# MeV related to Velocity ?

1. Dec 9, 2007

### Evenglare

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
v=E/B

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. Dec 10, 2007

### Avodyne

The weber is a unit of magnetic flux, equal to one telsa-meter^2. See
http://en.wikipedia.org/wiki/Tesla_(unit)

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