Chapter 17 Flashcards
What was Rutherford’s experiment?
Stream of alpha particles from a radioactive source was fired at a thin gold foil. Geiger and Marsden recorded number of alpha particles scattered at different angles, detected by a zinc sulphide screen. They were occasionally scattered at angles greater than 90*, meaning they must have collided with something bigger than themselves
What were the conclusions from Rutherford’s experiment?
1 - Most of the fast charged alpha particles went straight through the gold foil, meaning the atom is mostly empty space
2 - Some of the alpha particles were deflected back through significant angles, so the centre of the atom must be tiny but contain a lot of mass
3 - The alpha particles were repelled, so the nucleus must have a positive charge
4 - Atoms are neutral overall, so the electrons must be on the outside of the atom, separating one atom from the next
How do you calculate the distance of closest approach of an alpha particle to the nucleus?
At turning point, kinetic energy = electrical potential energy = Q(gold)q(alpha)/(4 pi epsilon(0) r). Now rearrange to find R
What are hadrons?
Baryons and masons. Made up of quarks.
What is the strong force of interaction?
Felt by hadrons, it holds the nucleons together (prevents protons repelling)
What are baryons?
Neutrons and protons are examples of baryons. They are made up of 3 quarks. The proton is the only stable baryon.
What is the baryon number?
The number of baryons in a reaction. Protons and neutrons have a baryon number of +1
What are leptons?
Fundamental particles that don’t feel the strong interaction. They interact through the weak interaction, gravity and electromagnetic force (if they’re charged)
What are the 3 different types of leptons?
The electron, muon and tau. You also get the neutrino version of these, with a charge 0, as opposed to -1. They each have a lepton number of +1, though you count them separately for each type of particle.
How do neutrons decay?
n -> p + e(-) + antineutrino. This is an example of beta decay
What are antiparticles?
Particles with the same mass but a different charge. Represented with the same symbol as the normal particle with a bar over the top. They also have opposite baryon and lepton numbers
How can energy be converted to mass?
E=mc^2. And you have to make equal amounts of matter and antimatter
How does creation work?
If a gamma ray photon has enough energy, an electron-positron pair may form (these more often than other particle pairs, as they have less mass). Often occurs near nuclei, as this helps to conserve momentum.
How does annihilation work?
When a particle meets its antiparticle, they ‘disappear’ and release a large amount of energy as 2 gamma rays, equivalent to the mass of the particles.
What is PET?
Positron emission tomography. Way of looking at tumours: inject patient with an isotope that releases beta plus decay, look at where it settles by detecting the gamma rays from the annihilation of positrons and electrons
What are quarks?
The particles that make up hadrons. They come in up, down, strange, top, bottom and charm. You also get antiquarks. They also have a new kind of charge called ‘colour’ that comes in 3 forms: red, green and blue.
What are the properties of up, down and strange quarks?
U: charge: +2/3 baryon number: +1/3 strangeness: 0
d: charge: -1/3 baryon number: +1/3 strangeness: 0
s: charge: -1/3 baryon number: +1/3 strangeness: -1
Their antiparticle have the opposite numbers
How are protons and neutrons made up?
p: uud
n: ddu
What is quark confinement?
The fact that you can’t make a free quark. The energy just goes into making more quarks and antiquarks
What are gauge bosons?
Exchange particles that are a way of explaining attraction and repulsion. They are virtual particles that only last for a very short time
What are the 4 fundamental forces and the gauge bosoms associated?
Strong: gluon (only affects hadrons)
Electromagnetic: photon (charged particles only)
Weak: W+, W-, Z0
Gravity: gravitons (?)
What is weird about the strong force?
The strength of it increases with distance: the gluon field increases in energy. Eventually, the energy is creates a quark, antiquark pair
How do linear particle accelerators work?
Long straight tube containing a series of electrodes; alternating current is applied across them, giving a continuously changing charge. The current is timed, so the particle is attracted to the next electrode and repelled from the previous one, increasing it in speed.
What is a cyclotron?
A circular particle accelerator with 2 semicircular electrodes to accelerate charged particles across a gap. Alternating pd and magnetic field is used to keep the particle accelerating and moving in a circular pathway. Particle spirals outwards
What is a synchrotron?
A particle accelerator in a circle that produces much higher energies that cyclotrons and linear accelerators. Beam is accelerated with electrodes; electromagnets keep particles in circular path. Particles can reach several TeV
Why can’t anything with mass reach the speed of light?
Its mass increases as you get closer to the speed of light as its kinetic energy increases and E=mc^2
What is the way of calculating the gamma factor based on energies?
Gamma = E(total)/E(rest) where E(rest)=mc^2. At low speeds, E(total) is very close to E(rest)
How are electrons in quantum shells modelled?
Standing waves. Number of nodes corresponds to increasing energy required to occupy that level.
How do electrons move between energy levels?
By absorbing or emitting a photon of exactly the right energy level. Leads to lines in absorption/emission spectra
What is important to remember about the energy of electrons?
They are negative because its a bound system: how much energy is needed to escape?
What is the Pauli exclusion principle?
The idea that no 2 fermions (electrons, protons and neutrons) can exist in the same quantum state at the same time
What is the evidence for electron energy levels?
Absorption and emission spectra: lines taken out/added where photons have been absorbed/emitted at specific frequencies (corresponding to energy)
How do you calculate the energy levels in a hydrogen atom?
E=-13.6eV/n^2
What is conserved in particle interactions?
Total energy, momentum, charge, lepton and baryon numbers
What are scattering experiments?
Experiments that reveal the structure of atoms, nuclei and the like. The smaller the scale, the greater the energy needed. At high energy, many new particles are created
What is the de Broglie wavelength equation?
lambda=h/p where h is plancks’ constant and p is momentum
What is the weak interaction?
The decay of protons into neutrons and vica versa by emission of one of W(0), W(+) or W(-)
n -> p by emission of W(-) (then p -> e(-) + antineutrino)
p -> n by emission of W(+) (then n -> positron + neutrino)
