Plates in a CRT and how they become charged

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    Charged Crt Plates
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Discussion Overview

The discussion revolves around the charging mechanism of the plates in a cathode ray tube (CRT) system, particularly focusing on how the plates become charged and the role of electric fields in the movement of electrons between them. The scope includes theoretical aspects of electric fields, potential differences, and practical applications in CRT technology.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant hypothesizes about the charging of the plates in a CRT, questioning how they become charged initially.
  • Another participant suggests that an electric field exists between the plates due to a potential difference, explaining that the positive plate causes an upward force on the electron.
  • A different participant mentions that the electric field on the deflector plates can be driven by a signal generator, which directs the electron beam as needed.
  • It is noted that modern CRTs often use magnetic deflection coils rather than electric field deflection plates, although acceleration and focusing plates are still present in the electron gun structure.
  • Participants reference historical experiments, such as those by J.J. Thomson, to illustrate concepts related to electron movement and deflection.

Areas of Agreement / Disagreement

Participants present multiple viewpoints regarding the mechanisms of charging and deflection in CRTs, indicating that there is no consensus on the initial charging process of the plates or the predominance of electric versus magnetic deflection methods.

Contextual Notes

Some assumptions about the nature of the plates and their charging mechanisms remain unaddressed, and the discussion does not resolve the specifics of how the electric field is established or the implications of using magnetic deflection in modern CRTs.

h20h
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I just am trying come up with a hypothesis regarding the plates in a CRT system? How do they become charged? Looking at when an electron travels between the two plates, say the plates are on the top and bottom and the top plate is positively charged and the bottom is negatively charged, we know that the electron will continue forward but also curve up because of it oppsite charges. The plate being positive and electron being negative, it will hit the screen (say in a tV) near the top of the display area...but how do the plates become charged to begin with?

ANy thoughts or help here?

h20h
 
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I would say there is an electric field between the plates. There is a potential difference with positive on the top, so it cause a force upward on the electron. If it is a capacitor plate, I'm assuming because of the opposite charges, the plates are conductors (free moving charges), on one plate the positive charges move out of the plate toward the emf source and leave a flow of negative charges and the other end the opposite, but in a similar fasion. Kinda reminds me of JJ Thomson's experiment, how he got electrons to curve. Yeah, his example is perfect, check it out!
 
Last edited:
The electric field on the deflector plates in a CRT can be driven by a signal generator, to make the beam go where required.

In an oscilloscope, a saw-tooth waveform will give a repeating line scan, for instance.
 
And keep in mind that most modern CRTs use magnetic deflection coils outside the neck of the tube, not electric field deflection plates inside the neck. There are still acceleration and focusing plates in the electron gun structure in the neck, though.

And the horizontal and vertical deflection B-fields are generated by driving signal currents through the deflection coils -- signal generator circuits are used for this as Mentz says.
 

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