An electron is accelerated from rest through a potential difference of 4000v. It enters a region where B=5.0*10^-3T perpendicular to its velocity. calculate the radius of the path it will follow.
The answer is : 0.042M but i dont know how to get it.

Question

An electron is accelerated from rest through a potential difference of 4000v. It enters a region where B=5.0*10^-3T perpendicular to its velocity. calculate the radius of the path it will follow.
The answer is : 0.042M    but i dont know how to get it.

anonymous · Answer

need to find velocity of the electron:
$$mv^2/2=eV$$
thus
$$v=\sqrt{2eV/m}$$
in magnetic field, there Lorentz force applies to electron, changes direction only
$$mv^2/r=Bev$$
thus 
$$r=mv/Be=m/Be*\sqrt{2eV/m}=\sqrt{2mV/eB^2}$$
here e andm m is charge and mass of the electron