AQA A-Level Physics: Electromagnetic Radiation & Particles

Electromagnetic waves

Transverse.

Photoelectric effect

When light above a particular frequency is shone on metal, electrons are released - these released electrons are "photoelectrons".

Threshold frequency

The minimum frequency of light required for an electron to be emitted.

Energy of a photon equation

E = hf = hc/𝜆; Energy = Planck's constant x frequency; Energy = (PC x 3 x 10^8) / wavelength.

Minimum frequency requirement for photon

The energy of the photon is determined by its frequency, the photon's energy must be greater than the work function (energy needed to break bonds holding the electron) in order for an electron to be emitted.

Effect of photon frequency higher than threshold

The electron will be liberated and the remaining energy is the kinetic energy of the electron.

Effect of increasing light intensity when photoelectric emission does NOT occur

If it is more intense then there would be more photons incident on the metal each second; However, each photon still carries the same amount of energy as before; Therefore it still does not contain enough energy to liberate an electron; No effect.

Photoelectric equation

Planck's constant x frequency = work function + maximum kinetic energy of the photoelectrons.

Work function

The energy required by an electron to overcome the metallic bond holding it in the metal.

Electron volt

The kinetic energy of an electron that has been accelerated from rest through a potential difference of 1V.

Conversion of electron volts to joules

X 1.6x10-19 for electron volt (eV) to joules; ÷1.6x10-19 for joules to electron volts.

How a fluorescent tube works

High voltage applied across mercury vapour accelerates fast moving free electrons which collide with the mercury atoms; Mercury electrons are excited and then return to the ground state, releasing a UV photon; The tube's phosphorus coating absorbs the UV photons and its electrons are excited, they cascade down the energy levels and emit visible light photons.

Evidence for discrete energy levels in atoms

Line emission and absorption spectra as the lines appear at discrete points which show where a light photon of specific frequency and wavelength has been absorbed or emitted, this shows electrons can only absorb an exact amount of energy to be excited to the next discrete energy level.

Wave particle duality

All particles have both particle and wave properties, waves can have particle properties e.g. light acts as a particle in the photoelectric effect and as a wave when it is diffracted.

de Broglie wavelength equation

𝝺 = h / mv, where mv is momentum.

Main constituents of an atom

Proton, Neutron, Electron

Specific charge

The charge to mass ratio: Specific charge = charge / mass. Units C/kg.

Specific charge of a proton

Specific charge = 1.6 x 10^-19/1.67 x 10^-27 = 9.58 x 10^7 C/kg

Letter associated with a proton number

Z.

Nucleon

A constituent of the nucleus: a proton or a neutron.

Letter representing nucleon number

A.

Correct notation

A Z X or Z X A.

Isotope

A version of an element with the same number of protons but a different number of neutrons.

Use of radioactive isotopes

Carbon dating - the proportion of carbon-14 in a material can be used to estimate its age.

Strong nuclear force

The fundamental force that keeps the nucleus stable by counteracting the electrostatic force of repulsion between protons.

Range of the strong force

Repulsive up to 0.5fm, Attractive from 0.5-3fm, Negligible past 3fm.

What makes a nucleus unstable?

Nuclei which have too many of either protons or neutrons or both.

Decay of nuclei with too many nucleons

Alpha decay (emission of a helium nucleus formed of 2 protons and 2 neutrons).

Decay of nuclei with too many neutrons

Beta minus decay in which a neutron decays to a proton by the weak interaction (quark character has changed from udd to uud).

Existence of the neutrino hypothesised

The energy of particles after beta decay was lower than before, a particle with 0 charge (to conserve charge) and negligible mass must carry away this excess energy, this particle is the neutrino.

Beta minus decay

When a neutron turns into a proton, the atom releases an electron and an anti-electron neutrino.

Alpha particle

A particle contains two protons and two neutrons, the same as a helium nucleus.

Antiparticle

For each particle there is an antiparticle with the same rest energy and mass but all other properties are the opposite of its respective particle.

True or false: 'Every particle has a antiparticle'

True.

Antiparticle of an electron

Positron.

Antiparticle of π0 (pion with 0 charge)

π0, its antiparticle is itself.

What occurs when a particle and antiparticle meet?

(No definition provided in the notes)

Annihilation

The mass of the particle and antiparticle is converted back to energy in the form of 2 gamma ray photons which go in opposite directions to conserve momentum.

Pair production

A gamma ray photon is converted into a particle-antiparticle pair.

Minimum energy for proton-antiproton pair

2 x proton rest energy; 2 x 938.257 = 1876.514 MeV.

Four fundamental forces

Gravity, Electromagnetic, Weak nuclear, Strong nuclear.

Exchange particle of electromagnetic force

The virtual photon.

Particles affected by strong nuclear force

Hadrons.

Exchange particle of weak nuclear force

The W boson (W+ or W-).

Electromagnetic force action

It acts on charged objects, for example when a positively charged ball repels another positively charged ball.

Weak nuclear interaction occurrence

When quark character changes (a quark changes into another quark), it affects all types of particles.

Conserved properties in particle interactions

Energy, Charge, Baryon number, Lepton number, Momentum, Strangeness (only for strong interactions).

Hadron

Both baryons and mesons are hadrons, hadrons are made of 2 or more quarks held together by the strong nuclear force.

Classes of hadrons

Baryons (three quarks), Mesons (1 quark, 1 antiquark).

Pion and kaon class

Mesons.

Pion as exchange particle

The strong nuclear force.

Kaon decay

A kaon decays into a pion.

Examples of baryons

Proton - uud, Neutron - ddu.

Significance of a proton

It is the only stable baryon; All baryons will eventually decay into protons.

Examples of leptons

Electron, Muon, Neutrino, (the antiparticles of the above).

Muon decay

An electron and two types of neutrino.

Strangeness value of strange quark

-1

Strangeness conservation in weak interaction

False. Strangeness is only conserved in the strong interaction, in weak interactions it can change by 0, -1 and +1.

Strange particles production and decay

Strange particles are produced through the strong interaction and decay through the weak interaction.