AS Particles Re-cap The stuff what you needs to know…

The atom

The Structure of Matter Atom: Nucleus and Electrons Nucleus: Protons and Neutrons (Nucleons) Nucleon: 3 Quarks |  m  | |  m  | |  m  |

ISOTOPES of an element, have the same number of protons, but a different number of neutrons in the nucleus. Specific Charge (Charge to mass ratio): of a charged particle is defined as its charge (C) divided by its mass (kg). Typical value 10 7 CKg -1

When the electrostatic force exceeds the strong nuclear force the nucleus is unstable and can 'fly' apart or decay.

Three Types of Radioactive Decay: 1. Alpha

Beta Radiation: Consists of fast moving electrons.

Photons Speed of light = c = 3 x 10 8 ms -1 Wavelengths typically in nanometres (10 -9 m) Frequencies Hz

Light can exhibit properties of both waves and particles. This property is referred to as wave-particle duality. h = 6.63 x Js Power of a beam (W or Js-1) = nhf

Particles and Antiparticles An antiparticle is a particle that has the: same rest mass and if charged the equal and opposite charge to the corresponding particle. When a particle and the corresponding antiparticle meet, they annihilate each other, converting their total mass into photons (Energy).

When a particle is stationary it has a rest mass or mo, which corresponds to its rest energy, locked up as mass. E o = m o c2 Whenever a particle and antiparticle meet and annihilate each other this rest energy is unlocked. The energy of a particle is often expressed in millions of electron volts (MeV) 1 MeV = 1.6 x J 1 ev = 1.6 x J

Annihilation Two photons produced. Minimum Energy of each photon produced hf min = E o Pair Production A photon creates a particle and corresponding antiparticle, each of rest energy, Eo, and vanishes in the process. Minimum energy required = 2E o = hf min

The Electromagnetic force Mediating particle The virtual photon. The quantity that varies Also is time Weak Nuclear Force There must be a different force at work in the nucleus, weaker than the strong nuclear force, otherwise it would affect stable nuclei, hence called the WEAK NUCLEAR FORCE. Responsible for decays.

Beta decay: You must be also able to describe beta positive decay, in which a proton rich nucleus will seek to decay.

Electron Capture Another way that a positive nuclei can become less positive.

Elementary Particles All ordinary nuclear matter is made out of quarks: Up-Quark Down-Quark (charge +2/3) (charge -1/3) In particular: Proton uud  charge +1 Nucleons Neutron udd  charge 0 (composite particles)

Hadrons Experience the strong nuclear force: Quarks are the constituents of hadrons There are two types of hadrons: Baryons made out of three quarks, include protons in their decay products. Mesons made out of one quark and one antiquark. Do not include protons in their decay products.

So decays can be expressed in quark changes.

Leptons Leptons - particles and antiparticles that do not interact through the strong nuclear force (interaction)

Rules 1.Conservation of charge Q. 2.Conservation of Lepton number. a.Overall b.Electron and Muon Branches 3.Conservation of strangeness. (Conserved in Strong interactions though not necessarily weak/decays) 4. Conservation of Baryon number.

The Big Question Why are hadrons as heavy as they are? Why is the proton mass (10) kg? Part of the answer: Because they are made of constituents that have mass There is also binding energy  Forces

The Fundamental Forces in Nature Force (wave) Gravity: couples to mass Electromagnetic force: couples to charge Weak force: responsible for radioactive decay Strong force: couples to quarks Carrier (particle) graviton (?) photon W +, W -, Z 0 8 gluons massless carriers  long ranged massive carriers  short ranged

It just doesn’t add up! If you add the mass of the constituents of baryons (three quarks) and mesons (two quarks) you are way short of typical masses Plus: quarks have never been observed as single particles (Color confinement)  Unique kind of interaction

Two completely different types of particles Force carriers have integer spin (bosons) Matter particles have half- integer spin (fermions)

The big Gap: Matter vs. Forces Matter particle are fermions, force carriers are bosons New type of symmetry: |fermion> = Q |boson> |boson> = Q |fermion>

Relativity Because of E=mc 2, new particles can be created from energy or mass can be transformed into energy  particle number is not conserved  Need states with different numbers of particles to describe physical system

Photoelectricity - An electron at the surface of a metal absorbs a single photon, gaining kinetic energy equal to hf, the energy of the photon. - An electron can only leave the surface if it gains energy from the photon that exceeds the work function (Φ) of the metal (energy needed for an electron to escape the surface).

Intensity Depends on energy of each photon and no. per second. No photoelectric emission takes place below the threshold frequency (particle model) Above threshold the emission increases if the number of photons incident per seconds increases.

Photocell Energy vs Frequency y = mx + c E k = hf - Φ y = E x = f m = h c = - Φ

Excitation Excitation by electron: Incident electron moves orbital electron to an energy level. Incident electron moves off with excess KE. If insufficient to excite to lowest level, then electron will deflect with no loss of energy

Ionisation by collision Atomic electron is knocked out of the atom by a colliding electron with sufficient kinetic energy. This can be achieved in a gas when electrons are emitted from a heated cathode and attracted to a metal anode. The gas needs to be at low pressure (low number of atoms), otherwise the electrons cannot reach the anode.

De-excitation A lower vacancy will be filled by an electron from an outer shell, and a photon is emitted, and the atom moves to a lower energy level. The atom may de-excite directly to the ground state or indirectly.

Excitation by photons An electron in an atom can absorb a photon and move to an outer shell where a vacancy exists. The photon energy must equal the difference between the two energy levels, if it is larger or smaller then the photon will not be absorbed by the electron.

The Fluorescent Tube

The fluorescent tube emits visible light because: Ionisation and excitation of the mercury atoms occurs as they collide with each other and with electrons in the tube. The mercury atoms emit ultraviolet photons, as well as visible photons of lower energy as they de-excite. The ultraviolet photons are absorbed by the atoms of the flourescent coating, causing excitation of the atoms. The coating atoms de-excite and emit further visible photons.

Light Wave/Particle Duality Wave – Diffraction. The nearer the gap size to the wavelength the greater diffraction. Particle – photoelectric effect, as one photon is absorbed by one electrons as long it has sufficient energy. Photon energy is not ‘stored’, so cannot be wave model.

Matter as Wave/Particle Particle – electron deflection in magnetic field. Wave – Diffraction by atoms in a thin foil. De Broglie wavelength