CATHODE RAYS AND CATHODE RAY TUBE - Form 4 Physics Notes

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Introduction

  • These are streams of electrons emitted at the cathode of an evacuated tube containing an anode and a cathode.


Production of Cathode Rays

  • They are produced by a set up called a discharge tube where a high voltage source usually referred to as extra high tension (EHT) supply connected across a tube containing air at low pressure thereby producing a luminous electron discharge between the two brass rods placed at opposite ends of the tube.
  • These electron discharges are called cathode rays which were discovered by J.J Thomson in the 18 th century.
    cathode ray discharge tube


Properties of Cathode Rays

  1. They travel in straight lines
  2. They are particulate in nature i.e. negatively charged electrons
  3. They are affected by both magnetic and electric fields since they are deflected towards the positive plates
  4. They produce fluorescence in some materials
  5. Depending on the energy of the cathode rays they can penetrate thin sheets of paper, metal foils
  6. When cathode rays are stopped they produce X-rays.
  7. They affect photographic plates.


Cathode Ray Oscilloscope (CRO)

  • It is a complex equipment used in displaying waveforms from various sources and measuring p.d.
  • It comprises of the following main components; - The cathode ray tubes (CRT)consists of a tube, electron gun, deflection plates and the time base (TB).
    cathode ray oscilloscope
  • The tube is made of strong glass to withstand the pressure difference between the outside atmospheric pressure and the vacuum inside.
  • It has a square grid placed in front of it to allow measurements to be made.
  • The electron gun produces the electrons with main parts consisting of a filament, a cathode, a grid and the anode.
  • Electrons are produced by the cathode when heated by the filament.
  • The grid is a control electrode which determines the number of electrons reaching the screen therefore determining the brightness of the screen.
  • The Y-deflection plates deflects the beam up or down. Clearly observable when low frequency inputs are applied i.e. 10 Hz from a signal operator.
  • The X-deflection plates are used to move the beam left or right of the screen at a steady speed using the time base circuit which automatically changes voltage to an a.c. voltage.
  • When time base control is turned the speed can be adjusted to produce a waveform.

    Examples
  1. If the time base control of the CRO is set at 10 milliseconds per cm, what is the frequency of the wave traced given wavelength as 1.8 cm?

    Solution
    Wavelength = 1.8 cm.
    time for complete wave = period = 1.8 × 10 milliseconds/cm  = 18 milliseconds = 1.8 × 10-2 seconds.
    Frequency ‘f’, is given=100/1.8 by =56 Hz or f = 1/T = 1/1.8 × 10-2 = 56 Hz

    NOTE: -
    The television set (TV) is a type of a CRT with both Y and X-deflection plates which control the formation of a picture (motion) on the screen.
    The colour television screen is coated with different phosphor dots (chemicals) which produce a different colour when struck by an electron beam.

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