From this book it has been also observed that the methods solves for the field sources (currents and charges). These sources can be either physical sources, or mathematical (equivalent) sources introduced through various electromagnetic field theorems. In the numerical analysis, the electromagnetic fields, or the related potentials, are expressed in terms of these sources, usually through the Lorentz potentials. The expressions are integral forms, where the sources appear under some integrals, multiplied by appropriate functions, which are referred to as kernels. For example, for fields in a vacuum, the kernel for the Lorentz potentials is the free-space Green's function. On the other hand, certain equations are imposed based on the boundary conditions or constitutive relations. The boundary conditions relate tangential and normal components of the field vectors at a surface of discontinuity. For example, on the surface of a perfectly conducing body, the tangential component of the electric field vanishes.