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Chapter 24 - Particle Physics I - Coggle Diagram
Chapter 24 - Particle Physics I
Rutherford's alpha scattering
Narrow beam of alpha particles from radioactive source were direction at thin piece of gold foil
Alpha particles were scattered and detected on zinc sulfide screen in front of microscope - each particle hitting the fluorescent screen produced a light speck
Microscope was moved around to count the particles scattered through dfferent angles, from 0 to 180
Observations + conclusion
Most particles passed straight through, 1 in 2000 were scattered -
atom is mostly empty space and mass is concentrated at center
1 in every 10000 were deflected through angles more than 90 -
nucleus must be positively charged as it repels alpha particles
Microscopic interactions
A - Alpha particle colliding head on with nucleus - rebounds back with scattering angle of 180
B - Oblique collisions, deflected through angle theta
C - Oblique collisions further away, deflected through smaller angle
Size of atom/ nucleus
Use alpha particles that collided head on with nucleus.
Conservation of energy - initial kinetic energy = electrical potential energy at distance d
Alpha particle momentarily stops
This distance gives upper limit for radius of gold nucleus, more energetic particles might get closer
Radius of nucleus is approximately 10^-15m
Radius of atom is 10^-10m
Nucleus
Nuclear model
Contains positive protons and neutral neutrons
Isotope
A - Mass/ nucleon number
Z - Proton number
Nuclei of same element that have same number of protons but different number of neutrons
Atomic mass units
One atomic mass unit (1u) = 1/12th mass of neutral carbon-12 atom
1u = 1.661 X 10^-27
Nuclear size
Radius of nucleus depends on mass number
r0 = 1.2fm radius of proton
Strong nuclear force
In helium 4 nucleus, two protons are separated by a distance of 10^-15m and exert large repulsive electrostatic force of 230N
Force holding protons nucleus is strong nuclear force
Repulsive at distances less than 0.5m
Attractive for 0.5 - 3 fm
Zero for distances greater than 3fm
Antimatter
Every particle has corresponding antiparticle
Two annihilate each other when they meet
Mass of particle and antiparticle converted into high energy photons
Antiparticle has opposite charge and exact same rest mass
Fundamental forces
Strong nuclear - experience by nucleons, counteracts repulsive electrostatic forces
Electromagnetic - experienced by static and moving charged particles, infinite range
Weak nuclear - responsible for beta decay, acts to change quark types over small distances
gravitational - experience by all particles with mass. Always attractive, infinite range, but very weak
