In the previous chapter we reviewed the static electric fields due to stationary charge distributions, and static magnetic fields due to charges moving at constant speed, i.e. dc currents.
With one exception, the two static fields are independent of each other, allowing us to study them separately. The only time the static fields are linked is in a lossy medium, where the current density J and the electric field intensity E are related by the conductivity of the medium as
The E field produces the current density J, which in turn creates the magnetic field.
When the charge distributions and currents vary with time the electric and magnetic fields will also vary with time. When the resulting fields are quasi‐static (slowly varying) we can study them separately. When the electric and magnetic fields vary rapidly with time they become coupled – the time‐varying electric fields produce the time‐varying magnetic fields, and conversely, the time‐varying magnetic fields produce the time‐varying electric fields.
This field coupling is the key factor in the study of the electromagnetic waves, transmission lines, and antennas, which is the subject of the next three chapters.
15.1 Eddy Currents
In this section we will discuss volume and surface induced electric currents in solid conducting bodies, when exposed to time‐varying magnetic field (flux).