• Electromagnetics I
  • Ch 6: Steady Current and Conductivity
  • Loc 6.2
  • Electromagnetics I
  • Ch 6
  • Loc 6.2

Current Distributions

In elementary electric circuit theory, current is the rate at which electric charge passes a particular point in a circuit. For example, 1 A is 1 C per second. In this view current is a scalar quantity, and there are only two possible directions because charge is constrained to flow along defined channels. Direction is identified by the sign of the current with respect to a reference direction, which is in turn defined with respect to a reference voltage polarity. 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.

However, this kind of thinking only holds up when the systems being considered are well-described as “lumped” devices connected by infinitesimally thin, filament-like connections representing wires and circuit board traces. Many important problems in electrical engineering concern situations in which the flow of current is not limited in this way. Examples include wire- and pin-type interconnects at radio frequencies, circuit board and enclosure grounding, and physical phenomena such as lightning. To accommodate the more general class of problems, we must define current as a vector quantity. Furthermore, current in these problems can spread out over surfaces and within volumes, so we must also consider spatial distributions of current.

    Line Current

    As noted above, if a current

    is constrained to follow a particular path, then the only other consideration is direction. Thus, a line current is specified mathematically as

    , where the direction

    may vary with position along the path. For example, in a straight wire

    is constant, whereas in a coil

    varies with position along the coil.

    Surface Current Distribution

    In some cases, current may be distributed over a surface. For example, the radio-frequency current on a wire of radius

    made from a metal with sufficiently high conductivity can be modeled as a uniform surface current existing on the wire surface. In this case, the current is best described as a surface current density

    , which is the total current

    on the wire divided by the circumference

    of the wire:

    (6.2.1)

    Volume Current Distribution

    Imagine that current is distributed within a volume. Let

    be the current passing through a small open planar surface defined within this volume, and let

    be the area of this surface. The volume current density

    at any point in the volume is defined as

    (6.2.2)

    In general,

    is a function of position within this volume. Subsequently the total current passing through a surface

    is

    (6.2.3)

    In other words, volume current density integrated over a surface yields total current through that surface. You might recognize this as a calculation of flux, and it is (but it is uncommon to refer to net current as a flux).1

    Exercise

    EXAMPLE 6.2.1: CURRENT AND CURRENT DENSITY IN A WIRE OF CIRCULAR CROSS-SECTION


    Figure 6.2.1 shows a straight wire having cross-sectional radius

    mm. A battery is connected across the two ends of the wire resulting in a volume current density

    A/m

    , which is uniform throughout the wire. Find the net current

    through the wire.

    The net current is

    We choose

    to be the cross-section perpendicular to the axis of the wire. Also, we choose

    to point such that

    is positive with respect to the sign convention shown in Figure 6.2.1, which is the usual choice in electric circuit analysis. With these choices, we have

    This answer is independent of the cross-sectional surface

    used to do the calculation. For example, you could use an alternative surface that is tilted

    away from the axis of the wire. In that case, the cross-sectional area would increase, but the dot product of

    and

    would be proportionally less and the outcome would be the same. Since the choice of

    is arbitrary – any surface with edges at the perimeter of the wire will do – you should make the choice that makes the problem as simple as possible, as we have done above.

    • 1

      However, it is not common to refer to net current as a flux; go figure!

    Additional Reading:

     

    Ellingson, Steven W. (2018) Electromagnetics, Vol. 1. Blacksburg, VA: VT Publishing. https://doi.org/10.21061/electromagnetics-vol-1 CC BY-SA 4.0

     
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