To translate theory into practice, we can use the TINA package from DesignSoft for Texas Instruments. It is a tool for building virtual analog and digital circuits. The basic version provides the minimum necessary set of elements that can be arranged in the form of a circuit. However, the most important function (at least for me) is the ability to run simulations.
All presented examples of simulations are prepared in TINA TI V9 (Schematic Editor), Special Complementary Basic Edition available free of charge. The schema sources are in the repository for this publication, for independent study github.com/michalasobczak/simple_hpc in the folder SeriesPartOne.
 For the purpose of studying a system, the concepts of linearity and non-linearity are adopted. A system with at least one nonlinear element will be a nonlinear system. Nonlinearity consists in the inability to describe the current-voltage characteristic of a given element in the form of a straight equation. When describing a mixed system, consisting of both linear and nonlinear elements, the point of intersection of their characteristics will be the working point.
Note: If we do not need to test the circuit, we can use other packages for electronics such as Frizting, LibrePCB or EAGLE.
Examples available at https://github.com/michalasobczak/simple_hpc/tree/master/SeriesPartOne