Particles Flashcards
Give the overall charge of an atom.
Neutral or zero
Give the overall charge of a nucleus.
Positive
Give the approximate size of an atom.
1 x 10-10 m
Give the approximate size of a nucleus.
1 x 10-15 m
What is found between the nucleus and electrons?
Empty space
In A-Z nuclide notation, what do A and Z represent?
A = nucleon number
Z = proton number / atomic number
Which number, A or Z, defines which element it is?
Z (proton/atomic number)
Give the definition for nucleon number.
Total number of protons and neutrons.
Complete: atoms have equal number of… and …
Protons and electrons.
How do atoms become positive ions?
They lose electrons.
How do atoms become negative ions?
They gain electrons.
How do we work out the mass of an atom, ion or nucleus in kilograms?
Number of nucleons x 1.67 x 10-27
(no need to include electrons)
How do we work out the charge of a nucleus in coulombs?
Number of protons x 1.6 x 10-19
How do we work out the charge of an ion in coulombs?
Relative charge x 1.6 x 10-19
How do we calculate specific charge?
Specific charge = charge / mass = Q /m
What are the units for specific charge?
C kg-1
What are isotopes?
Atoms with the same number of protons and different numbers of neutrons.
True or false: isotopes have different chemical properties.
False
True or false: isotopes have different nuclear stability.
True
Describe the role of the strong nuclear force in nuclear stability.
Balances the electrostatic repulsion between positively charged protons.
- Holds nucleons in an equilibrium position (stops them moving further apart or closer together).
Is the strong nuclear force associated with charge?
No – acts equally between protons and neutrons.
Describe how and explain why the strength of the nuclear force varies with nuclear separation.
Below 0.5 fm -> repulsive –> stops nucleons collapsing into a point.
Between 0.5 – 3.0 fm -> attractive –> binds nucleons.
Beyond 3.0 fm -> zero -> prevents nucleons in different atoms being attracted.
What type of nuclei normally undergo alpha decay?
Massive nuclei.
What is an alpha particle made up of?
2 protons and 2 neutrons (helium nucleus)
What type of nuclei normally undergo beta minus decay?
Neutron-rich nuclei (high neutron to proton ratio).
What is a beta minus particle?
An electron
Describe the change in the nucleons during beta minus decay.
A neutron changes into a proton.
Name the extra particle emitted in beta minus decay.
Electron anti-neutrino
Why did scientists hypothesise the extra particle?
- Total energy after the decay was less than the total energy before.
- Missing energy must be carried away by another particle to conserve energy.
Why was it difficult to detect the extra particle emitted in beta minus decay?
It has no mass and no charge.
What type of nuclei normally undergo gamma decay?
Nuclei that need to lose excess energy.
What is a gamma ray?
High energy EM wave.
Which decays lead to the formation of an atom of a different element? Why?
- Alpha and beta minus decay.
- As the proton number changes.
Describe the duality of EM radiation.
Can behave as a wave or a particle.
What is a photon?
A packet or quantum of EM energy.
(Has no mass and no charge)
Use the data sheet to write down 2 equations for photon energy.
E = hf = hc/ λ
The energy of a photon is directly proportional to…
Frequency
The energy of a photon is inversely proportional to…
Wavelength
Give the definition for the electron-volt.
The energy gained by an electron that is travelling through a potential difference of 1 V.
How do we convert eV to J?
Multiply by 1.6 x 10-19
How do we convert J to eV?
Divide by 1.6 x 10-19
Give two similarities between particles and anti-particles.
Mass and rest energy
Give one difference between particles and anti-particles.
Charge
What is energy-mass equivalence?
- Energy can be converted into mass.
- Mass can be converted into energy.
- Calculated by E= mc2
What happens in pair production?
A gamma photon interacts with a nucleus and the energy of the photon is used to create a particle and anti-particle pair.
Which pair of particles is most likely to be created during pair production? Explain why.
Electron and positron as they have the lowest rest energy.
Why does it need to be a gamma photon for pair production?
Highest energy.
How can we calculate the minimum energy of the photon required for pair production?
Emin = 2 x rest energy of particle/anti-particle
How can you convert MeV into J?
x 106 x 1.6 x 10-19
What happens in annihilation?
A particle meets its equivalent antiparticle.
Their mass is converted into energy in the form of two gamma photons.
Why are two gamma photons produced in annihilation?
They travel in opposite directions to conserve momentum.
How can we calculate the minimum energy of one of the photons produced in annihilation?
Emin = rest energy of particle/anti-particle
Why is Emin = rest energy of particle/anti-particle the minimum energy of the photon during annihilation?
The particle and anti-particle may have additional kinetic energy.
Describe the role of exchange particles in particle interactions.
- Move between particles.
- Give rise to the force between them.
Name the four fundamental forces from strongest to weakest.
- Strong nuclear force (or strong interaction)
- Electromagnetic force
- Weak nuclear force (or weak interaction)
- Gravity
Which of the four fundamental forces do particle physicists normally ignore? Why?
Gravity as it is so weak – negligible effect.
Give the exchange particle for the strong nuclear force.
Pions (π)
Give the exchange particle for the electromagnetic force.
Virtual photons (ϒ)
Give the exchange particle for the weak nuclear force.
W+ and W- bosons
Give the particles affected by the strong nuclear force.
Hadrons
Give the particles affected by the electromagnetic force.
Charged particles
Give the particles affected by the weak nuclear force.
All particles
Give the range of the electromagnetic force.
Infinite
Give the range of the weak nuclear force.
10-18 m
Describe and explain the relationship between the mass of the exchange particle and the range of the force.
Larger mass -> shorter range.
Requires more energy to create -> only exists for shorter time -> shorter distance.
Describe two differences between hadrons and leptons.
- Hadrons experience the strong interaction but leptons do not.
- Hadrons are made up of quarks but leptons are fundamental.
Give two examples of baryons.
3 quarks (or 3 anti-quarks)
Name the only stable baryon.
Proton
Give the baryon number of protons and neutrons.
+1
Give the baryon number of antiprotons and antineutrons.
-1
Data sheet: Give the quark combination for a proton.
uud
Data sheet: Give the quark combination for a neutron.
udd
Why do neutrons have a higher mass than protons?
d quark has a higher mass than u quark
Give two examples of mesons.
Pions and kaons
ive the quark structure of mesons.
A quark and an antiquark
True or false: All mesons are unstable.
True
Name the most stable meson. Explain why.
Pions –> lightest –> lowest energy.
Name the strange meson. Explain what strangeness tells you about quark structure.
Kaons -> contain a strange or anti-strange quark.
How are kaons produced?
By the strong interaction -> produced in pairs of strange particles.
How do kaons decay?
By the weak interaction -> decay into pions.
Give the strangeness of the four kaons: K+, K-, K0, _K0
K+= +1 K-= -1 K0= +1 _K0=-1
Why do kaons have a higher mass than pions?
s squark has higher mass than u and d quarks
Why do kaons have a higher mass than pions?
s squark has higher mass than u and d quarks
Give three examples of leptons.
Electrons, muons and neutrinos
Compare electrons and muons.
- Same charge.
- Muons are heavier than electrons.
- Electrons are stable, but muons decay into electrons.
Give the mass and charge of neutrinos.
Zero mass and zero charge.
Describe the four particle interaction conservation laws.
Baryon number (B) – always conserved.
Charge (Q) – always conserved.
Lepton number (L) - Le and Lµ always conserved separately.
Strangeness (S) – always conserved in strong interaction, in weak interaction changes by -1, 0 or +1.
State two other quantities that are conserved in all interactions.
Energy and momentum.
Use the conservation laws to explain why strange particles are always produced in pairs.
Strange particles produced by strong interaction -> strangeness must be conserved -> pair have equal and opposite strangeness to cancel to zero.
In which type of interaction can quark character change?
Weak interaction only.
Work out the change of quark in beta minus decay.
d -> u
Work out the change of quark in beta plus decay.
u -> d
What is quark confinement?
- Not possible for quarks to exist in isolation.
- The energy supplied to try separate a quark would cause pair production of another quark and anti-quark pair.
Why does particle physics research rely on the collaborative efforts of large teams of scientists and engineers?
- Particle accelerators are very expensive -> collaboration helps to spread the cost.
- Many skills and disciplines required.
- Lots of data to process.
- Results of experiments must be independently peer reviewed before confirmed.
