Discovery of the electron

• Cathode rays are produced in tubes of low pressure gas
• The discovery of cathode rays showed that there were particles with mass and a negative charge causing them which turned out to be electrons
• The work done on an electron by the electric field = the kinetic energy used by the electron to cross the plates
• Thermionic emission is when a heater warms up a cathode so that electrons are emitted
• The specific charge of an electron is about 1800 x the specific charge of a proton
• The equations for specific charge showed that specific charge is constant and so that all atoms contain electrons
• The pd can be adjusted in the electric field to make the oil drop stationary. It is stationary when QV/d = mg
• when the viscous drag force = the weight, the drop is travelling at its terminal velocity
• The viscous drag force is given by stokes law
• The terminal velocity of the oil drop can be found by measuring the distance travelled in a certain time through a microscope
• milikans experiment showed that the charge of oil drops was always an integer multiple of 1.6 x 10^-19C
• Milikans experiment showed that charge is quantised and exists in discrete packets of 1.6 x 10^-19C
• • To find specific charge by means of a magnetic field, use F=Bev and F=mv^2/r to get Be=mv/r. As we don't know the velocity of the particles, rearrange eV=1/2mv^2 for v and substitute into Be=mv/r. rearrange for e/m.
• the value of the specific charge of the electron was much higher than any other know ion showing that they must either have a very high charge or very small mass
• the specific charge being much smaller was evidence the cathode rays were made of different particles with a much smaller mass, giving evidence of subatomic particles
• milikans experiment

  • oil droplets are charged negatively by gaining electrons
  • the terminal velocity is measured when the oil drop is falling
  • the radius and so mass are measured here with mg=stokes law
  • an electric field is applied upward so that the drop becomes stationary
  • the charge is measured here with mg=Eq
• the charge measured in milikans experiment allowed the mass of an electron to be calculated