An electromagnetic field can also be divided into (SOBs). An electromagnetic field is no change to the time (Static Field or DC Field) for example. The electric field between the clouds and the earth oh. Magnetic field of a permanent magnet. " earth's magnetic field.The other is the electromagnetic field with a time-varying (Dynamic Field or AC Field) for example, electromagnetic fields caused by the alternating current power distribution system (50. Hz) and communication system, etc.!