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Determine the electric field that yields the desired result

  1. Jul 23, 2006 #1
    A beam of protons with various speeds is directed in the positive x direction. The beam enters a region with a uniform magnetic field of magnitude 0.45 T pointing into the page, as indicated in Figure 22-45. It is desired to use a uniform electric field (in addition to the magnetic field) to select from this beam only those protons with a speed of 1.60 105 m/s; that is, only these protons should be undeflected by the two fields.

    [​IMG]

    (a) Determine the electric field that yields the desired result.
    size: ____72000___ N/C
    dir: -y dir

    (b) Suppose the electric field is to be produced by a parallel-plate capacitor with a plate separation of 2.5 cm. What potential difference is required between the plates?
    ________ V

    for part B, I understand that C=EA/d and V=IR

    also, I can find I, by that I can use I=2pieRB/(4pie x 10^-7)
    but I am really not sure how to find this potential difference correctly, can anyone help?
     
  2. jcsd
  3. Jul 23, 2006 #2

    Andrew Mason

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    You seem to be just guessing at formulas. None of the three you have identified are used here, in either part.

    What is it that determines whether the protons get through the plates? What are the forces acting on the protons? How do you determine what those forces are?

    AM
     
  4. Jul 23, 2006 #3
    force acting on the proton is upwards and that should be force of the magnetic field, and im not sure what determines whether the protons get through the plates.
     
  5. Jul 23, 2006 #4

    Andrew Mason

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    There are two kinds of forces: motion through the magnetic field produces a Lorentz force. The electric field produces an electric force. They are in opposite directions. If they are not balanced, what happens to the proton? does it make it through?

    AM
     
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