Page 31

EDNE MAY 2015

Electromagnetics netic currents are accompanied by an electric (E) field. Magnetic current following a solenoid path would create a strong E field inside the solenoid analogously to an electric current following a solenoid path creating a strong H field inside the solenoid. All you need to know about magnetic currents is that they are just like ordinary electric currents except they are accompanied by an E field instead of an H field. • Displacement Current Densities: You may know that ∂D/∂t is called the electric displacement current density. You may have never heard of a name for ∂B/∂t but you probably would not be surprised to know that it is called the magnetic displacement current density. The value of ∂D/∂t field at a particular point in space and time is the partial derivative of D with respect to time at that same point in space and time. ∂B/∂t has a similar relation to B. According to textbooks and from where they occur in Maxwell’s equations, we know that ∂D/∂t and ∂B/∂t act exactly like an electric and magnetic current densities respectively. A classical vacuum is empty, so it may seem paradoxical that there could be a current in the vacuum. It may seem doubly paradoxical in the case of magnetic displacement current. Not only is there a current in a supposedly empty vacuum, but it acts like it is made of non-existent magnetic monopoles. In classical physics, there is no satisfactory physical explanation as to the composition of these fields in a vacuum. We will take the pragmatic approach and simply say that if the D and B fields exist, then the ∂D/∂t and ∂B/∂t fields also exist and can be illustrated in the same manner as any other field. All you need to know about ∂D/∂t and ∂B/∂t is to consider them to be ordinary electric and magnetic current densities respectively. Figure 1. Displacement currents as sources of E and H. Fig. 1 depicts the displacement currents and the fields that they create. There is an asymmetry in the handedness of the fields. In fig. 1(a) when ∂D/∂t creates H, it follows the right hand rule. In fig. 1(b) when ∂B/∂t creates E, it follows the left hand rule. This asymmetry is essential. Without it, the plane wave would be self-extinguishing instead of self-sustaining. Figure 2. Side view of the E fields radiated by a dipole antenna The article continues by applying this perspective to fields radiated by a dipole antenna (Figure 2, above) to aid comprehension of how the propagating plane wave is generated – click below. Download PDF of Article Find Electromagnetics on EETsearch 30 EDN Europe | MAY 2015 www.edn-europe.com


EDNE MAY 2015
To see the actual publication please follow the link above