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Particle physics (Inside the atom (Nucleus contains protons (+ve) and…
Particle physics
Inside the atom
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Neutrons provide stability to the atom (protons repel each other which neutrons help to overcome - also helped by strong nuclear force
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Isotopes are atoms of the same element with sane number of protons but different numbers of neutrons so different masses
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Isotopes cause small differences in physical properties - higher-mass isotopes have higher melting and boiling point as well as higher density
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Particles and antimatter
Positron
- When antimatter and matter particles meet they destroy each other and release radiation
- Positron emission happens when proton changes into a neutron in an unstable nucleus with too many protons
- Positron (β+) is antiparticle of the electron, and a neutrino (v) is emitted
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- Positron emitting isotopes don’t occur naturally – manufactured by placing stable isotope (liquid or solid)
in path of beam of protons – absorbs protons and becomes unstable
Annihilation
- When particle and antiparticle meet they annihilate and convert their total mass into 2 photons (radiation energy)
- Two photons produced to ensure total momentum after annihilation is zero
- Minimum energy need by photon = hf(min) = E (rest energy – written as E subscript 0)
Pair production
- Photon creates particle and its corresponding antiparticle and proton ‘vanishes’
- Minimum energy photon need = hf(min) = 2E (rest energy of particle)
- 1 eV (electron volt) = 1.60 × 10^-19 Joules
Discovery of positron
- Discovered using a cloud chamber by Carl Anderson – knew +ve particle would be deflected in opposite way to negative particle travelling in same direction
- Slower it went, more it would bend
- Beta particle went in opposite direction to the other beta particles – he had discovered the positron
Particle interactions
Electromagnetic force
- When two objects interact they exert an equal and opposite force on each other – momentum transferred by these forces if no other forces act on them
- Electromagnetic force between two objects due to exchange of virtual photons – virtual as we cannot detect them directly
- This is shown using a Feynman diagram (refer to the textbook) where virtual photon represented as a wave
Weak nuclear force
- Both β- and β+ decay, new particle and antiparticle created – one is an e- or e+, and other is a neutrino or an antineutrino
- Neutrino can interact with a neutron and cause it to change into a proton – β- particle created and emitted as a result
- Antineutrino can interact with proton causing it to change into a neutron – β+ particle emitted as a result
- These interactions due to exchange of W bosons which: Have a non-zero rest mass, Have a very short range of no more than around 0.001 fm, Are positively charged or negatively charged
- The force which causes nuclear decay - it is the mechanism of interaction between subatomic particles
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Electron capture
- Sometimes proton in proton rich nucleus turns into neutron due to weak interaction with an inner-shell electron from outside the nucleus (electron capture)
- W+ boson becomes neutrino
- Same change can happen when proton and electron collide at high speed
- W boson and photon known as force carriers because they are exchanged when EM and weak nuclear force act (respectively)
Photons
Electromagnetic waves
- Vacuum all EM waves travel at speed of light
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- Electromagnetic waves consists of an electric wave and a magnetic wave which travel together and vibrate:
- At right angles to each other and the direction they are travelling
- In phase with each other (reach a peak together)
Photons
- EM waves emitted by a charged particle when it loses energy – can happen when:
- A fast moving electron is stopped, slows down or changes direction
- An electron in a shell of an atom moves to a different shell of lower energy
- EM waves emitted as short bursts of waves, each leaving the source in a different direction
- Each burst is a ‘packet’ of EM waves called a photon
- Photon theory established by Einstin and used to explain the photoelectric effect – emission of electrons from a metal surface when light hits it
- Equation: (photon energy) E = f × h (Planck constant)
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