Physics - P2.7 - Energy from the Nucleus

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P2.7.1 - Nuclear Fission

  • Nuclear fission - splitting of atomic nucleus
  • Two fissionable isotopes used commonly in nuclear reactors: uranium-235 and plutonium-239
  • Most naturally occurinf uranium is uranium-238: non-fissionable - nuclear reactors used 'enriched' uranium containg 2-3% uranium-235
  • For fission to occur uranium-235/plutonium-239 must absorb a neutron -> nucleus splits into two smaller nuclei -> two/three neutrons emitted -> energy released - energ released in nuclear process is much greater than energy released in chemical process, eg. burning
  • Chain reaction occurs - each fission even causes more fission events - in nuclear reactor, process is controlled so one neutron/fission on average goes on to produce further fission
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P2.7.2 - Nuclear Fusion

  • Nuclear fusion - process of forcing two nuclei so close that they form a single larger nucleus
  • Fusion can be brought about by making two light nuclei collide at very high speed - process by which energy is released in stars
  • Enormous problems with producing energy from fusion - nuclei approaching each other repel due to positive charge: must be heated to very high temperatures to overcome repulsion and fuse - because of high temperatures, reaction can't take place in normal 'container': has to be contained by magnetic field
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P2.7.3 - Nuclear Issues

  • Radon gas - major source of backgound radiation - seeps through ground from radioactive substances in rocks underground - emits alpha particles: health hazard if inhaled
  • Other sources of background radiation: cosmic rays from outer space, food, drink, air travel, nuclear weapons testing, X-rays (have ionising effect), radioactive substances
  • Uranium and plutonium chemically removed from used fuel rods from nuclear reactors - can be used again - remaining radioactive waste must be stored in secure conditions for years
  • To reduce exposure to nuclear radiations, workers should:
    • keep as far as possible from sources of radiation
    • spend as little time exposed as possible
    • sheild themselves with materials, eg. concrete or lead
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P2.7.4 - The Early Universe

  • Believe Earth was created in the Big Bang ~13 billion years ago - universe was hot glowing ball of radiation at first - in first few minutes, nuclei of lightest elements formed - as universe expanded over millions of years, temperatures fell - uncharged atoms were formed
  • Before galaxies and stars, universe was cloud of hydrogen and helium - dust and gas pulled together by gravitational attraction, forming stars - resulting heat started off nuclear fusion reactions in stars: began to emit visible light and other radation
  • Large group of stars: galaxies - our sun is one of billions in the milky way
  • Galaxy: collection of billions of stars held together by their own gravity - billions of galaxies in the universe, with vast empty space between them
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P2.7.5 - The Life History of a Star

  • Protostar - gravitational forces pull dust and gas clouds together -> becomes denser and nuclei of hydrogen atoms and other light elements begin fusing -> energy is released so the core gets hot and bright
  • Main sequence star -  star radiates energy - stage can continue for billions of years, until star runs out of hydrgoen nuclei - stable as inward of force of gravity = outward force of radiation -> runs out of hydrogen nuclei, swells, cools down and turns red
  • Low mass - red giant - helium and other elements fuse to make heavier elements -> fusions stops: white dwarf -> no light emitted: white dwarf
  • High mass - red supergiant - continues to collapse -> star explodes in supernova - outer layers thrown into space, core left as neutron star -> high enough mass: black hole - gravitational field so strong, not even light escapes
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P2.7.6 - How the Chemical Elements Formed

  • Chemical elements formed by fusion in stars - nuclei of lighter elements fuse together to make nuclei of heavier elements - release energy
  • Elements heavier than iron only formed in final stages of life of big star: orocess requires input of energy - all elements distributed through space by supernova explosion
  • Presence of heavier elements in the sun and inner planets is evidence that they were formed from debris of a supernova
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