edVirtus Reference

What Are Electric and Magnetic Fields?

How Do the Two Components of an Electromagnetic Field Differ?

An electric field is a region in which an electric charge experiences force. A magnetic field is a region in which moving charges, electric currents, or magnetic materials experience magnetic force. Together, electric and magnetic fields form the physical foundation of electricity, magnetism, electromagnetic induction, and electromagnetic radiation.

Electric fields are produced by electric charge and differences in electric potential. Their strength is commonly expressed in volts per meter (V/m). Magnetic fields are produced by moving charge and electric current. Magnetic field strength is expressed in amperes per meter (A/m), while magnetic flux density is expressed in teslas (T).

Static fields do not change with time. Time-varying fields behave differently because changing electric and magnetic fields are coupled. A changing magnetic field can induce an electric field, while an electric current or changing electric field can produce a magnetic field. These relationships are described by Maxwell's Equations.

Close to a source, electric and magnetic fields may have complex spatial patterns and may not have a fixed relationship. The geometry of an antenna, conductor, enclosure, or nearby object can cause one field component to dominate at a particular location. Energy may also be stored temporarily in the reactive near field rather than propagating away.

Far enough from a radiating source, the fields approach the behavior of a propagating wave. In an ideal free-space far field, the electric and magnetic fields are perpendicular to one another and have a fixed ratio determined by the impedance of free space. Measuring one component can then allow the other component and power density to be inferred.

Electric and magnetic fields interact differently with the body. External fields can induce internal electric fields and currents, and the strength and distribution of those internal quantities depend on frequency, body dimensions, posture, grounding, and the surrounding environment. At higher RF frequencies, energy absorption and heating become increasingly important.

RF exposure assessment therefore considers both the source and the field region. Separate electric- and magnetic-field measurements may be necessary in the near field or at lower frequencies. In suitable far-field conditions, incident power density may provide a convenient measure of the energy flowing through an area.

The term electromagnetic field is sometimes used broadly for static, low-frequency, and radio-frequency phenomena. The applicable measurement quantities and safety limits nevertheless depend on frequency and field behavior. Identifying the field type is therefore an essential first step in any reliable RF safety assessment.