CORE SECTION 2: PARTICLES & RADIATION

 

CONTENTS:

2.1 ATOMIC STRUCTURE
2.2 PARTICLES AND FORCES BETWEEN PARTICLES
2.3 QUARKS AND LEPTONS
2.4 PHOTOELECTRIC EFFECT
2.5 SPECTRA & ENERGY LEVELS
2.6 WAVE-PARTICLE DUALITY

 

2.1  ATOMIC STRUCTURE

# Mass number A = number of neutrons & protons in nucleus

# Atomic number Z = number of protons in the nucleus

# Number of neutrons in the nucleus = A – Z

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# Isotopes: atoms sharing the same atomic number,

but with different mass numbers.

 

# Ion: an atom in which 1 or more electrons have been removed.

 

# Specific Charge of a particle is the ratio CHARGE / MASS unit is C kg-1

   You need to be able to calculate it for individual particles, nuclei, and ions.

   The electron has the highest specific charge.

 

Remember: Rutherford scattering experiment from GCSE – the

scattering of  α particles when fired at gold atoms.

 

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2.2  PARTICLES AND FORCES BETWEEN PARTICLES

# Stable and Unstable particles / nuclei. eg. α emitting nuclei are unstable

    but a proton is stable since it will not decay into anything else.

    The proton is the most stable baryon. All baryons ultimately decay into protons.

# Kaons decay into pions

# Muons decay into electrons

 

# Antiparticle – identical to its particle equivalent but of

  opposite charge (neutral antiparticles have opposite spin)

 

# Photon – a particle of em radiation containing an

   amount of energy (depending on its frequency E = hf or wavength E = hc / λ

 

# Electromagnetic force – long range force (but decreases

  fast with distance), caused by like / unlike charges,

  can be attractive / repulsive, holds atoms together.

# Gravity – long range, acts on masses, always attractive,

  atomic masses are so small that gravity is not significant

  on an atomic scale.

# Strong nuclear force – short range force, is attractive up to 3 fm

    but becomes repulsive below 0.5 fm. The strongest of the 4 forces,

   it acts on particles made of quarks (hadrons), it binds together

   nucleons in the nucleus.

 

# Weak interaction – very short range nuclear force, affects

  all particles, is most significant in β- and  β+ decay.

   significance of the neutrino to account for conservation of energy

   and momentum in β- decay.

 

# Exchange particles or bosons these cause one particle

   to exert a force on another particle during an interaction.

   eg photons between charged particles (ELECTROMAGNETIC),

   eg pions between protons and neutrons or gluons between

   quarks (STRONG), eg W+, W- Boson in β decay (WEAK),

   eg gravitons between masses (GRAVITY).

   NB gluons and gravitons will not be examined.

# Feynman diagrams are used to illustrate interactions, in

  this one a photon is exchanged between 2 electrons:

FEYNMAN DIAGRAMS YOU SHOULD KNOW:

 

# Annihilation – the conversion of mass to energy

  when a matter particle collides with its equivalent

  antimatter particle.

 

# Pair production – the conversion of a photon into a

  particle-antiparticle pair (energy is converted into mass).

 

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2.3 QUARKS AND LEPTONS

 

# Quarks–fundamental particles, building blocks of hadrons,

  experience strong nuclear force: up, down, strange, antiup,

  antidown, antistrange, never found singly.

   knowledge of u, d, s quarks and their antiquarks is expected.

 

# Hadron – high mass particles made of quarks, subject

  to the strong nuclear force. eg. Meson: pions / kaons

  (quark, antiquark pair), or Baryon: protons / neutrons( 3 quarks).

  All hadrons are unstable except the proton.

    

# Lepton – fundamental particles, only interact

  with other particles via the weak interaction

  eg. electron, neutrino, muon, tau.

 

# Strange particles are produced in pairs via the strong interaction

  but decay via the weak force.

 

# conservation rules :

quantity

when conserved?

charge Q

always

baryon number B

always

upness u

only in strong interactions

downness d

only in strong interactions

strangeness s

only in strong interactions

lepton number L

always

mass-energy

always

 

NB.Make sure that you know β- and  β+ thoroughly.

 

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 2.4 PHOTOELECTRIC EFFECT –

# the emission of electrons from the surface of a metal when

  electromagnetic radiation of high enough frequency hits it.

   You should understand WHY this is evidence for the particle theory of light.

# Work function Ψ = minimum energy needed to remove

  an electron from the surface of a metal.

  Kinetic Energy ½  m vmax 2 , E = maximum ke of a released electron.

  hf  is the incoming photon energy

 

# 1 electronVolt = 1.6  x  10-19 J,  1 keV = 1.6  x  10-16 J

 

# Threshold frequency = the minimum frequency needed

  to release an electron.

  Ensure you know Einstein's photoelectric equation thoroughly:

       hf  =  Ψ  +  Ek

   The kinetic energy Ek against frequency graph gives,

   threshold frequency f0 = x intercept, planck's constant h = gradient,

   work function  Ψ  = y intercept.

# Threshold wavelength = the maximum wavelength

  needed to release an electron

# Intensity of the source -  is the number of photons per

  second hitting the metal plate

 

# Stopping Potential Vs  = the minimum voltage needed to

   reduce the photoelectron current to zero:

 

  Consider carefully the following examples:

 

2.5 SPECTRA AND ENERGY LEVELS

 

# Ionisation = the removal of an electron from an atom,

  Needs a large amount of energy input to atom.

  eg. high energy particle / photon collision.

# Ground state – most stable state of an atom when all of

  its electrons are in their lowest possible energy states/levels.

# Excited state – when an electron has jumped into a

  higher energy level leaving a vacancy below it –

  atom becomes unstable.  It returns to its stable state

  by emitting the excess energy as a photon of

  certain frequency,  E2 – E1  = hf

   OR E2 – E1  = hc / λ

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

# Line spectrum – set of discrete wavelengths, seen as

  a set of parallel lines, each is an image of the light source.

  Each line/wavelength  corresponds to a particular electron

  transition. Only a few bright lines are seen – only a few

  transitions occur regularly.

# Continuous spectrum – all possible transitions occur,

  so all possible wavelengths are emitted, and lines

  blend together into a continuous spectrum.

 

# In a fluorescent tube, mercury atoms are first excited by collision with

    electrons, the subsequent de-excitation produces uv photons,

    these uv photons are absorbed by atoms in the

    tube's coating causing further excitiation, de-excitation of the coating atoms

    causes multiple emission of visible photons (continuous spectrum).

 

2.6 WAVE-PARTICLE DUALITY

 

# electron diffraction and electron interference both suggest

  that fast moving particles can behave like waves.

  Particles can have wavelengths calculated from the

  de Broglie formula  λ  =  h / mv

   NB the higher momentum of the particles the shorter is

   the wavelength of these 'matter waves'.

 

# Photoelectric effect (and solar wind) suggests that em

  waves can behave like particles and that photons can

  have momentum calculated from the formula:  mc  =  h / λ .

   Unit of momentum = kg  m / s

 

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