4.5 ELECTRON BEAMS ELECTRONS AND ELECTRONICS

Electron Beams Given enough energy, electrons can escape from a conductor and move through a vacuum space.Beams of electrons are used in oscilloscopes, TV picture tubes, computer monitors and X –Ray tubes.

ELECTRON BEAMS Emission of electrons from a metal surface when heated to a very high temperature is called thermionic emission.

Cathode Rays Cathode rays are beams of fast moving electrons

ELECTRON BEAMS VACUUM TUBES

Cathode Ray tube Cathode- it is a tungsten filament that is heated to C A 6V supply is used to heat the cathode that emits electrons The electrons are attracted to the anode and a current starts to flow With air in the tube, rather than vacuum, th electrons would collide with gas molecules and the white hot filament would burn up

ELECTRON BEAMS Deflection tubes

ELECTRON BEAMS Oscilloscope:

Cathode Ray Oscilloscope The CRO uses a narrow beam of electrons to trace out waveforms and other signals on a fluorescent screen A bright spot is formed on the screen where the beam strikes it If the beam is deflected, the spot moves If the spot moves fast enough, it appears as a line To deflect the beam, there are two sets of deflection plates

ELECTRON BEAMS Y – plates: The Y- plates move the beam vertically. This happens when an external source of voltage – for example, an AC supply is connected across the Y-input terminals

Gain control The amount of vertical movement can be amplified by the gain control

ELECTRON BEAMS X-plates: The X-plates move the beam horizontally.. The movement is produced by a circuit called the time base inside the oscilloscope. The time base automatically applies a changing voltage across the plates so that the spot moves from left to right across the screen at a steady speed.

Measuring with an oscilloscope The Y input is connected to an AC supply The gain control is set at 5V/cm If the amplitude of the waveform is 2cm, the peak voltage will be 10V The time base control is set at 10ms/cm The horizontal peak to peak is 4cm, so the time period is 40ms = 0.04s Frequency = 1/time pd = 25Hz

ELECTRON BEAMS Oscilloscope: