Matter and Radiation

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Inside the atom

  • Nucleon - a proton or neutron in the nucleus  
  • Because the nucleus is positively charged, the electrons are held in the atom by the electrostatic force of attraction between them and the nucleus.
  • Isotopes - atoms with the same number of protons and different number of neutrons
  • Nucleon number - (mass number) the total number of protons and neutrons in an atom
  • Nuclide -  a type of nucleus 

Specific charge:

  • specific charge of an atom is the charge divided by its mass. 
  • the electron has the largest specific charge of any particle because its mass is so small.
  • to calculate the SC of an ion: if it gains 2 electrons, the charge is -2 x charge -->

How many gained/ lost electrons x (1.6 x 10^-19)

number of P + N x (1.67 x 10^-27)

  • For nucleide -->  Number of protons x (1.6 x 10^-19)

                               number of P + N x (1.67 x 10^-27)

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Stable and unstable nuclei

  • The strong nuclear force- attractive force between neucleons that overcomes the electrostatic force between the protons and holds the nucleons together in the nucleus.
  • its range is between 0.5femtometres and 4 fm (1fm = 10^-15m) whwears the electrostatic force has an infinite range with slightly deceasing strength.  
  • it has the same effect between any nucleons
  • At separations smaller than 0.5fm it is a replusive force, preventing P and N being pushed into each other. 
  • Alpha radiation consists of alpha particles with 2 protons and 2 neutrons.
  • Beta radiation consists of fast-moving electrons. A beta particle is emitted instantly when a neutron changes into a proton in the nucleus. In addition, an antiparticle with no charge, an antineutrino is emitted. 
  • Gamma radiation is electromagnetic radiation emitted by an unstable nucleus. It has no mass or charge. It is emitted by a nucleus with too much energy following an alpha or beta emission. 
  • Neutrinos and antineutrinos are elusive particles that are emitted by the sun.

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Electromagnetic waves:

  • An EM wave consists of an electric wave and a magentic wave which travel together and vibrate at right angles to each other and to the direction at which they are travelling; in phase with each other. 
  • In a vacuum, all EM waves travel at the speed of light, c, 3 x 10^8 m/s. 

speed,c = frequency,f, x wavelength, ƛ

  • 1 nanometre, nm = 10^-9m
  • EM waves are emitted by an ion when it loses energy: when a fast moving electron is stopped/ slows down or changes direction OR an electron in a shell of an atom moves to a different shell of lower energy
  • A photon - a packet of EM energy

E = hf E - energy     h- Planck's constant (6.63 x 10^-34Js)      f - frequency

* for a beam with multiple photons of frequency f: power = nhf

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Particles and antiparticles


  • When matter and antimatter meet, they annihilate each other and release radiation. 
  • A positron is the antiparticle of an electron, with a +ve charge. It is emitted when a proton changes into a neutron in an unstable nucleus with too many protons. In addition, a neutrino (v) is emitted, which is uncharged. This emission does not occur naturally!

For every type of particle, there is a corresponding antiparticle that:

  • annihilates itself and the particle if they meet, converting their total mass into photons.
  • has exactly the same rest mass as the particle
  • has exactly opposite charge to the particle (if it has a charge)

The opposite process is known as pair production when a photon with enough energy can suddenly change into a particle-antiparticle pair, which would then separate.


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Particles, antiparticles and E=mc^2

  • The energy of a particle can be expresed in millions of electron volts (MeV) 
  • 1 MeV = 1.6 x 10^-13J
  • 1 MeV - the energy transferred when an electron is moved through a potential difference of 1 volt. 
  • When annihilation occurs, two photons are produced. Therefore, the min. energy of EACH photon, hfmin = E0
  • In pair production, a photon creates a particle and an antiparticle and vanishes in the process. Therefore, the min. energy of the PHOTON: hfmin = 2E0
  • This is the minimum energy needed for the photon to produce the pair. A photon with less energy could not create a particle and antiparticle.
  • *** The positron was discovered when it was deflected by a magnetic field in the opposite way to the electron due to its positive charge. 
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How particles interact

The EM force: 

  • When a single force acts on an object, it changes the momentum of the object. 
  • The momentum is its mass x its velocity. 
  • When two objects interact, they exert equal and opposite forces on each other and transfer momentum, providing no other forces act on the objects. 
  • The EM force between two charged objects is due to the exchange of virtual photons, which cannot be detected directly. 
  • The interaction is shown on a 'Feynman Diagram'  

The weak nuclear force:

  • The weak nuclear force is responsible for beta decay. It is weaker than the strong nuclear force, as otherwise, it would affect stable nuclei. 

Neutrinos and antineutrinos hardly interact with other particles but sometimes:

  • a neutrino can interact with a neutron and change into a proton, emitting an electron too.
  • an antineutrino can interact with a proton and change into a neutron and a positron. 
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How particles interact 2

The interactions between neutrinos/ antineutrinos and neutrons/protons is due to the exchange of particles known as W bosons. Unlike photons, these exchange particles:

  •  have a non-zero rest mass
  • have a very short range of no more than about 0.001fm
  • are charged


When no neutrino/ antineutrino is present:

  • the W- boson decays into a beta- particle and an antineutrino
  • the W+ boson decays into a beata+ particle and a neutrino

Electron capture: 

  • sometimes a proton in a proton-rich nucleus turns into a neutron as a result of interacting through the weak interaction with an 'inner-shell' electron from outside the nucleus. 
  • this can also happen when a proton and an electron collide at very high speed. The exchange particle is normally a W+ boson (could be W- with enough energy)
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