This lecture explores the concept of magnetic energy, demonstrating theoretically the process of storing energy in a transformer and calculating the magnetic energy in a coil. The instructor explains the equations for induced voltage, electrical power, and magnetic power, emphasizing the relationship between magnetic energy and inductance. The lecture concludes by relating magnetic energy to reactive power in electrical systems, highlighting the importance of creating and storing magnetic energy in coils. The audience is introduced to the calculation of magnetic energy in non-linear systems and complex geometries.