Probing the Heart of the Matter Flashcards
Original idea of fundamental particles:
protons, neutrons, electrons
Current standard model of particle physics:
there are 12 fundamental particles divided into two groups: quarks and leptons
What are the 2 groups of the fundamental particles?
quarks and leptons
How many quarks and leptons are there?
12 in total; 6 quarks, 6 leptons
What are the 6 quarks and their corresponding charges?
Up, +2/3 Down, -1/3 Charm, +2/3 Strange, -1/3 Top, +2/3 Bottom, -1/3
What are the 6 leptons and their corresponding charges?
Electron, -1 Electron neutrino, 0 Tau, -1 Tau neutrino, 0 Muon, -1 Muon neutrino, 0
What quarks is a proton made up of?
up, up, down
What quarks is a neutron made up of?
up, down, down
What are the 4 fundamental forces?
- gravity, acts between all objects
- electromagnetic, acts between charged objects
- strong force, acts between quarks
- weak force, acts between all fundamental particles
What is the range of a force?
the maximum distance by which two objects can be separated and still feel the force acting
What is the relative strength of a force?
a way of comparing how big an influence each force would have on the same pair of objects
What is a hadron?
any particle that is composed of quarks - a group of quarks
What are the two types of hadrons?
baryons and mesons
What is a baryon?
a group of 3 quarks (3 syllables)
What is a meson?
a group of 2 quarks (2 syllables)
Can fundamental particles be found alone?
Quarks will always be found in groups due to the strong force pulling quarks into groups from which they can never be separated
Leptons can exist on their own
What are mesons always made up of?
a quark and an antiquark
Antimatter
for every type of particle of matter that exists, there is a corresponding particle of anti-matter with the same mass but opposite charge
For every quark…
there is an anti-quark
For every lepton…
there is an anti-lepton
What are the 6 anti-quarks? Charges?
- Anti-up, -2/3
- Anti-down, +1/3
- Anti-charm, -2/3
- Anti-strange, +1/3
- Anti-top, -2/3
- Anti-bottom, +1/3
What are the 6 anti-leptons? Charge?
- Positron, +1
- Anti electron neutrino, 0
- Anti-muon, +1
- Anti-muon neutrino, 0
- Anti-tau, +1
- Anti-tau neutrino, 0
What happens when matter and anti-matter meet?
Annihilation, giving off electromagnetic radiation - gamma rays
Einstein’s equation
E=mc^2
What things must be conserved in a reaction?
- charge
- baryon number
- lepton number
- strangeness
- mass/energy (Einstein’s equation)
- momentum
Cosmology
the way the Universe has evolved since the Big Bang
particle physics
the standard model of the fundamental particles and the forces that act between them
What conclusions can be drawn from Rutherford’s experiment?
- there is a concentrated area of charge (we don’t know +ive or -ive yet)
- the mass is concentrated in the centre
- most of the atom is empty space
Rutherford’s experiment
Fired alpha particles at a sheet of gold foil. Most of the particles passed straight through without any deflection. Some particles are deflected through small angles. Few have large angle scatterings and even fewer back scatter completely
What can we say about the electrostatic forces on two objects of equal mass hanging a distance, d apart?
The pair of electrostatic forces is an example of Newton’s third law:
- they are equal in magnitude and opposite in direction
- they act on different bodies
- they are the same type of force
Coulomb’s law forces on one sphere
- the balls weight, mg
- the electrostatic force, F
- the tension of the string, T
What did coulomb investigate?
The force between two charged spheres
Coulomb’s law is an example of an inverse square law…
The force decreases in inverse proportion to the square of the distance between two charged spheres
Electric field
A region in which a charged object experiences a force
Electric field strength
Force per unit charge
What can a diagram of an electric field tell us?
- the strength of the field (closer together means stronger)
- the direction of the field
Field lines from a positive charge
Equidistant
At least 4
Arrows pointing outwards (give out positive energy)
Field lines from a negative charge
Equidistant
At least 4
Arrows pointing towards the charge (contain negative energy)
Closer to a point charge…
…More field lines so a stronger force
How do we prove the structure of atoms/ deduce the nature of sub-atomic particles? Who was the first person to do it?
By using collisions, Rutherford
What is a cloud chamber?
A device that can show up the tracks of charged sub-atomic particles
By analysing the tracks left in particle detectors…
Particle physicists can make deductions about the mass and energy of sub-atomic particles using the conservation of momentum and energy laws
Momentum is conserved in…
All collisions
What is an elastic collision?
Kinetic energy and momentum is conserved
Wave- particle duality
Sometimes has wave properties and sometimes has particle properties
How can we use diffraction of x-rays?
It can enable archaeologists to probe the structure of materials - from this we saw that electron beams behave like wavicles
What is electron diffraction used for?
To probe matter on an even smaller scale e.g. nuclei and presence of quarks
What happens when you place polycrystalline graphite in the electron beam?
You get an electron diffraction pattern - normally creates rings with light and dark spots (just like a laser beam through a diffraction grating)
What does the de Broglie wavelength allow us to do?
Convert from momentum - a particle property to wavelength - a wave property
Diffraction
The spreading out of waves as they pass through a gap
How are bright spots created in diffraction?
When many coherent waves interfere and supervise constructively and arrive at the same point together
Example of electron wavicality
Move freely as a beam and strike the screen at the end of the tube they behave as particles
When they pass through the graphite and diffract they act as waves
How is the electron beam produced??
In an electron gun electrons are released from a heated cathode by thermionic emission, then accelerated by a high voltage applied between cathode and anode. They pass through a hole in the anode and travel at a constant speed as they pass through the diffracting object then strike the fluorescent screen.
So long as there is no other energy transfer taking place what is the equation linking kinetic energy and the electrons?
KE = eV
e is the electrons charge
V is the potential difference between cathode and anode
For speeds approaching the speed of light…
…we need to use relativistic equations (normally non-relativistic is fine)
How can we alter the diffraction pattern of electrons?
- by changing the electrons de Broglie wavelength
- by changing the size of the particles (smaller particles creates wider rings)
Particle physics is often known as…
…high energy physics
Why do we use high energies?
- if you want a positively charged particle to get closer the the nucleus of an atom, a lot of energy must be supplied
- the more energy that can be given to particles the shorter their wavelength and the smaller the detail that can be investigated using them as a probe
- by colliding particles together the energy is re-distributed producing new particles. The higher the collision energy the larger the mass of the particles that can be produced
LINAC
A LINAC accelerates charged particles to very high energies without needing high voltages
How does a LINAC work?
The charged particles travel along a series of tubes separated by gaps and are given voltage kicks at each gap. When the particle emerges from the tube it repels the one it’s emerged from and attracts the next tube so is accelerated. Voltage swaps. Travels at a constant speed in the tube. No electric field inside the tubes
Why are LINACs so long?
Because the particles are travelling faster but must spend the same amount of time in each tube
Pros and cons of LINACs
Pros:
-don’t need high voltages
Cons:
-each accelerating section is only used once so to achieve high energies machines must be longer and are therefore more expensive
How does a cyclotron work?
There are 2 dees with a gap between them, the charged particles are accelerated across the gap between them by an electric field caused by a alternating potential difference. Rather than letting the particles move off in a straight line a magnetic field is used to bring the particles round in a circle to be accelerated across the gap again every half turn. 2 particles can be sent in opposite directions and then collided.
Circular motion
How particles can be steered in a circular path
To produce circular motion…
…a resultant force must be acting on it
An object in circular motion…
…is constantly changing direction and therefore constantly accelerating
Centripetal force
Any force that acts towards the centre of a circle
A charged particle moving in a magnetic field experiences a force at…
…right angles to its direction of motion - this is how a magnetic field steers an electron beam into a circular path for a particle accelerator
Angular displacement
The angle through which the object has moved relative to some fixed direction
Angular velocity
Just like linear velocity is related to linear displacement and time. The rate at which many devices turn in a circle can be written as angular velocity
How can you determine the direction of a charged particle in a magnetic field?
By using Flemings left hand rule
thuMb - Movement
First finger - Field
seCond finger - Current
Key points
- only charged particles cause ionisation
- only charged particles make tracks in detectors
- the direction of a particles track in a magnetic field indicates the sign of its charge
- the radius of a track provides information on the particles charge and momentum
- momentum is always conserved
- energy is always conserved in collisions and interactions (Einstein’s equation)
The kinetic energy of an electron accelerated through 1V is…
1eV
In a cloud chamber…
Particles will be faster if they’re lighter. In cloud chambers the particles are being ionised. Thicker tracks are left if the particle is more massive and moving slower as they’re more ionised. During each ionising collision particles lose kinetic energy so the tracks Spurs inwards due to being more affected by the field
