Lesson 13, Topic 1
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Evidence of the Electron

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Early Experiments with Cathode Rays

J. J. Thomson conducted a number of early experiments with cathode rays.

  • Found that the cathode ray tube glowed more when the gas was at a lower pressure.
  • Experimented with phosphorescent paint. The paint opposite the negatively charged cathode sparked and glowed.
  • Use electric plates above and below the tube to deflect the cathode ray. This allowed Thomson to determine that the rays were negatively-charged.

Thomson’s Charge-to-Mass Experiment

Thomson was also measured the electron’s charge-to-mass ratio. He achieved this by creating perpendicular electric and magnetic fields around a cathode ray tube. The experiment involved two stages.

  1. The magnetic and electric fields were varied until their forces on the particles cancelled, causing no ray deflection. With no net force, the magnetic and electric forces could be equated and re-arranged to find the velocity of the electrons.
  2. The magnetic field was then applied alone to the tube, causing a circular deflection of the particles perpendicular to the field. The radius of curvature was measured.

Thomson combined the results of the two stages to calculate the charge-to-mass ratio for the electron.

Millikan’s Oil Drop Experiment

Millikan observed the behaviour of oil droplets to determine a value for the charge of the electron.

  1. Oil was atomised into tiny droplets. A beam of X-rays were passed through the oil so that some droplets became charged.
  2. The droplets fell through a gap in the higher of two parallel charged plates, entering a uniform electric field.
  3. The voltage was adjusted until some droplets were suspended, achieved when the force due to gravity was equally opposed by the force due to the electric field.
  4. Millikan calculated the mass of the suspended oil droplets by measuring their radii and using known values for the density of oil.
  5. The charge of the oil droplets could then be found be equating the electric and gravitational forces, and re-arranging to find q.

The experiment found that all charges on the droplets were multiples of a fundamental charge, the charge of an electron. This was calculated to be [katex]1.6 \times 1-^{-19} \text C[/katex], known as the elementary charge.