Physics AS unit 1 (AQA) particle physics

revsion notes on all of unit 1- particle physics

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the atom

Isotope- the same no. of protons and electorns, different no. of neutrons

specific charge= charge/mass

e.g. for a nucleus of hydrogen (one proton)

charge= +1.6x10-19C mass= 1.67x10-27 Kg

specific charge= +1.6x10-19/1.67x10-27 = 9.58x107 C Kg-1

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stable and unstable nuclei

strong nuclear force- the force that holds the nucleus together, it has an increadibly small range. It overcomes the electrostatic force that repels protons and neutrons.

Radio Active decay

Alpha decay consists of 2 protons and 2 neutrons- helium nucleus

Beta decay consists of a fast moving electron

Where a neutron changes into a proton and releases an electron and an anti neutrino.

Gamma radiation a form of electromagnetic radiation, has no mass or charge.

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λ=c/f where c is the speed of light =3.0x108

electromagnetic waves are emitted when a charged particle loses energy:

  • a fast moving electron slows or stops
  • an electron in a shell moves to a lower energy level

-emitted as bursts of energy, also known as 'photons'

E=h/f where h is planck's constant = 6.63x10-34 Js

laser beams are just photons of the same frequency emitted at the same time.

power of a laser=nhf where n is the no. of photons per second

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particles and antiparticles

when matter and antimatter meet, they annhilate each other and emit 2 photons. The mass of the particles is converted into photons.

  • every particle has an antiparticle with the exact same rest mass, but exact opposite charge.

Pair production is where a single photon creates a particle and antiparticle pair.

hfmin = E0 where E0 is the rest mass of each particle

hfmin = minimum photon energy

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Weak nuclear force

Weak nuclear force is how leptons interact and how hadrons decay, it is also how hadron-lepton interactions take place.

The weak nuclear force is represented by the W bosons, these have:

  • non-zero rest mass (unlike photons)
  • A very short range
  • And a charge (W- , W+ )

Feyman diagrams represent an interaction between particles, the y-axis represents time.

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Hadrons and leptons

Hadrons- Particles and antiparticles that interact through the strong interaction, e.g. protons, neutrons, K mesons

Leptons- Interact via the weak interaction

But all charged particles also interact via through electromagnetic waves.

Hadrons, also decay via the weak interactions


Baryons protons and all other hadrons that decay, directly or indirectly to protons. They are made of 3 quarks or 3 antiquarks

Mesons hadrons that don't include protons in their decay. made of one quark and one antiquark.

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There are 3 different types of Mesons: electrons (e-), Muon (μ), Tau (τ)

Each have their own neutrinos: νe, νμ, ντand they must each be counted seperatly so that lepton no. is always conserved.

In muon decay μ- ---> e- + νe + νμ

But not μ- ---> e- + νe + νμ

As lepton no. is not conserved

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Quarks and Antiquarks

There are 3 types of Quarks- up (u), down (d), and strange, each with their corresponding antiquarks.

In the strong interaction strangness is always conserved.

u d s u d s

charge +2/3 -1/3 -1/3 -2/3 +1/3 +1/3

Strangness 0 0 -1 0 0 +1

stangness does not have to be conserved in the weak interaction

a baryon consists of 3 quarks, e.g proton= uud, antiproton=uud

a meson consists of a quark and an antiquark e.g K0 ds

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Conservation rules

1. Conservation of energy and charge

-applies to all science

2. Conservation of lepton no.

3. Conservation of Baryon no.

4. Conservation of strangness

5. Conservation of Quark no.

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Photoelectric emission of electrons from a metal surface only takes place if the frequency of the incident electromagnetic radiation is above the value known as the threshold frequency (depends on the metal)

Einstien suggested light consists of wave packets, or photons.

Energy of photon=hf where his planck's constant = 6.63x10-34 Js

An electron can leave the surface of the metal if the energy gained from a single photon exceeds the work function (ϕ) of the metal.

Ekmax =hf-ϕ where Ekmax is the max energy of the electron

hf=Ekmax +ϕ Fmin=ϕ/h

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Energy levels in atoms

An electron in a shell near the nucleus has less energy than one further away.

The lowest energy state of an atom is called its ground state, when an atom absorbs energy, one or more electrons move to a higher energy level. This means the atom is now in an excited state.

There are only discrete energy levels an electron can have.

When an electron de-excites, it emits a photon, for example in flourescent tubes, visible light is emitted.

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Energy levels and spectra

When light is passed through a prism, it seperates out.

A line absorbtion spectra is formed when white light is passed through cold gas. it shows the whole spectra with a few black lines, this shows the atoms which have absorbed wavelengths of the white light.

A line emission spectra shows bright lines on a black background, these are the wavelengths emitted from the excited gas.

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Wave particle Duality

Light demonstrates both wave-like and particle-like nature.

Wave-like: diffraction of light when light is passed through a slit.

Particle like: Photoelectric effect

Electrons also show wave like nature. The wave-like behaviour is characterised by its wavelength, its De Broglie Wavelength (λ)

λ= h/mv h=planck's constant m=mass


Everything has a de broglie wavelength, so in theory everything can behave like a wave

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Frank Dong


            (u)     (d)    (s)    (anti u)  (anti d)  (anti s)

charge  +2/3  -1/3   -1/3     -2/3      +1/3      +1/3

Strangness 0     0      -1        0             0           +1

(Is that mean to be that?   I might be wrong as I have not learnt yet, but I wish the mistakes (if there are some) can be modified ASAP. The content below makes no sense to me as well. Wish someone can change those mistakes)

a baryon consists of 3 quarks, e.g proton= uud, antiproton=uud

a meson consists of a quark and an antiquark e.g K0 ds

lauren felton


is an antiproton not two anti-ups and one anti-down ?



Strangeness is NOT conserved

lee marshall


Lauren you are correct, an anti proton does consist of and anti down and two anti up quarks.

_ _ _

u u d

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