In this work, a single-mode resonant microwave cavity (2.45 GHz) is loaded with spherical particles. A weakly coupled electromagnetic (EM)-thermal solver is used iteratively to determine how the microscopic geometry (local curvatures between the particles, grain size and neck size) modifies the EM field, and in turn the thermal field in the particles and also to quantify the microwave effect for an experimental process. The modelling is performed with the conformal finite element solver COMSOL Multiphysics. Moreover, this study will show that the electric field norm increases for one spherical particle of ceria with different sizes and consequently the temperature increases. In addition, the electric field and heating behaviour have been studied in the case of three particles with different neck sizes. Finally, the effect of the configuration versus the E-field direction has been studied for three particles and has a significant result.