The flux momentum is a physical magnitude that represents the amount of linear momentum that crosses a surface. The density of such magnitude can be defined locally in space by means of a vector field. The density of flux momentum of an electromagnetic field is defined in all the regions of the space where the electromagnetic field is present. The Maxwell's Stress tensor is the rigorous expression for the density of electromagnetic flux momentum and is given in Eq. A.10. This appendix shows a less formal way to derive the Maxwell's Stress tensor in order to provide a more intuitive meaning of such quantity.