Physics

• Mass number


  Number of protons and neutrons in the nucleus of an atom
• Number of protons


  Defines the element
• Neutrons


  Contribute to the mass number but not the atomic number
• Atomic number


  Number of protons in the nucleus of an atom
• Atomic number is specific to the element
• Mass number can change to make a different isotope
• When a nucleus has an excess of neutrons or protons

  It becomes unstable
• Radioactive decay

  1. Radiation is emitted

  2. Causes the nucleus to lose energy

  3. Becomes more stable
• Radioactive decay is a completely random process
• Naturally occurring radioactive minerals and water produce background radiation. The human body contains some of these naturally occurring radioactive minerals.
• Irradiation


  When the outside of an object is exposed to radiation. The object does not become radioactive.
• Contamination


  When radiation is inside an object. The object will then be radioactive with the source removed.
• Types of Radiation


  • Alpha
  • Beta
  • Gamma
• Alpha particles


  • Made up of two neutrons and two protons
  • Overall charge of +2
  • Mass number of 4
  • Lowly penetrative, highly ionising
  • Can be stopped by paper
• Beta particles


  • High energy, high speed electrons
  • Don't come from an atom's nucleus, but rather as a neutron transforming into a proton
  • Charge of -1 and negligible mass
  • Can be stopped by a few mm of aluminium
• Gamma rays


  • High frequency electromagnetic waves
  • Have no charge
  • Highly penetrative, low ionising
• Rutherford's Experiment


  1. Directed a beam of positively charged alpha particles at a thin gold foil

  2. Most particles went straight through the foil

  3. A small number were deflected at large angles (>40°)

  4. This suggested the atom has a very small, concentrated positive charge
• Alpha Decay


  1. Mass number decreases by 4

  2. Atomic number decreases by 2
• Beta Decay


  Atomic number increases by 1 but mass number does not change
• Activity


  Number of decays per second by the radioactive sample
• Half-life


  Time taken for half the unstable nuclei in a radioactive isotope sample to decay
• Types of Forces


  • Weight
  • Friction
  • Air resistance
  • Electrostatic
  • Reaction
  • Tension
  • Magnetic
  • Upthrust
• Non-contact force


  Force applied to an object by another object that is not in direct contact with it, e.g. weight
• Contact force


  Force applied to an object by another object that is in direct contact with it
• Free body diagram shows only the forces acting on an object, simplified to see the forces easily
• If an object is at constant velocity, the resultant force acting on it is zero
• Scalar

  Quantity that only has a magnitude
• Vector

  Quantity that has both magnitude and direction
• Weight


  Mass x Gravitational Field Strength
• Work done

  Force x distance moved
• Hooke's Law


  The extension of a spring is directly proportional to the force applied, as long as the elastic limit is not exceeded
• Spring constant


  Property of a material that determines how stretchy it is
• Energy efficiency


  Useful output energy / Total input energy x 100%
• Types of Waves


  • Transverse
  • Longitudinal
• Mechanical waves

  Require a substrate/particle to oscillate for the wave to pass through, e.g. sound, seismic, water
• Electromagnetic waves


  Can pass through where there are no particles (or a vacuum) as well as where there are particles, they are always transverse, e.g. microwaves, X-rays, visible light
• Reflection


  When a wave meets a boundary and changes direction
• Refraction


  When a wave moves from a less dense material to a more dense material and bends
• Electrical Quantities


  • Charge
  • Current
  • Potential Difference
  • Resistance
• Potential Difference


  Amount of energy transferred by each coulomb of charge