chapter 24 - particle physics Flashcards
plum pudding model
atom contained neg electrons embedded in a sea of uniform pos charge
Rutherfords alpha scattering experiment
- fired a narrow beam of alpha particles with the same KE at at piece of thin gold foil - so particles are defelected once - one layer of atoms
- alpha was scattered by foil and detected on a screen - when they hit the screen produced a speck of light
Rutherfords alpha scattering experiment observations/ conclusion
- most passed through with little scattering - mostly empty space
- very few were deflected by large angles- mass was mostly concentrated in a small volume - of positive charge
size of atom/ nucleus
the fraction of the particles scattered a lot = the fraction of the atom taken up by the nucleus
nucleus radius = 10^-14
atom radius = 10^-10m
1mm dot on a 100m running track
how is radius of atom estimated
use distance of closest approach - fire alpha particle directly at it
KE > EPE - where it stops
1/2mv^2 = kQq/r^2
solve for r
neutrons discovered
Chadiwck noticed that alpha particles hitting beryllium nuclei knock off neutrons
nuclear model of atom
nucleus contains positive protons and uncharged neutrons
proton and neutron have about the same mass
isotopes
same number of protons with different number of neutrons
isotopes of same element undergo same reactions
atomic mass unit
one atomic mass unit is 1/12 the mass of a neutral C-12 atom
nuclear size
radius depends on nucleon number (A)
R = r0A^1/3
where r0 = 1.2fm (10^-15) - radius of a hydrogen nucleus (one proton)
strong nuclear force
acts between all nucleons
short range force (effective over a few fentometres)
attractive to 3fm
and repulsive below 0.5fm
large - much more than electrostatic
antimatter
every particle has a corresponding antiparticle
has same mass and opposite charge
if the two meet they destroy each other - annihilation - mass of poth particles are converted into a high energy pair of photons
symbol is bar over letter
fundamental forces
weak nuclear force - responsible for beta decay
strong nuclear - experienced by nucleons
electromagnetic - experienced by static and moving charges
gravitional - experienced by masses
weak nuclear force
responsible for beta decay within unstable nuclei
fundamental particles
has no internal structure - cant be divided smaller
eg quarks, electrons, neutrinos (leptons)
hadrons
affected by strong nuclear force
decay by weak nuclear force
if charged exoerience EM force
made up of quarks
heavy
eg protons, neutrons, mesons
2 types of subatomic particle
hadrons
leptons
leptons
not affected by strong nuckear force
if charged experience EM force
fundamental particels
light
eg electrons, neutrinos, muons
electron, muon tau charge -1
neutrinos charge 0
all lepton number 1
quarks
make up hadrons - fundamental particle
6 quarks with antiquarks
up, down
charm, strange
top, bottom
up quark
charge + 2/3
baryon +1/3
lepton 0
down quark
charge -1/3
baryon +1/3
lepton 0
strange quark
charge -1/3
baryon +1/3
lepton 0
antiquarks
same mag of charge/ baryon but opposite sign
class of quarks
up, down
charm, strange
top, bottom
2/3, -1/3 charge
1/3, 1/3 baryon
increase in mass/ energy as you go down
protons
+1 charge
uud
neutrons
neutral charge
udd
2 types of hadrons
mesons
baryons
baryons
- all hadrons with 3 quarks
eg proton, neutrons
mesons
- hadrons made of a quark and antiquark
eg pion, kaon
neutrino
no charge - tiny mass
leptons
3 types - electron, muon and tau
each also has its own antineutrino
beta decay
due to weak nuclear force
either beta - or beta +
beta - decay
neutron changes to proton and electron
neutron > proton + electron + anti electron neutrino
d > u + e- + anti electron neutrino
beta + decay
proton changes to a neutron and positron
proton > neutron + positron + electron neutrino
u > d + e+1 + electron neutrino
conservation
in particle interactions have to conserve:
- charge
- baryon number
- lepton number
- stangeness (-1 strange, 0 for every other quark)
