Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Electrons in television tube

  1. Jan 29, 2007 #1
    1. The problem statement, all variables and given/known data
    In a typical television tube, the electrons are accelerated through a potential difference of 25,000 volts. a) What speed do the electrons have when they strike the screen? And b) What is their kinetic energy in joules?

    2. Relevant equations
    [tex]K_m_a_x = hf - \varphi[/tex]
    [tex]KE = \gamma mc^2 - mc^2[/tex]

    3. The attempt at a solution

    Well, i don't know where to start. How do you use the volts to obtain the velocity of the electron, so far, i haven't encountered any formular that can do that.(at least i didn't notice in the lecture)

    Any help will be appreciated. Thanks
  2. jcsd
  3. Jan 29, 2007 #2
    First off, the photoelectric effect is not part of the problem at all.

    The energy gained by an electron moving through a 1 volt potential is 1 eV (electron volt). So then how much energy is gained moving through a 25,000 V potential? Also a constant may be useful is the rest mass of an electron, [tex]{m_0}=511 \frac{KeV}{c^2}[/tex]. Can you solve for v now, using your kinetic energy relationship?
  4. Feb 2, 2007 #3
    Ok, so i am going to use this equation : [tex]KE = \gamma mc^2 - mc^2[/tex]

    And then since the electrons are accelerated through 25,000 Volts it gains 25,000 eV. This 25,000eV = KE in the above equation is that correct?

    so i obtain :
    [tex]\gamma mc^2 = 25,000eV + mc^2[/tex]
    [tex]v = 9.055*10^7[/tex]

    Is this correct can anyone confirm?
  5. Feb 2, 2007 #4


    User Avatar
    Science Advisor
    Homework Helper

    Aside from the fact you should specify units on v, it is correct.
    Last edited: Feb 2, 2007
  6. Feb 2, 2007 #5
    Thanks for the help.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook