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Cathode Ray Tube

  1. May 21, 2007 #1
    1. The problem statement, all variables and given/known data
    I have to find out a relationship between the deflection of an electron beam in a cathode ray tube and the accelerating voltage.

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

    3. 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.
  2. jcsd
  3. May 21, 2007 #2


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    Staff: Mentor

    Here's a backround info page on CRTs:


    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?
  4. May 21, 2007 #3
    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.
  5. May 21, 2007 #4


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    Staff: Mentor

    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.
  6. May 21, 2007 #5
    Thank you, again.
    The higher the voltage the higher the acceleration and the associated velocity.
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