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HSC Physics

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  1. Module 1: Kinematics
    1.1 Motion in a Straight Line
  2. 1.2 Motion on a Plane
  3. Module 2: Dynamics
    2.1 Forces
  4. 2.2 Forces, Acceleration and Energy
  5. 2.3 Momentum, Energy and Simple Systems
  6. Module 3: Waves and Thermodynamics
    3.1 Wave Properties
  7. 3.2 Wave Behaviour
  8. 3.3 Sound Waves
  9. 3.4 Ray Model of Light
  10. 3.5 Thermodynamics
  11. Module 4: Electricity and Magnetism
    4.1 Electrostatics
  12. 4.2 Electric Circuits
  13. 4.3 Magnetism
  14. Module 5: Advanced Mechanics
    5.1 Projectile Motion
  15. 5.2 Circular Motion
  16. 5.3 Motion in Gravitational Fields
    2 Topics
  17. Module 6: Electromagnetism
    6.1 Charged Particles, Conductors and Electric and Magnetic Fields
  18. 6.2 The Motor Effect
    1 Topic
  19. 6.3 Electromagnetic Induction
  20. 6.4 Applications of the Motor Effect
    1 Topic
  21. Module 7: The Nature of Light
    7.1 Electromagnetic Spectrum
    3 Topics
  22. 7.2 Light: Wave Model
  23. 7.3 Light: Quantum Model
    2 Topics
  24. 7.4 Light and Special Relativity
  25. Module 8: From the Universe to the Atom
    8.1 Origins of the Elements
    5 Topics
  26. 8.2 Structure of the Atom
    3 Topics
  27. 8.3 Quantum Mechanical Nature of the Atom
    2 Topics
  28. 8.4 Properties of the Nucleus
    2 Topics
  29. 8.5 Deep Inside the Atom
    4 Topics
Lesson 26, 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 1.6 \times 1-^{-19} \text C, known as the elementary charge.