Cathode Ray Tube (Triple Science GCSE)

How it works and the function of each part.

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Thermionic Emission


Therm - This emission is triggered by heat

Ionic - Loss of electrons causes the atoms to become ions.

So basically, a thermionic emission is where heat causes atoms in a filament to emit, or 'boil off' outer electrons.

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All in a Vacuum

1. This all happens in a vacuum.

This is because firstly, the filament has to be heated to pretty high temperatures, and if air was present, the oxygen would cause it to burn out in a blink of an eye.

Also, the electrons just wouldn't be able to travel in a straight beam without the vacuum, they would be bumping into air particles left right and centre.

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Boiling off electrons.

2. The low voltage unit is used to heat up a filament at one end of the tube.

This heat causes the filament to emit, or 'boil off' negatively charged electrons and therefore is the cathode. 

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Focussing the electrons to make a beam.

3. When these outer electrons are emitted, they fly off in all directions so in order for them to become a focused beam, we need something to focus them.

This is where the anode/electron accelerator comes in. In the vacuum, a little way from the filament, is the anode which is positively charged and attracts the negative electrons towards it.

It has a tiny hole in it where the electrons then travel through in a beam.

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4. Once the electrons are in a beam, for them to be of any use in TVs/computers, we need for the beam to be deflected. There are two ways the beam can be deflected.

Magnets can deflect the beam by being held close to it; North attracts the beam, South repels it.

Two parallel high-voltage charged plates, a pair of them horizontally and a pair vertically, placed around the path of the beam. If the plates are equally positive/negative, then the beam will carry on in the same direction, but if one of the plates has a higher positive/negative voltage, it will deflect the beam. The horizontal plates will affect the beam's up-and-down movement, the vertical plates will affect the beam's side-to-side movement.

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Usage in TVs

TVs have these components inside, but they also have a phosphoric screen at the end of the tube, which releases light when hit by the electron beams. There are 3 electron guns in the TV, one for red light, one for green, and one for blue. Combined these can make all the different colours of visible light.

There are either big coils inside which create an electromagnetic field to deflect the electron beams, or charged plates which repel or attract the beam to send it in a particular direction.

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