This paper describes an approach to solve the thermal-electromagnetic problem inside a single mode applicator loaded with a ceramic sample. The FEM is combined with analytical techniques to solve the electromagnetic problem. We obtain a fast, simple and accurate method that allows performance investigation of the single mode cavity during the whole heating process. A key feature of the method is to permit iris aperture and short position to vary during the heating. The dependence upon temperature of the complex permittivity of the load is included in the model. The simulation of a thin cylinder ceramic shows that the approach proposed is capable to provide accurate results with low computational cost.