Ricardo Lopes Coelho did his PhD and Habilitation at the Technical University Berlin, where he taught for several years. He has taught History and Philosophy of Science at the Faculty of Science, University of Lisbon for more than two decades. More recently, he created the History and Philosophy of Physics Laboratory, where historical experiments are carried out. His main research interest is the understanding of scientific concepts and principles through their history. On the concept of energy, he has published numerous articles over the last fifteen years. In the present book, he revisits some of those theses in a more in-depth context, among other things, among other things performing an analysis of contemporary textbooks and presenting the results of experiments carried out in his laboratory, material that has not appeared in the literature to date.