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Homework Help: MeV related to Velocity ?

  1. Dec 9, 2007 #1
    MeV related to Velocity ??

    1. The problem statement, all variables and given/known data
    An electron beam from a 2.0 MeV Van De Graff enters at right angles to a uniform magnetic field of .05 weber/m^2. What is the radius of curvature of the path of the elctrons?

    2. Relevant equations
    A couple of them. First off what the hell is a weber... after taking 3 years of physics in college this is my first encounter with such a unit. But more importantly... how do i convert MeV into Velocity?

    3. The attempt at a solution

    I know

    qv x B = (Mv^2)/r

    I tried converting MeV into joules and then plugging it into the Kinetic energy equation .. no go.. i started getting velocities much bigger than the speed of light. Any help is appreciated, as this also is used in the next problem.
  2. jcsd
  3. Dec 10, 2007 #2


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    Science Advisor

    The weber is a unit of magnetic flux, equal to one telsa-meter^2. See

    You need to use a relativistic formula to get the velocity, [itex]E=\gamma mc^2[/itex], with [itex]\gamma=(1-\beta^2)^{-1/2}[/itex] and [itex]\beta=v/c[/itex]. For the electron, [itex]mc^2=0.511\,\rm MeV[/itex]. (This is how particle physicists always quote particle masses, by the rest energy [itex]mc^2[/itex]; it's much more convenient than converting everything to MKS units.)
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