1.5. Magnetic Field
When electrons move (that is, when current flows), a magnetic field is generated around that current flow. This is the basis of an electromagnet. The magnetic field is normally oriented in concentric circles around the current flow, and (like an electric field) it decreases in strength inversely proportional to the square of the distance from the current flow. It is polarized, and its direction (toward its north pole) can be found by the right-hand rule: Point your thumb in the direction of the current flow, and the fingers of your right hand will curl in the direction of the field. See Figure 1-4.
Figure 1-4. If current is flowing, there is a magnetic field around the current; the direction of the field can be found by the right-hand rule.
This magnetic field is well known to boaters and airplane pilots. It can be a significant safety problem if current paths related to radios or lights create magnetic fields that change the direction in which a compass points.6
When current flows (for example, on a circuit board trace), both of these fields (magnetic and electric) exist at the same time. Electric fields can radiate not only into space but also to adjacent traces or planes. Magnetic fields circulate around the trace(s). The pattern of the fields depends on the relative direction and magnitudes of currents on nearby traces. The Hyperlynx signal integrity tool from Mentor Graphics can illustrate these fields when traces are coupled together in the tool. Figure 1-5a illustrates a case of a pair of traces carrying differential signals (the signals are equal and opposite on the two traces). Figure 1-5b illustrates the common-mode case, where the signals are identical on both traces. Notice how the like charges and fields repel (a) and unlike charges and fields attract (b) each other.
Figure 1-5. The Hyperlynx simulation tool can illustrate how the radial electric and circular magnetic fields form around a pair of coupled Microstrip traces above an underlying plane.