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Homework Help: Finding E field from potential

  1. Mar 11, 2010 #1
    Two long conducting cylindrical shells are coaxial and have radii of 20 mm and 80 mm. The electric potential of the inner conductor, with respect to the outer conductor, is +600V.

    In the situation provided, an electron is in circular motion around the inner cylinder in an orbit of 30mm radius. Find the speed of the electron in orbit.

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    So where I'm stuck is mostly just in finding the E field at said point

    The electron has a force radially inward putting the electron in uniform circular motion.
    Therefore:

    [tex]eE=\frac{mv^{2}}{r}[/tex]

    [tex]v=\sqrt{\frac{erE}{m}} [/tex]

    So, I can't seem to figure out how to find the e field around the point r =30mm

    What I tried to do was the following

    [tex]V_{a}-V_{b}=\int \vec{E} d\vec{l} [/tex]

    Then I rewrote it as


    [tex]-\int dV =\int \vec{E} d\vec{l} [/tex]

    So [tex]E = -\frac{dV}{dl} [/tex]


    However I think all I've just done is derived the gradient, and I don't know how to use this without a function.

    Give me a hint on how to continue my calculation or give me an easier way to calculate E
     
  2. jcsd
  3. Mar 11, 2010 #2

    rl.bhat

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    Homework Helper

    Find the capacitor per unit length of the co-axial cable. Then find charge per unit length λ by using Q = C*V formula.
    The electric field between the coaxial cylinder is given by
    E = λ/(2*π*εο*r)
     
  4. Mar 11, 2010 #3
    Well I can't possibly answer with that solution though. I know nothing about the length of the cylinder except that it is very long. To answer in terms of lamba would be an incomplete answer since I still have unknown variables.
     
  5. Mar 11, 2010 #4
    Thank you rl.bhat

    I took a look at the lamba again and figured out I should have simply just solved for lamba instead of q using my potential integration. Thanks!
     
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