surroundings dissipating energy in the form of heat. The moving charge bumps into 'non-moving'objects and transfers mechanical energy which appears in the form of heat in the medium. Conduction currents represent an energy dissipating mechanism for an electromag-netic field. Energy is extracted from the electromagnetic field and transferred irreversibly into the medium as heat.

Mathematically one describes the relationship between conduction current and the applied electric field as indicated

in Equation 3-1.

（3-1） =

In simple materials, the relationship is linear and the proportionality constant is referred to as the electrical conductiv-ity. Electrical conductivity has units ofSiemens per meter (S/m). For many applications, however, it is more useful to work with units of milliSiemens per meter (mS/m). Conductivity is dependent on the charge density and the inter-nal statistical mechanical interaction ofthe charge with its surroundings. These details are beyond the scope of this discussion.

It should be noted that electrical conductivity and resistivity are directly related. Refer toFigure 3-3 for the relation-ship and the expression of Ohm's law. Electrical resistivity is the inverse of electrical conductivity.

Figure: 3-3 Relationship between current and applied field as well as the relationship will Ohm's

law and resis-tively.

It is important to note that there are simplifications in the above discussion from the general form shown in Chapter 2.

The conductivity is shown as being a constant. In fact it can be a function of the rate of change of the electric field,the amplitude of electric field itself, as well

as