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Homework Help: Magnetic and electric fields

  1. Nov 25, 2004 #1
    i have no idea how to do this problem:

    a velocity selector consists of electric and magnetic fields described by the expressions E=E(k hat) and B=B(j hat), with B=14.5 mT. Find the vale of E such that a 828-eV electron moving along the positive x axis is undeflected.

    hmm..since it is undeflected we can say that qE=qvB, right? and v=E/B...

    but how could u solve for E? and how does the 828 eV electron fit into this problem?
  2. jcsd
  3. Nov 25, 2004 #2

    Andrew Mason

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    You have correctly stated the Lorentz force law:
    [tex]\vec{F} = q \vec{E} + q \vec{v} \times \vec{B}[/tex]

    Determine electron speed from the electron kinetic energy of 828 eV. Since the net force = 0,
    [tex]\vec{v} = \frac{-\vec{E}}{\vec{B}}[/tex]

  4. Nov 25, 2004 #3
    [tex]1eV=1.602 \times 10^{-19} J[/tex]
    [tex]m_e=9.11 \times 10^{-31} Kg[/tex]

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