomic structure even when you cannot see the particle itself.

It also gives historical context for the atomic model. Once electrons were identified and their charge was measured, scientists could build better models of atoms, leading into later topics like nuclear structure and the distribution of charge inside matter.

## Connections

### Electron

Millikan's experiment is one of the major pieces of evidence for the electron as a real, negatively charged particle. Thomson had already shown that cathode rays behaved like charged particles, but Millikan helped quantify the electron's charge. That made the electron easier to place into later models of atoms and electricity.

### Quantization of Charge

This is the main result of the oil drop experiment. When Millikan measured many droplets, the charges always came out as whole-number multiples of one smallest unit. That pattern is what makes charge quantized, and it is one of the earliest clear examples of a discrete property in atomic physics.

### [Cathode Ray Tube](/intro-college-physics/key-terms/cathode-ray-tube)

Cathode ray tube experiments came before Millikan and helped reveal that electrons exist. Thomson used deflection of cathode rays to show they were negatively charged particles. Millikan's work did not replace that result, but it completed the picture by measuring the actual charge on the electron.

### Atomic Structure

Millikan's experiment fits into the bigger shift from indivisible atoms to structured atoms with internal parts. Once the electron's charge was known, it became easier to model atoms as containing smaller charged pieces. That opened the door to later ideas about the nucleus, electron arrangement, and the overall electrical behavior of matter.

## On the AP Exam

A quiz or problem set may ask you to identify what Millikan measured, explain why the oil drops could be held motionless, or state what conclusion came from the repeated charge values. The move is usually to connect the observed motion of a droplet to the balance between electric force and gravitational force, then explain why the result supports quantized charge.

You may also see a short answer that asks how the experiment supports the existence of the electron. In that case, mention that the measured charges came in integer multiples of a smallest unit, which points to a fundamental particle carrying one elementary charge. If a figure or description is provided, look for the suspended droplet, the plates, and the electric field direction, since those features tell you what forces are acting.

## Millikan's Oil Drop Experiment vs Thomson's Cathode Ray Experiments

Thomson's experiments showed that cathode rays were made of negatively charged particles, which identified the electron. Millikan's oil drop experiment came after that and measured the charge of the electron itself. Thomson found the particle, while Millikan measured one of its fundamental properties.

## Key Takeaways

- Millikan's oil drop experiment measured the charge on tiny oil droplets and showed that electric charge comes in discrete units.
- The experiment works by balancing gravity with an electric field, so a droplet can be suspended when the upward and downward forces match.
- Millikan found that the droplet charges were always whole-number multiples of one smallest charge, which is evidence for quantization.
- The experiment gave physicists a value for the electron's charge and strengthened the case that electrons are real subatomic particles.
- In College Physics I, this term connects electric force, field strength, and atomic structure in one famous experiment.

## FAQs

### What is Millikan's oil drop experiment in College Physics I?

It is an experiment that measured the charge on tiny oil droplets and showed that electric charge comes in fixed units. By balancing electric force against gravity, Millikan could calculate the droplet's charge and see that the values were multiples of one smallest charge.

### How did Millikan's oil drop experiment measure electron charge?

Millikan adjusted the electric field between two plates until a charged oil drop hovered or moved at a known rate. From the force balance, he calculated the drop's charge, and repeated trials showed a smallest unit equal to the charge of one electron.

### What does quantization of charge mean in this experiment?

It means charge does not vary continuously in this context. The measured charges on the oil drops always came out as integer multiples of a basic unit, so you could write them as 1e, 2e, 3e, and so on.

### How is Millikan's experiment different from the cathode ray experiment?

The cathode ray experiment showed that electrons exist and carry negative charge. Millikan's experiment went further by finding the size of that charge. Together, they moved physics from discovering the particle to measuring one of its basic constants.

## Related Study Guides

- [30.2 Discovery of the Parts of the Atom: Electrons and Nuclei](/intro-college-physics/unit-30/2-discovery-parts-atom-electrons-nuclei/study-guide/VE7712e9WQVi1AZJ)

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