Cathode Rays

• Cathode rays can be deflected by magnetic fields or electric fields.
• Cathode rays are negatively charged particles that travel from the cathode to the anode.
• Thermionic emission


  Emission of electrons from a metal surface by the application of heat energy
• Thermionic emission


  1. Filament heated

  2. Cathode produces electrons

  3. Electrons attracted to anode

  4. Current flows through circuit
• The current causes a heating effect
• The filament heats the cathode
• The cathode produces electrons by thermionic emission
• Electrons are attracted to the anode
• This causes current to flow through the circuit
• Suitable material for filament

  • Tungsten
  • High melting point to withstand high temperatures
• Suitable material for cathode

  • Mixture of barium and strontium oxides
  • Low work function so electrons are easily emitted by thermionic emission
• Increasing potential difference (voltage) across the glass bulb from 12V to 20V


  Increases the speed of emitted electrons
• Increasing voltage across the heater circuit from 6V to 10V

  Increases the intensity of emitted electrons
• Reason for evacuating the glass bulb

  To prevent collision between cathode rays and air molecules which reduce their kinetic energy (prevent oxidation of the filament)
• Cathode rays


  A stream of electrons moving from the cathode to the anode
• Application of cathode rays

  • Cathode ray tube (CRT)
• Working of a cathode ray tube

  1. Filament electrically heated

  2. Filament heats cathode

  3. Cathode produces electrons by thermionic emission

  4. Electrons attracted to screen by anode

  5. Screen glows on impact with electrons
• Reason for screen glowing

  It is coated with a fluorescent material
• Function of the vacuum

  Prevent collision between cathode rays and air molecules which reduce their kinetic energy
• Function of extra high tension (EHT) voltage

  To provide potential difference that accelerates electrons from the cathode to the screen
• Properties of cathode rays

  • Travel in straight lines
  • Cast shadows when an opaque object is placed in their path
  • Cause certain substances to glow
  • Possess energy (can set a paddle wheel into rotation)
  • Cause heating effect when striking a metal surface
  • Negatively charged (deflected by electric and magnetic fields)
• Cathode ray oscilloscope (CRO)


  A modification of a cathode ray tube to perform various measurements
• Major parts of a CRO

  • Electron gun
  • Deflection plates
  • Evacuated glass envelope
  • Fluorescent screen
• Function of filament in CRO

  Used to heat the cathode, made of tungsten with high melting point
• Function of cathode in CRO

  Produces cathode rays by thermionic emission when heated by the filament
• Function of grid in CRO

  Controls the intensity of the electron beam
• Function of anodes in CRO

  Control the brightness of the screen by controlling the intensity of the electron beam
• Function of Y-plates in CRO

  Give electrons deflection in the vertical direction
• Function of X-plates in CRO

  Give electrons deflection in the horizontal direction, time base circuit applies AC signal to X-plates
• Function of the screen in CRO

  Coated with fluorescent material that glows on impact with electrons, has persistence of vision
• Functions of a CRO

  • Measure voltage of a signal
  • Measure frequency of a signal
  • Display the waveform of a signal
• Advantages of CRO over conventional meters

  • Can measure both AC and DC voltages
  • Can measure larger voltages without getting damaged
  • Has infinite resistance and does not interfere with the circuit
  • Pointer does not swing about due to inertia
• Differences between CRO and TV tube

  • Deflection in CRO is by electric field, in TV tube is by magnetic field
  • Signal fed on Y-plates of CRO, on grid of TV tube
• TV tubes are slightly shorter than CRO tubes because deflection in TV tubes by magnetic field gives wider deflection compared to electric field in CRO