THE FIELDS OF MOVING CHARGES — Edward Purcell & David Morin
Physics — E&M · synchrotron-current
Problem
Synchrotron current *
In a 6 gigaelectron-volt (1 GeV =109 eV) electron synchrotron, electrons travel around the machine in an approximately circular path 240 meters long. It is normal to have about 1011 electrons circling on this path during a cycle of acceleration. The speed of the electrons is practically that of light. What is the current? We give this very simple problem to emphasize that nothing in our definition of current as rate of transport requires the velocities of the charge carriers to be nonrelativistic and that there is no rule against a given charged particle getting counted many times during a second as part of the current.