In an electron cyclotron resonance ion source, ions are produced from a plasma generated and sustained by microwaves with a proper frequency. Some experiments showed that the plasma formation, the consequent amount of particles extracted from the source, and the related beam shape strongly depend on the frequency of the electromagnetic wave feeding the cavity. In order to have a better understanding of these phenomena, in this work we deal with the description of the motion of a charged particle inside the plasma chamber model of the SERSE ion source operating at INFN-LNS in Catania, the analysis being applicable to any similar apparatus. The electromagnetic fields inside the vacuum filled chamber were determined theoretically and, together with proper simulations, their fundamental role on the particle motion, on their confinement, and on the energy transfer they are subjected to during their motion within the cavity is shown.