C5 Electronics

• Thermionic emission is the emission of free electrons from a hot cathode.
• Cathode Ray: high velocity electron beam produced using thermionic emission in a vacuum tube.
• Production of cathode ray:

  • cathode is heated by passing current until it gets hot
  • hot cathode emits free electron through thermionic emission
  • free electrons accelerates with a high velocity towards positively charged anode in vacuum tube
  • beam of high velocity electrons known as cathode rays
  • cathode rays complete the EHT circuit and milliammeter reading shows current on EHT circuit
• Characteristics of cathode ray:

  • travel in straight line in vacuum tube
  • have momentum
  • have kinetic energy
  • negatively charged
  • produces fluorescence when hit phosphor
  • can be stopped by Maltese Cross
  • can be deflected by electric and magnetic field
• Deflection of cathode rays

  • not deflected in absence of electric field - travels straight line
  • has electric field - deflects toward positively charged plate in a parabolic path
• Effects of magnetic field on the direction of the cathode rays with a Maltese Cross:

  • with torchlight - opaque Maltese Cross blocks light passing in a straight line
  • electron gun with both switches on - shadow of Maltese Cross forms when cathode rays are blocked - fluorescent effect on cathode rays due to momentum and kinetic energy
  • North pole placed near cathode ray - Maltese Cross deflects downwards following Fleming LHR - cathode rays are deflected by magnetic field