alpha particles are accelerated through a potential of 1000volts and then enter a magnetic field 0.2T perpendicular to the their direction of motion.Calculate the radius of the circular path that they traverse.An alpha particle has a mass 6.68x 10^-27kg and a charge of +2e.
Review and apply this article:
http://en.wikipedia.org/wiki/Gyroradius
You will need to convert the kinetic energy (2,000 eV in this case) to velocity