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A-Level Physics - Particles and Radiation Cheat Sheet (DRAFT) by

AQA A-Level Physics Topic 2 - Particles and Radiation; made directly in accordance with the AQA 7408 specification

This is a draft cheat sheet. It is a work in progress and is not finished yet.

Consti­tuents of an Atom

An atom is formed from 3 consti­tuents: protons, neutrons and electrons.
Protons and neutrons (called neutrons) are found in the nucleus at the centre
Electrons orbit around the nucleus in shells­/energy levels.
The diameter of the nucleus is about 1 femtometre (10-15 m)
The diamerer of an atom is roughly 100,000 times larger, or 10-10 m
Specific charge is the charge­-mass ratio, calculated by dividing a particle's charge by its mass
Specific charge (C kg-1) = charge of partic­le/mass of particle

Particle Properties

Particle
Proton
Neutron
Electron
Charge (C)
+1.6×10-19
0
-1.6×10-19
Relative Charge
+1
0
-1
Mass (kg)
1.67×10-27
1.67×10-27
9.11×10-31
Relative Mass
1
1
0.0005
Specific Charge
9.58×107
0
1.76×1011

Atom Notation

Isotopes

Atoms of the same element always have the same number of protons, and therefore the same atomic number
However, they can have different amounts of neutrons, which are called isotopes
We can use isotopes for carbon­-da­ting, a method of estimating the age of living organisms like fossils
Organisms are made of carbon, which has a radioa­ctive isotope (carbo­n-14) and decays at a known half-life once the organism is dead
Therefore we can use the amount of carbon-14 left to determine how old it is by how much carbon remains

Stable and unstable nuclei

The nucleus is held together by the strong nuclear force (one of 4 fundam­ental forces)
It provides an attractive force between nucleons with a range of about 3 femtom­etres ( 3x10-15 m)
This overcomes the repulsive electr­ostatic force exerted by positively charged protons on each other
At distances less than about 0.5 fm the strong nuclear force is repulsive and prevents the nucleus collapsing into a point

Variation of strong nuclear force with distance

Alpha and beta decay

Unstable nuclei have too many proton­s/n­eut­ron­s/both, where the SNF is not enough to keep them stable
They will often decay via α (alpha) or β- (beta minus) emission in order to become stable, where the type of decay is dependent on the number of each nucleon
Alpha decay occurs in large nuclei with too many of both nucleons.
Beta-minus decay occurs in neutro­n-rich nuclei.
Beta-plus decay occurs in neutro­n-d­efi­cient nuclei.
The existence of the neutron was hypoth­esised in the conver­sation of energy law in the beta decay equation

Alpha decay equation

Beta- decay equation

Beta+ decay equaion

Particles and antipa­rticles

For every type of particle, there is a corres­ponding antipa­rticle
Examples of these include:
electron and positron
proton and anitproton
neutron and antineutron
neutrino and antine­utrino

Comparison of partic­les­/an­tip­art­icles

Electron (e^-)
mass=9.11×10-31 kg
rest energy=0.51MeV
relative charge=-1
Positron (e^+)
mass=9.11×10-31 kg
rest energy=0.51MeV
relative charge=+1
Neutron
mass=1.67x10-27
rest energy=940MeV
relative charge=0
Antineutron
mass=1.67x10-27
rest energy=940MeV
relative charge=0
Neutrino
mass=0
relative charge=0
Antineutrino
mass=0
relative charge=0
In short, particles and their corres­ponding antipa­rticles will have the same mass and rest energy, but different relative charges
The antine­utron and antine­utrino symbols are the same as the particle ones but with a line above them

Photon model of Electr­oma­gnetic (EM) Radiation

EM Radiation, or light, travels as small packets of energy known as photons
Photons transfer energy but have no mass themselves
Since EM waves travel at the speed of light and follow Planck's constant, we can use the following equation:
Energy of a photon = (Planck's Constant x Speed)­/Wa­vel­ength

Partic­le/­Ant­ipa­rticle intera­ctions

Pair production is where a photon is converted into an equal amount of matter and antimatter
This only happens when the photon has a energy greater than the total rest energy of both particles, and any excess energy is converted into kinetic energy of the particles.
Annihi­lation is where a particle and its corres­ponding antipa­rticle collide, resulting in both of their masses being converted into energy (in the form of 2 photons moving in opposite directions as to conserve momentum).

Pair Production diagram

Annihi­lation diagram

Fundam­ental Intera­ctions

There are 4 main fundam­ental forces: strong nuclear, weak nuclear, electr­oma­gnetic and gravity.
Forces between particles are caused by exchange particles, which carry energy and momentum between the particles experi­encing the force.
Each fundam­ental force has its own exchange particles.

Particle Intera­ctions

Intera­ction
Exchange Particle
Range (m)
Acts on
Strong
Gluon/­Pions
3x10-15
Hadrons
Weak
W boson (both +/-)
10-18
All particles
Electr­oma­gnetic
Virtual photon (λ)
Infinite
Charged particles
Gravity
Graviton (not on spec)
Infinite
Particles with mass

Feynman Diagrams