Physics (university) · Part 10
Relativity and quantum physics
How physics changes at very high speeds and very small sizes, plus radioactivity.
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- Postulates of relativityEinstein's two ideas: the laws of physics are the same for everyone moving at constant velocity, and light in a vacuum has the same speed c for all observers.
- Time dilationA moving clock runs slow as seen by a stationary observer, by the factor gamma = 1/sqrt(1 - v squared / c squared).
- Length contractionObjects moving fast are shorter along the direction of motion, by the factor 1/gamma.
- Mass-energy equivalenceMass is a form of energy, E = m c squared. A tiny mass is a huge amount of energy.
- PhotonA packet of light with energy E = h f = h c / lambda, where h = 6.63e-34 J s.
- Photoelectric effectLight above a threshold frequency knocks electrons out of a metal. Maximum kinetic energy = photon energy minus the work function.
- de Broglie wavelengthMatter has a wavelength, lambda = h/p. Only tiny particles have a noticeable one.
- Uncertainty principleYou cannot know both a particle's position and momentum exactly: change in x times change in p is at least h-bar / 2.
- Wave functionA mathematical description of a quantum particle. The square of its size at a point gives the chance of finding the particle there.
- Bohr energy levelsElectrons in hydrogen have fixed energies, E = -13.6 eV / n squared. Light is emitted when an electron drops a level.
- Radioactive half-lifeThe time for half of a radioactive sample to decay. It does not depend on how much you start with.