Near field and far field
In addition to Faraday's law displacement current is another physics concept that transcends electronic circuit theory. Displacement current adds to Ampere's law and when combined with Faraday's law provides the wave equations for electromagnetic radiation, e.g., the transmission of electromagnetic waves by a radio station. AC currents produce electric and magnetic fields and these fields appear differently depending on how far the observer is from the source. Electric and magnetic fields resulting from charges or currents in nearby conductors are called near fields and are consistent with capacitance/inductance concepts whereas fields resulting from the radiation by distant transmitters are called far fields and are known as radio waves. The approximate dividing line for near field v. far field behavior is the distance l/4. For distances much less than l/4 from an oscillating current in a conductor, the near field is dominant whereas for distances much greater than l/4 the far field is dominat. For example, a 1 MHz signal has l=300 meters (since the speed of light is 300,000 km per second) so for distances much less than 75 meters from a 1 MHz transmitter the near field dominates, but for distances much greater than 75 meters the far field dominates. The near-field can react back on the source, but the far-field does not. The near field falls off as 1/r^2 where r is the distance from source to observer, but the far-field falls off only as 1/r. Picking up the far-field is essentially a receiving antenna problem, whereas picking up the near field is analogous to the problem of how a transformer or capacitor works.