Modern Physics

Modern Physics

1. The Electron

   • History of the electron
     • The word 'electron' was first used by Irish physicist George Stoney
     • The value of the charge was first measured by Robert Millikan in 1911
     • The charge of the electron was measured as 1.6x10^-19
     • The mass of the electron was measured as 9.1x10^-31
   • Electrons orbit around the central part of an atom
   • Energy of an electron
     • 1 eV = 1.6x10^-19
     • 1 keV = 1,000 eV
     • 1 MeV = 1,000,000 eV
     • 1 GeV = 1,000,000,000 eV

2. Thermionic Emission

   • Thermionic emission is the giving off of electrons from the surface of a hot metal
   • A cathode ray tube is a tube that cause streams of high speed electrons to move
   • The filament heats up the cathode due to thermionic emission. This causes electrons to be emitted from the cathode. There is a large voltage between the cathode and the anode which helps the electrons to speed up. The electrons then pass through the hole of the anode and it travels to the end of the tube. The end of the tube is made from a flourescent screen and once the electrons hit it, the kinetic energy is converted into light and a bright dot appears on the screen
   • The beam of electrons passes through a set of parallel plates with opposite charges. This causes the beam to be attracted towards the positive plate

3. The Photoelectric Effect

   • The photoelectric effect is the emission of electrons from the surface of a metal by electromagnetic radiation of a suitable frequency
   • The photoelectric effect experiment
   • E=hf
     • E = energy
     • h = Planck's constant
     • f = frequency
   • A photocell is a device that conducts electric current when light of a suitable frequency shines on it
   • The threshold frequency of a given metal is the frequency below which photoemission will not occur. Light above the threshold frequency will cause photomission
   • Photocells used in burglar alarms, automatic doors and sound track on films

4. X-Rays

   • An x-ray is a high frequency electromagnetic radiation produced when high speed electrons in a cathode ray tube strike a metal target that has a high melting point
   • In a hot cathode x-ray, thermionic emission occurs at the cathode and a beam of electrons is produced. The electrons are accelerated towards the anode. When it hits the anode, less than 1% of the electrons are converted into x-rays. The rest is turned into heat and the anode gets cooled by a coolant
   • X-rays are used in medicine to photograph organs and bones and also destroy cancerous cells

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