3.2.1 PARTICLES

Question 1

What are the four fundamental forces?

Answer 1

Gravity
Electromagnetic
Strong nuclear force
Weak nuclear force

Question 2

What is the specific charge of a particle?

Answer 2

The charge per unit mass CKg^-1

Question 3

What does the strong nuclear force act on?

Answer 3

Quarks

Question 4

Describe the effects of the strong nuclear force with a range of values

Answer 4

under 0.5 fm, extremely repulsive, over 3/4 fm significantly less attractive. Between that range is very attractive

Question 5

What is the difference between the effect of the strong nuclear force between 2 protons, 2 neutrons and a proton + neutron?

Answer 5

No difference, acts the same

Question 6

What is the importance of the strong nuclear force in nuclear stability?

Answer 6

Equilibrium point of the strong force - so nucleons don’t move closer to/further from each other

Question 7

Why does nuclear fusion require a lot of energy?

Answer 7

Large amount of energy needed to get two nuclei close enough and overcome the strong electrostatic force, then close enough within the range of the strong force to fuse them together

Question 8

What is pair production, and describe the conditions for it to take place

Answer 8

A photon interacts with the strong electric field around the nucleus and turns into a particle and its corresponding particle.

The energy of the photon is converted to mass in the particles, and Ek (if there is remaining) in order to obey conservation laws

Question 9

What is rest energy?

Answer 9

The energy stored in the mass of a particle

E = mc^2

Question 10

What is anhilitation?

Answer 10

When a subatomic particle collides with its corresponding antiparticle, producing 2 other particles (e.g. photons) which travel in opposite directions

(as a single photon only would take away momentum which isn’t allowed, as no outside forces act)

Question 11

Describe the difference it would make to the particles produced during annihilation if the annihilating particles had a greater amount of energy

Answer 11

They could be more numerous
They could be more massive (e.g. muons)
They could have a greater Ek

Question 12

What is an electronvolt?

Answer 12

The energy transferred by an electron accelerated over a potential difference of 1 volt

1eV = 1.6x10^-19 J

• There will always be fewer joules

Question 13

Describe why pair production cannot occur without the presence of a heavy nucleus

Answer 13

In a vacuum, a photon travelling with a momentum p1 is converted into an electron and positron with momentums p’ and p’’ respectively, which are equal and opposite, travelling at 90 degrees to the photon, conservation of momentum isn’t obeyed, as the initial momentum = pi, while final momentum = 0

A nucleus is required to interact with the photon to absorb/donate momentum in such a way that momentum is conserved

Question 14

Describe the possible outcomes of pair production based on the energy of the photon

Answer 14

hf < 2mc^2 - No particles can be produced
hf = 2mc^2 - Particle and antiparticle produced w/o Ek
hf > 2mc^2 - Particle and antiparticle produced with remaining energy converted to Ek

Question 15

Name 4 exchange particles and the forces they govern

Answer 15

Virtual photon - EM force
Pion - Strong nuclear force
W+/- Boson [also Z0 ] - Weak force
Graviton [theorised] - Gravity

Question 16

What is an exchange particle/gauge boson?

Answer 16

Particles exchanged between two other particles in order to make a force happen, by transferring energy/momentum (sometimes charge)

Question 17

What are the properties of each exchange particle

Answer 17

• Virtual photon - zero rest mass, zero electric
  charge, stable
• W bosons - non zero rest mass, can be
  charged, very short range, no more than
  0.001fm
• Pion

Question 18

Describe the two categories that particles can be split into

Answer 18

Hadrons - are comprised of quarks, and experience the strong nuclear force

Leptons - are fundamental particles that don’t experience the strong nuclear force

Question 19

Define baryons and mesons, and give examples of both

Answer 19

Baryons are Hadrons made up of 3 quarks, e.g. Protons, Neutrons
B = 1

Mesons are Hadrons made up of 2 quarks, with a quark antiquark combination
B - 0

Question 20

What is the baryon number of a quark?

Answer 20

1/3

Question 21

Define an antiparticle

Answer 21

A subatomic particle having the same mass as another but having an opposite charge/lepton number/ baryon number or strangeness

Question 22

What properties are always conserved in particle interactions, and which aren’t

Answer 22

Baryon number, lepton number and charge are always conserved, however strangeness in only conserved during the strong interaction

Question 23

Are the lepton numbers of the electron and muon the same?

Answer 23

Yes but no, they both have a lepton number of +1, but lepton number is specific to a “family”

a muon has a muon lepton number of +1, but an electron lepton number of 0

Question 24

Are lepton and baryon number always integers?

Answer 24

Yes

Question 25

State the quark composition of an antibaryon

Answer 25

Antiquark antiquark antiquark

Question 26

Using the beta minus decay of carbon 14 into nitrogen 14 , explain the purpose of the neutrino

Answer 26

6C —- 7N + e- + ν’e
(only showing atomic number)

The antielectron neutrino conserved lepton family number, as LHS lepton number = 0, and e- has an electron lepton number of 1

Question 27

What are the properties of an antineutrino

Answer 27

Lepton number -1,
Zero mass
Zero charge

Question 28

What are the charges on different quarks

Answer 28

Up 2/3
Down -1/3
Strange -1/3

Question 29

State the quark compositions of a proton and a neutron

Answer 29

Proton - uud
Neuton - udd

Question 30

State the quark composition of different pions

Answer 30

π⁺ = ud’
π⁻ = u’d
π⁰ = uu’ dd’ (quark antiquark pair)

Question 31

State the quark composition of different Kaons

Answer 31

K⁺ = us’
K⁻ = su’
K⁰ = ds’ antikaons are opp

Question 32

What are the rules for strangeness in weal interactions?

Answer 32

It can only change by 1 (or zero), and can’t change by 2 or more

Question 33

What do kaons decay into?

Answer 33

Pions, or a muon and antineutrino, or an antimuon and neutrino

Question 34

What do different pions decay into?

Answer 34

Charged pions can decay into a muon and antineutrino (+ anticombo)

A neutral pion decays into high energy photons

Question 35

What do muons and antimuons decay into?

Answer 35

µ → e + νe + νµ

(anti for anti)

Question 36

How are strange particles produced and how do they decay?

Answer 36

They are produced via the strong interaction and decay via the weak interaction

Question 37

How is knowledge validated in particle physics?

Answer 37

Collaborative
efforts of large teams of scientists and engineers

Question 38

Describe the change in quark character in beta plus and minus decays

Answer 38

p quark changes into down quark with the emission of a positron and a neutrino, while in beta minus decay a down quark changes into a up quark with the emission of an electron and an anti-neutrino

Question 39

How are quarks held together in the nucleus?

Answer 39

By the strong nuclear force

Question 40

Describe ways a charged particle can lose energy to emit em radiation

Answer 40

A fast moving electron is stopped, e.g. in an X ray tube

An electron is excited

Question 41

What is transferred between 2 particles exerting equal and opposite forces on each other?

Answer 41

Momentum

Question 42

Examples of neutrino/antineutrino interactions

Answer 42

Neutrino + neutron — Proton + electron
Antineutrino + proton — neutron + positron

Question 43

What can leptons and antileptons interact to produce?

Answer 43

Hadrons, by making quarks when they annihilate