Section 1 - Particles

Question 1

What are the relative charges and the relative masses of protons, neutrons and electrons?

Answer 1

Protons
-Relative charge: +1
-Relative mass: 1
Neutrons
-Relative charge: 0
-Relative mass: 1
Electrons
-Relative charge: -1
-Relative mass: 0.0005

Question 2

What are isotopes?

Answer 2

Atoms with the same number of protons but different numbers of neutrons
Changing the number of neutrons doesn’t affect the atom’s chemical properties, however they do affect the stability of the nucleus, which may mean an atom is radioactive and will decay overtime into different nuclei that are more stable

Question 3

How can radioactive isotopes be used to find out how old something is?

Answer 3

1. All living things contain the same percentage of carbon-14 taken in from the atmosphere
2. When they die, the amount of carbon-14 in them decreases over time as it decays to form stable elements
3. Scientists can calculate the approximate age of archaeological finds made from dead organic matter, by using the isotopic data to find the percentage of carbon-14

Question 4

What is the specific charge of a particle?

Answer 4

The ratio of its charge to its mass, given in coulombs per kg
Specific charge = Charge / Mass

Question 5

What is the strong nuclear force?

Answer 5

1. To hold nucleons together, it must be an attractive force that’s stronger than the electrostatic force
2. Experiments have shown it has a very short range, it can only hold nucleons together when they’re separated by up to a few femtometres (1x10^-15 m)
   3.The strength falls quickly after this distance
3. Experiments also show that the strong nuclear force works equally between all nucleons
4. At very small separations, the strong nuclear force must be repulsive otherwise it would crush the nucleus to a point

Question 6

What does the graph of the strong nuclear force look like?

Answer 6

Up to 0.5fm - Repulsive
0.5fm - 3fm - Attractive
3fm+ - Falls rapidly towards 0

Question 7

When does alpha emission occur?

Answer 7

1. Only happens in very big nuclei, like uranium and radium
2. The nuclei of these atoms are just too massive for the strong nuclear force to keep them stable

Question 8

What happens when an alpha particle is emitted?

Answer 8

The proton number decreases by 2 and the nucleon number decreases by 4

Question 9

How do we know the range of alpha particles?

Answer 9

They have a very short range- only a few centimeters in air
1. This can be seen by observing the tracks left by alpha particles in a cloud chamber
2. You can also use a Geiger counter by bringing it close to the alpha source and then moving it away slowly and observing how the count rate drops

Question 10

How does Beta- emission occur?

Answer 10

1.An electron is emitted from the nucleus along with an anti-neutrino.
2.This happens in isotopes that are unstable due to being neutron-rich.
3. When a nucleus ejects a beta-particle, one of the neutrons in the nucleus is changed into a proton

Question 11

What happens when a beta particle is emitted?

Answer 11

The proton number increases by 1 and the nucleon number stays the same.
The anti-neutrino that’s also released carries away some energy and momentum for conservation laws

Question 12

How were neutrinos first hypothesised?

Answer 12

Observations of beta decay
1. It was originally thought that only an electron was emitted
2. Observations showed that the energy of the particles after the decay was less than it was before, which didn’t fit with the principles of conservation of energy
3. Wolfgang Pauli suggested that another particle was emitted as well that carried away the missing energy
4. The particle had to be neutral, and had to have zero or almost zero mass as it had never been detected
5. The particle was named the neutrino

Question 13

What are photons?

Answer 13

Packets of electromagnetic radiation

Question 14

What are the trends in the electromagnetic spectrum?

Answer 14

From radio waves to gamma rays, wavelength decreases and frequency increases

Question 15

What is an anti-particle?

Answer 15

Every particle has an anti-particle which has the same mass and rest energies but opposite charges

Question 16

How can you create matter and antimatter from energy?

Answer 16

From Einstein’s theory of relativity, energy can turn into mass and mass can turn into energy. The rest energy of a particle is the energy equivalent of the particle’s mass, measured in MeV. E=mc^2

Question 17

When can pair production happen?

Answer 17

1. If one photon has enough energy to produce that much mass - only gamma ray photons have enough
2. Tends to happen near a nucleus which helps conserve momentum

Question 18

What is pair production?

Answer 18

When a particle-antiparticle pair is produced from a single photon
1. Usually get an electron-positron pair because they have a relatively low mass
2. The minimum energy for a photon to undergo pair production is the total rest energies of the particles produced (hf x 2 or 2E)

Question 19

What is annihilation?

Answer 19

When a particle meets its antiparticle and all the mass of the particle and antiparticle gets converted back to energy. Antiparticles can usually only exist for a fraction of a second so you don’t get them in ordinary matter
See physical flashcards for diagram

Question 20

What are gauge bosons?

Answer 20

Exchange virtual particles that only exist for a very short time
1. You can’t have instantaneous action at a distance
2. When 2 particles interact something must happen to let one particle know the other one us there

Question 21

What are the 4 fundamental forces?

Answer 21

1. Electromagnetic
2. Weak
3. Strong
4. Gravity

Question 22

What are the exchange particles and the particles affected when there is the electromagnetic force?

Answer 22

Gauge boson - Virtual photon γ
Particles affected - Charged particles only

Question 23

What are the exchange particles and the particles affected when there is the weak force?

Answer 23

Gauge bosons - W+, W-
Particles affected - All types

Question 24

What are the exchange particles and the particles affected when there is the strong force?

Answer 24

Gauge bosons - Pions (π+. π- and π0)
Particles affected - Hadrons only

Question 25

How does the mass of the gauge boson affect the range of the force?

Answer 25

The larger the mass of the gauge boson, the shorter the range of the force
1. The W bosons have a mass of about 100 times that of a proton which means the weak force has a very short range. Creating a virtual W particle uses so much energy that it can only exist for a very short time and can’t travel far
2. The photon has zero mass so the electromagnetic force has infinite range

Question 26

What are hadrons?

Answer 26

1. The particles which can interact through the strong force.
2. They aren’t fundamental particles, as they’re made up of quarks
3. There are 2 types of hadrons - baryons and mesons, which are classified according to the quarks they’re made up of

Question 27

How do baryons become stable?

Answer 27

1. Protons are the only stable baryons and so don’t decay
2. All other baryons decay into other particles, depending on what they started as, but a proton is always included

Question 28

What is a baryon number?

Answer 28

A quantum number that must be conserved in any interaction - can only take on a certain set of values
When an interaction happens, the baryon number on either side of the interaction has to be the same, which means you can use this to predict whether a reaction will happen

Question 29

What are mesons?

Answer 29

1. A type of hadron
2. They are all unstable and have a baryon number of 0
3. Pions (π mesons) are the lightest mesons - in high-energy particle collisions
4. Kaons (K-mesons) are heavier and more unstable than pions - have a very short lifetime and decay into pions
5. Pions and kaons were discovered in cosmic rays
6. Interact with baryons through the strong force

Question 30

What are leptons?

Answer 30

1. Fundamental particles that don’t interact through the strong nuclear force - mainly interact through the weak interaction
2. Electrons are stable
3. Muons are heavier electrons that are unstable and eventually decay into ordinary electrons
4. Neutrinos are nearly massless and so only interact through the weak interaction

Question 31

What is a lepton number?

Answer 31

A conserved quantum number in any interaction. The electron and muon leptons have to be conserved separately

Question 32

What are quarks?

Answer 32

The building blocks for hadrons

Question 33

When is strangeness conserved?

Answer 33

1. Strange particles (kaons) are created via the strong interaction but decay via the weak interaction
2. Strangeness is conserved in the strong interaction but not in the weak interaction
3. Strange particles are always produced in pairs: one with strangeness of +1 and one with strangeness -1 so the overall strangeness is 0

Question 34

What was the evidence for quarks?

Answer 34

Hitting protons with high-energy electrons, the way the electrons scattered showed that there were three concentrations of charge (quarks) inside the proton

Question 35

What are the quark compositions of protons and neutrons?

Answer 35

Protons: udu
Neutrons: dud

Question 36

What is the quark structure of mesons?

Answer 36

A quark and an anti-quark, kaons have strangeness and so must have a strange quark
π0 is the antiparticle of itself (2 compositions)

Question 37

What changes quark type?

Answer 37

The weak interaction can change quark type for example in B- decay when a neutron changes into a proton
Some unstable isotopes like carbon-11 decay by B+ emission and a proton changes to a neutron

Question 38

What 4 properties are conserved in particle interactions?

Answer 38

1. Charge
2. Baryon number
3. Strangeness in strong interactions
4. Lepton numbers separately

Question 39

What is quark confinement?

Answer 39

Quarks cannot exist freely, on their own

Question 40

Where is there collaboration in the discovery of new particles?

Answer 40

Experiments in particle physics require particles travelling at very high speeds which can only be achieved using particle accelerators, which are very expensive to run and build so large groups of scientists and engineers from all around the world have to collaborate to fund these experiments