Class 12 Physics Dual Nature Radiation Matter Discovery of Electron

Roadmap to the Discovery of Electron:

  • William Crookes, in 1870, discovered that on applying a strong electric field between cathode and anode (kept under discharge tube at low pressures), some rays were emitted by the cathode. These rays were noticed as a bright fluorescent on the glass adjacent to the cathode
  • These rays were called as cathode rays by William Crookes
  • In 1879, he proposed that these cathode rays are a bunch of fast moving negatively charged particles

  • Then, J. Thomson (1856-1940) validated the above hypothesis. He found that when ultraviolet radiation falls on a metal surface (or when metal surface is heated), few negatively charged particles are released by the metal surface.
  • He used electric and magnetic fields (across the discharge tube), mutually perpendicular to each other and the emitted electrons
  • He was the first to find the values of; speed(v), and the charge to mass ratio (e/m) of the cathode ray particles, experimentally
  • He found the speed of particles to be about 0.1 to 0.2 times the speed of light (), and the charge to mass ratio (e/m)
  • He also found that the e/m ratio to be constant, irrespective of the material used as an emitter. This proved the universal nature of cathode ray particles.
  • In 1897, he termed this negatively charged cathode ray particles as electron, and this name was used for the first time.

  • A. MIlikan, in 1913 performed experiments to calculate the charge on an oil drop (popularly known as Milikan’s oil drop experiment)
  • He observed that, every time the charge came to be an integral(whole number) multiple of a small charge (), which is actually the charge on an electron
  • MIlikan concluded that charge on a matter is quantized (discrete)

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