physics

• Energy dissipation in different systems

  • Radio or speakers - electrical work transferred into useful sound waves, infrared radiation dissipated as heat
  • Tumble dryer - electrical work transferred into useful internal (thermal) energy to dry clothes
• Atom


  The building block of matter
• Atom


  • A collection of smaller particles
  • Not a single particle about 1 × 10‾¹º m across
• The idea of the atom as the building block of matter has developed over time
• Rutherford


  Conducted an experiment in 1905 to test the plum pudding model
• Alpha particles


  Subatomic particles comprising two protons and two neutrons (the same as a helium nucleus)
• The vacuum is important because any deflection of the alpha particles would only be because of collisions with the gold foil and not due to deflections off anything else
• Gold was used because it was the only metal that could be rolled out to be very, very thin without cracking
• It was thought that the alpha particles could pass straight through the thin foil, or possibly puncture it
• If the plum pudding model had been correct then all of the fast, highly charged alpha particles would have whizzed straight through undeflected
• Rutherford's observations


  • Most alpha particles passed straight through the foil
  • A small number of alpha particles were deflected by large angles (> 4°) as they passed through the foil
  • A very small number of alpha particles came straight back off the foil
• Rutherford's conclusions


  • The fact that most alpha particles went straight through the foil is evidence for the atom being mostly empty space
  • A small number of alpha particles being deflected at large angles suggested that there is a concentration of positive charge in the atom - like charges repel, so the positive alpha particles were being repelled by positive charges
  • The very small number of alpha particles coming straight back suggested that the positive charge and mass are concentrated in a tiny volume in the atom (the nucleus) - the tiny number doing this means the chance of being on that exact collision course was very small, and so the 'target' being aimed at had to be equally tiny
• Rutherford's discovery


  The nuclear atom - a small, positively-charged nucleus surrounded by empty space and then a layer of electrons to form the outside of the atom
• Atom


  The building block of matter, previously thought to be a single particle about 1 × 10‾¹º m across, now known to be a collection of smaller particles
• Development of the atomic model

  1. Idea of the atom as the building block of matter

  2. Rutherford proved existence of the nucleus

  3. Bohr suggested electrons orbit the nucleus in different energy levels

  4. Chadwick proved existence of the neutron
• The discovery of the make-up of the nucleus (protons and neutrons) came much later, and was not made by Rutherford
• The nucleus was calculated to be about 1/10,000th the size of the atom
• Bohr's 'solar system' model of the atom

  • Electrons orbit the nucleus in different energy levels or at specific distances from the nucleus
  • Explains why particular chemicals burn with certain-coloured flames
  • Electrons have fixed levels of energy within each type of atom
• When atoms absorb energy

  Electrons at a particular level are pushed up to higher levels (at bigger distances from the nucleus)
• Electrons jump back down to a lower level


  They release light of definite frequencies
• Proton


  Nuclear particle responsible for the positive charge of the nucleus and some of the nuclear mass
• Neutron


  Neutral particle the same size as a proton, keeps the nucleus stable and makes up the mass
• Chadwick proved the existence of the neutron in 1932
• Chadwick used a version of Rutherford's experiment, using a sheet of beryllium and a paraffin block instead of gold foil
• Radioactive decay


  The process in which unstable atomic nuclei break apart or change, releasing radiation as they do so
• Unstable nucleus


  • Can decay by emitting an alpha particle, a beta particle, a gamma ray, or in some cases a single neutron
• Alpha particle


  A subatomic particle comprising two protons and two neutrons (the same as a helium nucleus)
• Beta particle


  A type of ionising radiation consisting of a single electron
• Gamma ray


  A type of ionising radiation that is also part of the EM spectrum, with no mass
• Neutron emission


  Occasionally it is possible for a neutron to be emitted by radioactive decay
• Properties of nuclear radiations

  • Penetrating power
  • Ionising power
  • Range in air
• Alpha radiation


  • Penetrating power: Skin/paper
  • Ionising power: High
  • Range in air: < 5 centimetre (cm)
• Beta radiation


  • Penetrating power: 3 mm aluminium foil
  • Ionising power: Low
  • Range in air: ≈ 1 metre (m)
• Gamma radiation


  • Penetrating power: Lead/concrete
  • Ionising power: Very low
  • Range in air: > 1 kilometre (km)
• All types of radioactive decay can be detected by a Geiger-Muller tube, or G-M tube
• Radioactive decay


  The process in which unstable atomic nuclei break apart or change, releasing radiation as they do so
• Radioactive decay is a random process
• Not all nuclei in a block of radioactive material are likely to decay at the same time
• It is impossible to tell when a particular nucleus will decay
• Scientists cannot tell when a particular nucleus will decay, but they can use statistical methods to tell when half the unstable nuclei in a sample will have decayed