6: Steady Current and Conductivity

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In practice, we deal with two physical mechanisms for current: convection and conduction. The distinction between these types of current is important in electromagnetic analysis. Convection current consists of charged particles moving in response to mechanical forces, as opposed to being guided by the electric field (Sections 2.2 and/or 5.1). An example of a convection current is a cloud bearing free electrons that moves through the atmosphere driven by wind. Conduction current consists of charged particles moving in response to the electric field and not merely being carried by motion of the surrounding material. In some materials, the electric field is also able to dislodge weakly-bound electrons from atoms, which then subsequently travel some distance before reassociating with other atoms. For this reason, the individual electrons in a conduction current do not necessarily travel the full distance over which the current is perceived to exist. The distinction between convection and conduction is important because Ohm’s Law (Section 6.3) – which specifies the relationship between electric field intensity and current – applies only to conduction current.

6.1: Convection and Conduction Currents
In practice, we deal with two physical mechanisms for current: convection and conduction. The distinction between these types of current is important in electromagnetic analysis.
6.2: Current Distributions
The convention in electrical engineering defines positive current as the flow of positive charge into the positive voltage terminal of a passive device such as a resistor, capacitor, or inductor. For an active device such as a battery, positive current corresponds to the flow of positive charge out of the positive voltage terminal.
6.3: Conductivity
Conductivity is one of the three primary “constitutive parameters” that is commonly used to characterize the electromagnetic properties of materials . Conductivity is a property of materials that determines conduction current density in response to an applied electric field.
6.4: Resistance
The concept of resistance is most likely familiar to readers via Ohm’s Law for Devices; i.e., V=IR where V is the potential difference associated with a current I . This is correct, but it is not the whole story.
6.5: Conductance
Conductance, like resistance, is a property of devices. Therefore, conductance depends on both the conductivity of the materials used in the device, as well as the geometry of the device.
6.6: Power Dissipation in Conducting Media
The displacement of charge in response to the force exerted by an electric field constitutes a reduction in the potential energy of the system. If the charge is part of a steady current, there must be an associated loss of energy that occurs at a steady rate. Since power is energy per unit time, the loss of energy associated with current is expressible as power dissipation. Here. we address two questions: (1) How much power is dissipated in this manner, and (2) What happens to the lost energy?