An alternating EMF is generated at a power station by a coil rotating in a magnetic field. The root mean square voltage is the square root of the average voltage square.
Power is lost no matter which direction the current flows.
Average over time. An AC voltage V1 generates a magnetic flux f in the second circuit proportional to the number of turns N1. We can step up a voltage to a high value. Apply a DC voltage to coil one and a magnetic field is created. Given P and R, high voltage maximizes efficiency.