Static fields (0 Hz)

Static fields (0 Hz) occur e.g. as in the Earth's magnetic field and with permanent magnets, at direct current transmission lines (HVDC lines), batteries, and between objects with different electrical charges. In medicine, strong static magnetic fields are used in magnetic resonance imaging.
Static fields don’t change their polarity or only very slowly, thus, the frequency is 0 Hz or almost 0 Hz (< 0.1 Hz).