chapter 17 - matter Flashcards
What was the model of the atom before the nuclear model?
A positive mass with electrons in it
What was Rutherford’s experiment?
Firing a beam of alpha particles at a thin gold foil and recording the number of particles scattered at different angles.
What was the unexpected result in Rutherford’s experiment?
Some alpha particles were scattered at angles greater than ninety degrees, which could only happen if they hit something more massive than themselves
What does it mean that most alpha particles went straight through the gold foil?
The atom must be mostly empty space
What does it mean that a small number of alpha particles are deflected back through significant angles (more than ninety degrees)?
The centre of the atom must be very massive but very small
What does it mean that the alpha particles were repelled from the nucleus?
The nucleus has a positive charge
What is the name for protons and neutrons?
nucleons
What is the nuclear model of the atom?
A nucleus made up of protons and neutrons, orbited by electrons
Ho do you estimate the closest approach of a particle scattered by a nucleus?
The initial kinetic energy is known
The particle will stop briefly when initial kinetic energy is equal to potential energy from the charged nucleus
Use this to find r
What is the equation for electrical potential energy?
EP = Q1Q2/4πϵ0r
charge1 x charge2/4π x permittivity of free space x distance between particle and nucleus centres
What is ϵ0?
The permittivity of free space
8.9 x10^-12
How do you find the charge on a nucleus?
Multiply the number of protons (proton number) by e
1.6 x10^-19 C
What is Z?
The proton number of an element
What is the charge on a proton?
+e
1.6 x10^-19 C
What makes a particle a hadron?
- Feels the strong interaction
- Not fundamental, made up of quarks
- Behave like matter, obey the Pauli exclusion principle and cannot occupy the same space, giving them a volume
What are the two types of hadron?
Baryons and mesons
Name two hadrons
proton, neutron
Name two baryons
proton, neutron
What is the only stable baryon?
Proton
What do all baryons decay to?
Protons
What is the number of baryons in a reaction called?
The Baryon number
What has to happen to the baryon number in a reaction?
It is conserved, so cannot change on either side
What is the Baryon number of a proton?
+1
What is the Baryon number of a neutron?
+1
What makes a particle a lepton?
- Fundamental particles
- Do not feel the strong interaction
How can leptons interact with other particles?
Via the weak interaction or gravity
or electromagnetic force if they are charged
Name the three kinds of lepton
electron, muon tau
How can muon and tau leptons be described?
Like heavy, unstable electrons
What will muon and tau leptons decay into?
electrons
What are the three kinds of neutrino?
electron, muon, tau
Are neutrinos leptons?
yes
Why are neutrinos so hard to detect?
- No electric charge
- Zero or almost zero mass
- Interact very weakly with matter
Why are there three different lepton numbers for a particle reaction?
One lepton number for electron, muon and tau leptons
What is the lepton number of an electron?
+1
What is the lepton number of a muon neutrino?
+1
What is the formula for the decay of a neutron?
neutron –> proton + electron + electron antineutrino
Why is this formula wrong?
neutron –> proton + electron
The lepton number is not conserved
0 –> 1
What is an antiparticle?
A particle with the same mass but an opposite charge to an existing particle
What is an antielectron?
A positron
What is the baryon number of an antiproton?
-1
What is the lepton number of a positron?
-1
What is the lepton number of a tau antineutrino?
-1
What has to be made when energy is converted to mass?
An equal amount of matter and antimatter has to be made
What is the equation which shows that matter can be converted to energy and energy can be converted to mass?
E = mc^2
What is pair production?
When a particle and its antiparticle are created from the energy of a gamma ray photon, and travel away in opposite directions
What is the minimum energy a gamma ray photon can have for pair production to happen?
An energy equal to the rest energy (mc^2) of two particles
What will happen if pair production happens at the minimum possible energy?
The particles will annihilate each other immediately as they have no extra energy (which would be the kinetic energy needed to move away from each other)
What pair is usually produced in pair production?
Why?
An electron positron pair
Because their masses are lowest, so the least amount of energy is needed
Why are particle tracks usually curved in pair production experiments?
Why are they curved in opposite directions?
There is usually a magnetic field present in particle physics experiments
The opposite charge on the particle and antiparticle means they will curve in different directions
What is the opposite of pair production?
Annihilation
What happens when a particle meets its antiparticle?
They annihilate
Their mass is converted to energy, carried away by a pair of gamma ray photons moving in opposite directions
What are quarks?
Fundamental particles which make up hadrons
What kind of quarks are nucleons made up of?
up and down
Other than up and down, what are the other kinds of quark?
strange
top, bottom, charm
What kind of particles do strange quarks let you make?
Particles with the property ‘strangeness’
What are the antiparticles of hadrons made up from?
Antiquarks
What is the charge on an up quark?
+ 2/3
What is the charge on a down quark?
- 1/3
What is the charge on a strange quark?
- 1/3
What is the baryon number of up, down and strange quarks?
+ 1/3
What is the strangeness of up and down quarks?
0
What is the strangeness of a strange quark?
-1
What is the charge on an anti-down quark?
+ 1/3
What is the charge on an anti-up quark?
- 2/3
What is the charge on an anti-strange quark?
+ 1/3
What is the baryon number of anti-up, anti-down and anti-strange quarks?
- 1/3
What is the strangeness of anti-up and anti-down quarks?
0
What is the strangeness of an anti-strange quark?
+1
How many quarks make up a baryon?
3
What quarks is a proton made up of?
up, up, down
uud
What quarks is a neutron made up of?
up, down, down
udd
What quarks is an anti-proton made up of?
anti-up, anti-up, anti-down
What quarks is an anti-neutron made up of?
anti-up, anti-down, anti-down
Where did the evidence for quarks come from?
Hitting protons with high energy electrons
Electron scattering showed three concentrations of charge inside the proton
Can you ever have free quarks individually?
no
What holds hadrons together?
gluons/ the gluon field
What are exchange particles called?
gauge bosons
What are the four fundamental forces and the gauge bosons which cause them?
gravity - gravitons (?)
strong force - gluons
weak force - W0 W- W+
electromagnetic - photons
Why can’t quarks be separated?
The more energy that is put into the gluon field (in trying to pull apart quarks), the stronger the field gets and the more attraction the quarks feel to each other
When the energy is sufficient it goes to producing a quark-antiquark pair
This is called QUARK CONFINEMENT
What are particle accelerators used for?
Causing high energy particle collisions
How do linear particle accelerators work?
There is a straight tube containing a series of electrodes
An alternating current is applied to the electrons and timed so that the charged particle is always repelled from the electrode behind it and attracted to the electrode in front of it
The particles speed increases each time it passes an electrode until it collides with a fixed target at the end
What is a cyclotron?
A circular particle accelerator
How do cyclotrons work?
Two semi-circular electrodes accelerate particles across a gap
An alternating pd means charged particles are attracted from one side to the other and their energy increases
A magnetic field keeps them moving in a circle
Particles spiral outwards because of the electrical and magnetic fields as their energy increases
What are synchotrons used for?
Producing very high energy particle beams
Ho do synchotrons work?
Electromagnets keep particles moving in a circular path in focused beams
The particles travel between electrodes, timed so that they are attracted to the electrode in front of them and repelled from the electrode behind them, causing them to accelerate
Two beams with oppositely charged particles can be accelerated at the same time and made to collide for experiments
What happens when an object travels at speeds approaching the speed of light?
It has a greater kinetic energy
Because energy is equivalent to mass, the object’s mass increases
(this happens to all objects with kinetic energy but is only noticeable at speeds approaching the speed of light)
What is the formula for finding the relativistic factor?
relativistic factor = total energy / rest energy
What is the symbol for the relativistic factor?
γ
What is the name for the lowest possible energy level an electron in an atom can exist at?
Ground-state
n =1
How do the energies of electrons in atoms exist?
In discrete energy levels
What is the name for the number of an electron energy level?
The principal quantum number
eg for n = 2 the principal quantum number is 2
How can electrons move down an energy level?
By emitting a photon
How can electrons move up an energy level?
By absorbing a photon
What is an electron volt?
The energy carried by an electron accelerated through a potential difference of one volt
What is one electron volt in joules?
1.6 x10^-19J
Why are electron energies negative?
The energy given is the energy which would be needed for the electron to escape the atom
What potential energy must an electron have to leave an atom and ionise it?
zero potential energy
no longer bound
What is the energy carried by a photon emmitted from an atom equal to?
The difference between the two energy levels the electron has moved between
What is the formula for the energy of a photon emitted from an atom?
ΔE = E2 - E1 =hf =hc/λ
What are two fomulae for photon energy?
E = hf E = hc/λ
What is h?
Planck’s constant
What kind of particle obeys the pauli exclusion principle?
fermions
Are protons and neutrons fermions?
yes
Are electrons fermions?
yes
What is the Pauli exclusion principle?
No two fermions can be in exactly the same quantum state at the same time
What does the Pauli exclusion principle mean for electron energy levels?
No more than two electrons can be in the same energy level at the same time
