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Homework Help: Charge and Current AS Question

  1. Apr 28, 2009 #1
    The beam current in a television tube is 32mA. The electrons travel at a speed of 4.2x10^7m/s through a distance 21cm.

    a) The number of electrons striking the screen in 1 hour is?

    I did: Q=IT = 0.032A x (60x60)s = 115.2 C

    Number of electrons = 115.2/1.6x10^-19 = 7.2x10^20

    b) The number of electrons in the beam at any instant is?

    I tried doing the same as above but for one seconds but couldn't work it out at all. Anyone give me a clue?
  2. jcsd
  3. Apr 28, 2009 #2


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    Homework Helper

    The length of your beam is .21 m.

    In 1 sec you would have a beam column containing of .032ma.

    A 1 sec column would contain 3.2*10-5*6.241*1018 electrons of which only .21/4.2*107 are actually in motion within the tube.
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