Overview
This lecture explains the Millikan oil drop experiment, highlighting how it measured the electron's charge and contributed to calculating the electron's mass.
Thomson’s Contribution
- J.J. Thomson measured the ratio of charge to mass (e/m) of the electron using a cathode ray tube in 1897.
- Thomson's work established that electrons are negatively charged subatomic particles.
Millikan Oil Drop Experiment
- Conducted between 1906 and 1914 by R.A. Millikan to determine the electron's charge (e).
- An atomizer created oil droplets that passed through an electrically charged condenser plate.
- Forces acting on oil drops: gravitational force, electrostatic force due to electric field, and viscous drag.
- The falling speed of oil droplets was measured with a micrometer eyepiece through a telescope.
- X-rays ionized air, allowing oil droplets to pick up discrete charges.
- The voltage applied to the plates controlled the motion of the oil drops.
Key Results and Findings
- Millikan found the charge (Q) on oil droplets was always a whole number multiple of a smallest unit (e), i.e., Q = n·e, where n = 1, 2, 3…
- The experimentally determined charge of an electron is about 1.6 × 10⁻¹⁹ coulombs (C).
- By using Thomson's e/m ratio and Millikan's value of e, the mass (mₑ) of an electron was calculated.
Key Terms & Definitions
- Electron — a negatively charged subatomic particle.
- Charge of electron (e) — fundamental unit of electric charge, approximately 1.6 × 10⁻¹⁹ C.
- Millikan oil drop experiment — experiment to measure the electron’s charge using charged oil drops.
- Electrostatic force — force between charged objects.
- Viscous drag — resistance experienced by droplets moving through air.
- Ionization — process by which atoms gain or lose electrons to form ions.
Action Items / Next Steps
- Review calculation steps for determining electron charge from oil drop experiment data.
- Read more about how the e/m ratio and electron charge are used to calculate electron mass.