Unit 5 ~ Optional topic

Unit 5 ~ Optional topic

1. The discovery of the electron

   • Thermionic emission of electrons
     • Cathode rays
       • Definition: a beam of electrons emitted from the cathode of a dischrage tube.
       • Gasses at sufficiently low pressure can conduct electricity and emit light.
       • Changing the gas pressure causes the glow of light to come from different parts along the tube.
         • If a magnet was brought near a discharge tube, then the light would become distorted. This observation showed that there were charged particles move through the gas when conducting electricity.
         • The discharge tube can turn a 'paddle wheel' when it is placed next to the cathode, whilst turned on. This shows that something was being emitted by the cathode.
     • The emission of light from a discharge tube
       • When electricity is passed through a discharge tube, electrons are fired out of the cathode and into the gas.
         • This ionises the atoms closest to the cathode, and creates a 'negative glow' at the cathode. The electrons knocked during ionisation then (mostly) recombine with the gas atoms.
       • Some of the electrons do not recombine.
         • They are then pull towards the positive anode.
           • This causes excitation by collision, and creates the light near the anode.
     • The principle of thermionic emission
       • This is a much easier way of producing electrons than the discharge tube
       • When a metal is heated, some of its free electrons gain sufficient kinetic energy to leave the metal at is surface.
         • By doing this, we can pull the released electrons into one place, by using a positively charged anode.
           • If a small hole is made in the anode, then the electrons will accelerate towrds it then through the whole, creating a beam of electrons.
       • The speed of each electron can be found by:
         • Linking work done to kinetic energy
           • 1/2 m v ^2 = e V
   • Deflection of an electron beam
     • Using a uniform electric field
     • Using a uniform magnetic field
     • Balancing fields
   • Use of electric and magnetic fields to determine e/m
   • The determination of the charge of the electron, e, by Millikan's method

2. Wave-particle duality

   • Early theories of light
     • Newton's corpuscular theory of light
   • The discovery of electromagnetic waves
   • The development of the photon theory of light
   • Matter waves

3. Special relavtivity

   • About motion
   • Einstein's theory of special relativity