Atomic Phyics — E.1-E.5

electron volt definition

the energy required to move an electron through a p.d. of 1V

alpha decay

the emission of a helium nucleus (alpha particle) from an unstable nucleus

beta- decay

the emission of a fast-moving electron and an antineutrino when a neutron turns into a proton

beta+ decay

the emission of a fast-moving positron and a neutrino when a proton turns into a neutron

gamma decay

an excited nucleus de-exciting by emitting a gamma ray

radioactive decay definition

the spontaneous disintegration of a nucleus to form a more stable nucleus, resulting in the emission of an alpha, beta, or gamma particle

activity definition

the number of decays per second, measured in becquerel (nucleus/second)

half-life

the time taken for half the undecayed nuclei to decay or the activity of a source to decay by half

decay constant

represents the probability that an individual nucleus will decay per unit of time

uses of radioactive isotopes

• metastable technetium-99 and cobalt60 in medicine

• leaks in underground pipes

• controlling thickness of materials in factories

• carbon-14 dating

• americium in smoke detectors

natural sources of background radiation

• radon gas from rocks & buildings

• cosmic rays

• carbon-14 in biological material

artificial sources of background radiation

• nuclear medicine

• nuclear waste

• nuclear weapons & accidents

binding energy definition

the energy required to entirely separate the nucleons of a nucleus

unified atomic mass unit

1/12 the mass of a neutral carbon-12 atom

threshold frequency

the frequency of incident light in which electrons are beginning to be liberated

photoelectric effect definition

the spontaneous emission of electrons from a metal surface when hit by light of a suitable frequency

compton scattering definition

the interaction of a high-energy photon with an orbital electron which causes an increase in the wavelength of the photon and the ejecton of the electron

lyman series

• ground state

• highest energy photons

• shortest wavelength (UV)

pfund series

• 5th state

• lowest energy

• longest wavelength (IR)

order of bohr’s hydrogen series

lyman → balmer → ritz-paschen → brackett → pfund

fission definition

the splitting of a large, unstable nucleus into two smaller nuclei

induced fission

thermal neutron + unstable nucleus = 2 smaller nuclei, 1+ neutrons, gamma rays

critical mass definition

the minimum mass of a fuel required to maintain a steady chain reaction

luminosity of a star

total power output of a star

main sequence stars

stars that fuse hydrogen into helium and are in hydrostatic equilibrium

red giants

have a hot central core surrounded by an envelope of a thin tenuous gas

supergiants

large and very bright with a radius 300x that of the sun

white dwarfs

remnants of old stars

instability strip

narrow, nearly vertical region on the HR diagram that shows variable stars

variable stars

unstable stars that pulsate, changing their size cyclically

life-cycle of a low-mass star

protostar → main sequence star → red giant → helium burning star → double-shell burning red giant → planetary nebula → white dwarf

hydrostatic equilibrium is made up of:

• inwards force due to the gravitational attraction between the star’s interior and outer layers

• outwards force of thermal and radiation pressure that are trying to expand the star

electron degeneracy pressure

the pressure exerted by electrons in a white dwarf that prevents further gravitational collapse due to the pauli exclusion principle

pauli exclusion principle

forbids to electrons from having the same quantum state

life-cycle of a high-mass star

protostar → blue main sequence star → red super-giant → hellium burning supergiant → multiple-shell burning supergiant → supernova → neutron star/black hole

chandrasekhar limit

stars with a remnant mass > 1.4 solar masses cannot form a white dwarf

oppenheimer-volkhoff limit

stars with mass >20 solar masses form black holes if the core mass is > 3 solar masses

other ways to infer the existence of a black hole

• x-rays emitted as mass spirals towards the edge and heats up

• jets of matter emmited from some galaxies’ cores

• some stars influenced by invisible strong gravitational fields

star

a sphere of gas at whose centre nuclear fusion takes place

binary star system

two stars are bound gravitationally and orbit a common centre of gravity

constellation

a pattern formed by stars not gravitationally bound

stellar cluster

a group of stars which are gravitationally bound

galaxy

a collection of billions of stars orbiting a center; diameter 10^5 ly

galaxy cluster

several galaxies gravitationally bound together; diameter 10^7 ly

super cluster of galaxies

larger scale network of gravitationally bound falaxies

diameter of the observable universe

10^11 ly

1° in arcseconds

3600 arcseconds