--- title: "Millikan Oil Drop Experiment | Principles of Physics II" description: "Millikan Oil Drop Experiment measures the elementary charge by balancing gravity and electric force on tiny oil drops in Principles of Physics II." canonical: "https://fiveable.me/principles-physics-ii/key-terms/millikan-oil-drop-experiment" type: "key-term" subject: "Principles of Physics II" unit: "Unit 1" --- # Millikan Oil Drop Experiment | Principles of Physics II ## Definition The Millikan Oil Drop Experiment is a lab method used in Principles of Physics II to measure the elementary charge, e, by balancing gravity and electric force on tiny charged oil droplets. ## What It Is The Millikan Oil Drop Experiment is a classic physics lab that measures the charge on tiny oil droplets, which led to a precise value for the elementary charge, e. In Principles of Physics II, it shows how electric force can be measured and compared directly with weight. The basic setup is simple but very clever. Tiny oil drops are sprayed into a chamber between two metal plates. Some drops pick up extra electrons, so they carry a net negative charge. When the drops fall, gravity pulls them downward, but an electric field between the plates can pull upward if the field points the right way. Millikan adjusted the voltage until a drop hovered in place or moved at a steady speed. That means the forces were balanced, so the electric force qE matched the droplet’s weight mg, or nearly matched it when air resistance was included. Once you know the field strength E and estimate the drop’s mass from its size and density, you can solve for the charge q. The real breakthrough was not just getting one charge value. Millikan found that the measured charges always came in small whole-number multiples of the same basic unit. That pattern showed charge is quantized, not continuous. A droplet might carry 1e, 2e, 3e, and so on, but never 1.4e. That is why this experiment matters in electrostatics. It turns electric charge from an abstract property into something you can measure with a force balance. It also connects directly to electric force, because the whole method depends on the relationship between charge, electric field strength, and force. The experiment gives you a concrete example of how a field can be used to control motion at the microscopic scale. ## Why It Matters This experiment sits right at the point where Principles of Physics II moves from naming charge to measuring what charge actually does. You use it to see that electric force is not just a theory on paper. It can exactly counteract gravity on a tiny object when the field is tuned correctly. It also gives you one of the cleanest physical examples of charge quantization. Instead of treating charge like any random amount, the data show that net charge comes in discrete packets. That idea shows up again when you study electrons, conductors, charging by transfer, and how current is built from moving charge. For the course, the Millikan setup is a model of force balance. If you can identify the forces, write the electric field relation, and decide whether the drop is suspended, rising, or falling, you are using the same reasoning physics labs rely on. It is a good bridge between electrostatics and problem solving with fields and forces. ## Connections ### Elementary Charge The experiment helped establish the value of the elementary charge, e, which is the smallest unit of free charge you usually see in physics problems. Millikan’s results showed that measured droplet charges were integer multiples of this value. That is why the experiment is tied so closely to the idea that electrons carry a fixed amount of charge. ### Electric Field The oil drop is controlled by an electric field between the plates. In the lab, changing the voltage changes the field strength, which changes the electric force on the droplet. This makes the experiment a direct application of the field relation F = qE, not just a background idea. ### [Quantization of Charge](/principles-physics-ii/key-terms/quantization-of-charge) Millikan’s measurements gave clear evidence that charge comes in discrete steps instead of any possible value. If you see a question about why a droplet has charge values like 2e or 5e, this is the concept behind it. The experiment is one of the most famous demonstrations of quantization in physics. ### Electric Force The whole setup depends on balancing electric force against gravitational force. If the droplet is stationary, the upward electric force equals the downward weight. If it rises or falls, you can tell which force is larger and use that motion to reason about the charge. ## On the AP Exam A lab question may give you the drop’s mass, the plate separation, and the voltage, then ask you to find the charge or decide whether the drop is positively or negatively charged. You should identify the forces on the drop, use qE for the electric force, and compare it with mg for weight. If the drop is suspended, that balance is the whole clue. You might also see a conceptual item asking why repeated measurements produce charges like 3e, 4e, or 7e instead of random decimals. The move there is to connect the data pattern to quantization of charge. In a written lab explanation, you would describe how changing the electric field changes the drop’s motion and how that reveals the charge on the drop. ## Millikan Oil Drop Experiment vs Faraday Cup A Faraday cup is an instrument used to collect charge and measure it indirectly, while the Millikan Oil Drop Experiment measures charge by balancing forces on a tiny droplet. Both deal with electric charge, but Millikan’s setup is about force balance and quantization, not simple collection. If the question focuses on suspended drops in an electric field, it is Millikan, not a Faraday cup. ## Key Takeaways - The Millikan Oil Drop Experiment measures the charge on tiny oil droplets by balancing electric force against gravity. - Its biggest physics result is that electric charge is quantized, so measured charges come in whole-number multiples of e. - The experiment is a clean example of using an electric field to control motion and solve for an unknown charge. - In Principles of Physics II, this term connects directly to electric force, electric field, and the nature of charge. - If a drop is suspended in the lab setup, the force balance qE = mg is the main relationship to use. ## FAQs ### What is the Millikan Oil Drop Experiment in Principles of Physics II? It is a physics experiment that measures the charge on tiny oil droplets by observing how they move in an electric field. By balancing the electric force with gravity, you can calculate the droplet’s charge and see that charge comes in discrete steps. ### How did Millikan measure the charge of an electron? He used charged oil droplets between two plates and adjusted the voltage until a droplet hovered or moved at a steady speed. That let him balance forces and solve for the droplet’s charge, which led to the elementary charge because all measured values were multiples of the same smallest unit. ### Why does the Millikan Oil Drop Experiment show charge is quantized? The measured charges never came out as random values. They always matched integer multiples of one basic charge, which showed that electric charge is not continuous in this context. That pattern is the evidence for quantization of charge. ### Is the Millikan Oil Drop Experiment about electric force or electric charge? It is about both. The experiment uses electric force to balance gravity, but the reason it is famous is that the force balance lets you determine the droplet’s charge. So the setup is a force problem with a charge result. ## Related Study Guides - [1.1 Electric charge](/principles-physics-ii/unit-1/electric-charge/study-guide/IwrNy3XwHgXUY1BQ) - [1.3 Electric force](/principles-physics-ii/unit-1/electric-force/study-guide/WFQj8lINkcGLtfOf) ## About This Document Canonical Fiveable pages are available as Markdown at the same path plus `.md`. - [llms.txt](https://fiveable.me/llms.txt): index of Fiveable's sections and URL patterns - [llms-full.txt](https://fiveable.me/llms-full.txt): complete subject and unit listing - [MCP server](https://fiveable.me/mcp): call Fiveable as tools instead of fetching pages (`https://fiveable.me/api/mcp`) - [MCP server for AP teachers](https://fiveable.me/mcp/teachers): a teacher's classes, assignments and AP-rubric grading (`https://fiveable.me/api/mcp/teacher`) ## Structured Data ```json {"@context":"https://schema.org","@graph":[{"@type":"LearningResource","@id":"https://fiveable.me/principles-physics-ii/key-terms/millikan-oil-drop-experiment#resource","name":"Millikan Oil Drop Experiment | Principles of Physics II","url":"https://fiveable.me/principles-physics-ii/key-terms/millikan-oil-drop-experiment","learningResourceType":"Concept explainer","educationalLevel":"AP® / High School","about":{"@id":"https://fiveable.me/principles-physics-ii/key-terms/millikan-oil-drop-experiment#term"},"audience":{"@type":"EducationalAudience","educationalRole":"student"},"dateModified":"2026-07-03T02:23:59.888Z","isPartOf":{"@type":"Collection","name":"Principles of Physics II Key Terms","url":"https://fiveable.me/principles-physics-ii/key-terms"},"author":{"@type":"Organization","@id":"https://fiveable.me/editorial#content-team","name":"Fiveable Content Team","url":"https://fiveable.me/editorial","parentOrganization":{"@id":"https://fiveable.me/#organization"},"member":[{"@type":"Person","name":"Amanda DoAmaral","sameAs":["https://www.linkedin.com/in/adoamaral"]},{"@type":"Person","name":"Becky Bahr","sameAs":["https://www.linkedin.com/in/becky-bahr-b52743132"]}]},"publisher":{"@type":"Organization","name":"Fiveable","url":"https://fiveable.me"}},{"@type":"DefinedTerm","@id":"https://fiveable.me/principles-physics-ii/key-terms/millikan-oil-drop-experiment#term","name":"Millikan Oil Drop Experiment","description":"The Millikan Oil Drop Experiment is a lab method used in Principles of Physics II to measure the elementary charge, e, by balancing gravity and electric force on tiny charged oil droplets.","url":"https://fiveable.me/principles-physics-ii/key-terms/millikan-oil-drop-experiment","inDefinedTermSet":{"@type":"DefinedTermSet","name":"Principles of Physics II Key Terms","url":"https://fiveable.me/principles-physics-ii/key-terms"}},{"@type":"FAQPage","mainEntity":[{"@type":"Question","name":"What is the Millikan Oil Drop Experiment in Principles of Physics II?","acceptedAnswer":{"@type":"Answer","text":"It is a physics experiment that measures the charge on tiny oil droplets by observing how they move in an electric field. 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