Finding Deflection of Electron Beam in Cathode Ray Tube

[husky88]

Homework Statement

I have to find out a relationship between the deflection of an electron beam in a cathode ray tube and the accelerating voltage.

Homework Equations

I know that theoretically D is inversely proportional to the accelerating voltage.

The Attempt at a Solution

I have to do this experimentally and they are giving me data tables. To find out the deflection (D) they give me:
Accelerating Voltage: 400 V
First Position: 5.00 cm
Second Position: 9.00 cm
To find out the deflection (D), they tell me to subtract the two positions and divide by 2. I don't understand what first position and second position refer to. Why do I need to divide by 2? I actually don't know how a cathode ray tube works and why would it provide 2 different positions for the same accelerating voltage.

I don't know if I provided enough details, but any suggestions or ideas are greatly appreciated.

 

[berkeman]

Here's a backround info page on CRTs:

http://en.wikipedia.org/wiki/Cathode_ray_tube

You may want to search the "Lorentz Force" at wikipedia as well.

What can you say about how the Lorentz Force varies with the velocity of the charged particle? How does the velocity of the charged particle vary with the applied voltage between the cathode and anode?

 

[husky88]

Thank you. That really helped shed some light on cathode ray tubes. :)
The Lorentz force increases when velocity increases. And I think the velocity is inversely proportional with the voltage. (?)

But I still don't know what to make of this data that I have.
Deflection = (Second Position - First Position)/2
at a certain voltage. More specifically what are the conditions for the first position and for the second position and why are they different if the voltage is the same.

 

[berkeman]

Thank you. That really helped shed some light on cathode ray tubes. :)
The Lorentz force increases when velocity increases. And I think the velocity is inversely proportional with the voltage. (?)

No. The energy of the electron accelerated across some voltage (on the anode-cathode capacitor, basically) is expressed in electron-volts (eV). The higher the voltage the ______ the acceleration and the associated velocity.

 

[husky88]

Thank you, again.
The higher the voltage the higher the acceleration and the associated velocity.
Hmmm...