PACS 3.50De, 41.20
Electromagnetism, or the unified theory of electric and magnetic fields, could be said to be the workhorse of technology. With applications as widely diverse as the function of integrated circuits, the generation of gigawatts of electric power, and the transmission of cell phone signals, electromagnetism governs many of the devices and systems that contemporary civilization uses to go about its business. The fact that electromagnetism is so pervasive is perhaps now not so great a mystery: the electromagnetic forces are, along with gravity, the predominant forces governing the interactions of matter at everyday scales. Moreover, chemistry (including biomolecular interactions) and the strength and behavior of materials are ultimately traced to electromagnetic forces.
The physics of electromagnetism rests on a fundamental property of matter: electric charge. Electrostatic forces exist between stationary charges and magnetic forces exist between moving or spinning charges. A remarkable conceptual framework for understanding such forces relies on the ideas of electric and magnetic fields. These concepts allow us to separate the source of electromagnetic forces from the action at a distance of these forces on charges or currents. Far from being a useful abstraction, the idea of a field takes on special significance when discussing the spatially-extended storage and transmission of energy. The prediction, by electromagnetic theory, of the existence of electromagnetic waves rests on the idea that energy resides in the fields themselves and can propagate through space as electromagnetic radiation.
Another key element in the conceptual arsenal of electromagnetism is the idea of potential: both scalar potential and vector potential. The scalar potential, measured in volts, is perhaps the most important concept in the practical application of electric fields to circuit design. Much of our description of electronic technology is based on the behavior of devices in response to applied potentials. Most instrumentation is based on sensitive voltmeter technology, including analog-to-digital conversion technology that is used to connect sensor systems to computers. A less commonly encountered concept, the vector potential, is important to the understanding of electromagnetic radiation and, in particular, to the detailed modeling of sources of such radiation. From both analytical and computational points of view, potential distributions in space and time are an efficient way to encode the configuration of electromagnetic fields, especially electric fields that derive from the scalar potential.
Since the classification of the state of a charged particle as stationary or moving depends on an observer's reference frame, one expects there is a deep connection between electric and magnetic fields through the theory of relativity. Indeed, the fact that Maxwell's grand synthesis of the theory of such fields is consistent across different inertial reference frames essentially forced the issue of bringing the rest of physics – notably mechanics – into agreement with the idea that there is no preferred reference frame. This led Einstein to formulate his special theory of relativity.
The theory of electromagnetism might be seen embodied in two late themes. On one hand, we are interested in the behavior of electric and magnetic fields in matter, understood through such ideas as electric conductivity, electric and magnetic polarization, storage and gating the release of electric charge, etc. On the other hand, we are interested in the origin of electromagnetic radiation from the motions of electric charge and the propagation of such radiation through matter and empty space. Both areas of investigation are profoundly important to science and technology. At the microscopic scale and at the scale of the universe and its origin, we might bring these themes together into a coherent whole.
As a domain in experimental physics, electromagnetism is useful in several ways. The vast majority of scientific instrumentation relies on electronic technology that is based on fundamental ideas about the manipulation and measurement of electric charge and electric currents. The remarkable sensitivity of electronic instrumentation attests to the existence of a fundamental unit of charge, the charge of the electron, that is extremely small by everyday measures and yet which can produce effects that can be amplified by clever detection schemes. Of course, the routine technological uses of electromagnetism, from electric motors to communications, are essential components to many experimental systems. Finally, we should recognize that electromagnetism is so integral to the behavior of matter that electromagnetic effects readily serve as fundamental probes of material systems, ranging from biological tissues to geologic deposits.