Millikans Oil Drop Experiment Flashcards
(12 cards)
What was the primary purpose of Millikan’s oil drop experiment?
To determine the charge of a single electron by observing the behavior of charged oil droplets in an electric field.
How do oil droplets acquire charge in Millikan’s oil drop experiment?
The oil droplets become negatively charged due to friction when sprayed into the chamber using an atomizer.
Describe the basic setup of Millikan’s oil drop apparatus.
It includes an atomizer to spray oil droplets into a chamber with two horizontal parallel plates that create a uniform electric field. A microscope with a graticule is used to observe the droplets.
What condition must be met for an oil droplet to become stationary in the electric field?
The electric force upward must equal the gravitational force downward: EQ = mg. Since E = V/d, the equation becomes QV/d = mg.
Why can’t the charge on the oil droplet be calculated directly from the balanced force equation?
Because the mass m of the droplet is unknown and must be measured separately.
How is the mass of an oil droplet determined in the experiment?
By switching off the electric field and measuring the terminal velocity of the falling droplet, using Stokes’ Law and the known viscosity of air.
State Stokes’ Law and identify each variable.
F = 6πηrv
F: viscous drag force, η: viscosity of air, r: radius of the droplet, v: terminal velocity.
What force balance occurs when the oil droplet reaches terminal velocity?
The viscous drag force equals the gravitational force: 6πηrv = mg.
How is the mass of the droplet related to its radius and density?
m = (4/3)πr³ρ
Where ρ is the density of the oil.
How is the radius of the oil droplet calculated from terminal velocity?
r² = (9ηv²)/(ρg)
Where ρ is the density of the oil.
Once mass and radius are known, how is the charge on the droplet calculated?
Using: QV/d = mg = (4/3)πr³ρg
So: Q = (4/3)πr³ρg⋅(V/d).
What did Millikan discover about the charge values on the oil droplets?
All measured charges were integer multiples of 1.60×10⁻¹⁹ C, showing that electric charge is quantised, and this value is the elementary charge, the charge of a single electron.
