This volume exposes and develops the essential topics for understanding the physical foundations and calculation methods of alternating current circuits. The text is a useful tool for the acquisition of skills necessary for professionals as well as students of Electrical and Electronic Technologies, also representing a preparatory resource for university courses. Specifically, it is composed of the first six chapters of the larger work: "From Alternating Current to the Fourier Transform" and is recommended for those who intend to limit themselves to the study or review of only the aspects relating to the steady state of single-phase alternating current systems. The maximum possible self-sufficiency was one of the guiding elements in the conception of this work, with references and detailed explanations of the most critical aspects of the subject, with the aim of making the reader as autonomous as possible and quickly. Knowledge of the basic concepts relating to direct current networks and the possession of elementary notions of electromagnetism are considered prerequisites for tackling this text, but as regards the topic of alternating currents, as mentioned, the text is substantially self-sufficient. For each chapter there is a certain number of examples and for all there are verification problems. 1 SINUSOIDAL ELECTRICAL QUANTITIES 2 SINUSOIDS, PHASORS AND COMPLEX NUMBERS 3 IMPEDANCE AND REACTANCE 4 ADMITTANCE AND AC NETWORKS 5 RESONANCE 6 ELECTRICAL POWER IN AC SYSTEMS SANDRO RONCA After studying Physics at the University of Padua, he dedicated himself to teaching Electrical and Electronic Technologies and Computer Science at Technical and Technological Institutes, paying attention to the didactic aspects of the subject. He has studied computer networks in depth and designed, at the request of industrial associations, courses for systems engineers and IT security workers.