The entire field collapses and rebuilds in the magnetically opposite direction. If current alternates continuously, the field is never static. Alternating currents do, in a sense, copy their changes of energy into nearby circuits, making energy available there. For example, power present in the primary as a large current at a low voltage may be transformed into low current at high voltage in the secondary. AC Advantages Generally, engineers would much prefer to send power over long lines at a very high voltage, with comparatively lower current, but deliver it to most users at a safer, lower voltage.
Transformers make that possible.
Resistance in AC circuits works differently, too, so that with good design, losses in power lines are dramatically lower than in DC lines. The same basic AC ideas, a magnetic transfer and transformation of power, can also make highly efficient, reliable motors.