- #1

felixthecat

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## Homework Statement

a beam of particles of charge q with velocity v is emitted from a point source, roughly parallel with a magnetic field B, but with a small angle of angular dispersion. Show that the effect of the field is to focus the beam to a point at a distance z = 2[/pi]mv/|q|B.

## Homework Equations

We know the force F= q vxB

dv/dt = -q/m Bxv

## The Attempt at a Solution

I try to solve the problem with the force. However, since the beam is roughly parallel to B, the cross product tends to 0. I am now like clueless...