Cathode ray tube- thermionic emission

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Discussion Overview

The discussion revolves around the operation of cathode ray tubes (CRTs), specifically focusing on the electron gun and the behavior of electrons as they strike the fluorescent screen. Participants explore the implications of electron loss and the circuitry involved in the process.

Discussion Character

  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant questions whether there is a loss of electrons when they strike the fluorescent screen, noting that the screen is not connected in a circuit to return the electrons to their source.
  • Another participant asserts that there is a circuit connecting the fluorescent screen back to the electron gun, explaining that a conductive layer in the phosphor screen conducts electrons away to prevent charging.
  • A subsequent post seeks clarification on whether the circuit returns electrons to the electron gun or if they are earthed, referencing information found on a website.
  • A later reply clarifies that both the electron gun and the phosphor screen are typically connected to ground potential, allowing electrons to flow back to the electron gun, while noting that the specifics of the connection do not affect the overall function.

Areas of Agreement / Disagreement

Participants express differing views on the specifics of how electrons are returned to the electron gun, with some uncertainty about whether they are earthed or returned directly. The discussion remains unresolved regarding the exact nature of the circuit connections.

Contextual Notes

There are unresolved aspects regarding the assumptions about the circuitry and the potential differences between components, which may affect the understanding of electron flow in CRTs.

Nemika
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I have just learned about the cathode ray tube in the class room. There is a part called electron gun in it which emits electrons and these electrons finally strike the fluorescent screen.(I hope its correct till here.) But I want to ask that won't there be a loss of electrons in this process since the fluorescent screen is not connected in a circuit to pass these electrons to the initial atoms from which they where lost.
Thanks in advance for any reply.
 
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There is a circuit connecting the fluorescent screen back to the electron gun. There is a conductive layer incorporated into the phosphor screen to conduct the electrons away. Without this, the phosphor screen would charge up to a large negative potential until it repels the incoming electrons and the CRT ceases to function. The conductive layer conducts the electrons back to ground so there is a complete circuit between the electron gun and the phosphor screen.
 
phyzguy said:
There is a circuit connecting the fluorescent screen back to the electron gun.
So does this circuit takes these electrons back to the electron gun or are they earthed(I found the second one on a website).
 
Nemika said:
So does this circuit takes these electrons back to the electron gun or are they earthed(I found the second one on a website).

Typically both the electron gun and the phosphor screen are connected to a ground (earth) potential. This gives a path for the electrons to flow from the phosphor screen back to the electron gun. They can't be directly connected together because they are at different potentials. It doesn't really matter whether the electrons flow from the phosphor screen, through a power supply and back to the electron gun, or whether electrons flow from the phosphor screen to ground and other electrons flow from ground to the electron gun. The net result is the same.
 
Thanx for clearing my doubt.
 

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