# Plates in a CRT and how they become charged

1. Apr 22, 2007

### h20h

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 becuase 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

2. Apr 22, 2007

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: Apr 22, 2007
3. Apr 25, 2007

### Mentz114

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.

4. Apr 25, 2007

### Staff: Mentor

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.