Electromagnetism

The fundamental role of special relativity in electromagnetism

Technological uses of electromagnetism

Electromagnetism comprises all physical phenomena in which electric and magnetic fields are involved. The rules of electromagnetism are responsible for the way charged particles of atoms interact, and are employed in all electrical and electronic machines.

Some of the rules of electrostatics, the study of electric charges at rest, were first noted by the ancient Romans, who observed the way a brushed comb would attract particles. It is now known that electric charges occur in two different types, called positive and negative. Like types repel each other, and differing types attract.

The force that attracts positive charges to negative charges weakens with distance, but is intrinsically very strong. The fact that unlike types attract means that most of this force is normally neutralized and not seen in full strength. The negative charge is generally carried by the atom’s electrons, while the positive resides with the protons inside the atomic nucleus. There are other less well-known particles that can also carry charge. When the electrons of a material are not tightly bound to the atom’s nucleus, they can move from atom to atom and the substance, called a conductor, can conduct electricity. On the contrary, when the electron binding is strong, the material is called an insulator.

When electrons are weakly bound to the atomic nucleus, the result is a semiconductor, often used in the electronics industry. It was not initially known if the electric current carriers were positive or negative, and this initial ignorance gave rise to the convention that current flows from the positive terminal to the negative. In reality we now know that the electrons actually run from the negative to the positive.

Electromagnetism is the theory of a unified expression of an underlying force, the so-called electromagnetic force. This is seen in the movement of electric charge, which gives rise to magnetism (the electric current in a wire being found to deflect a compass needle), and it was a Scotsman, James Clerk Maxwell, who published the theory unifying electricity and magnetism in 1865. The theory arose from former specialized work by Gauss, Coulomb, Ampe`re, Faraday, Franklin, and Ohm. However, one factor that did not contradict the experiments was added to the equations by Maxwell so as to ensure the conservation of charge. This was done on the theoretical grounds that charge should be a conserved quantity, and this addition led to the prediction of a wave phenomena with a certain anticipated velocity. Light, which has the expected velocity, was found to be an example of this electromagnetic radiation.

Light had formerly been thought of as consisting of particles (photons) by Newton, but the theory of light as particles was unable to explain the wave nature of light (diffraction and the like). In reality, light displays both wave and particle properties. The resolution to this duality lies in quantum theory, where light is neither particles nor wave, but both. It propagates as a wave without the need of a medium and interacts in the manner of a particle. This is the basic nature of quantum theory.

Classical electromagnetism, useful as it is, contains contradictions (acausality) that make it incomplete and drove scientists to consider its extension to the area of quantum physics.

There is much symmetry between electric and magnetic fields. A changing electric field gives rise to a magnetic field, and a changing magnetic field gives rise to an electric field. It is a back-and-forth exchange of this kind that allows the propagation of electromagnetic waves. These waves, although they do not need a medium of propagation, are slowed when traveling through a transparent substance. The induction of magnetic and electric fields by each other is also used in transformers, which allow alternating currents of one voltage to be transformed into alternating currents of another voltage, either lesser or greater.

Electromagnetic waves differ from each other only in amplitude, frequency and orientation (polarization). Laser beams are particular in being very coherent, that is, the radiation is of one frequency, and the waves coordinated in motion and direction. This permits a highly concentrated beam that is used not only for its cutting abilities, but also in electronic data storage, such as in CD-ROMs and DVD-ROMS.

The differing frequency forms are given a variety of names, from radio waves at very low frequencies through light itself, to the high frequency x and gamma rays.

Many miracles depend upon the broad span of the electromagnetic spectrum. The ability to communicate across long distances despite intervening obstacles, such as the walls of buildings, is possible using the radio and television frequencies. X rays can see into the human body without opening it. These things, which would once have been labeled magic, are now ordinary ways we use the electromagnetic spectrum.

The unification of electricity and magnetism has led to a deeper understanding of physical science, and much effort has been put into further unifying the four forces of nature. The remaining known forces are the so-called weak, strong, and gravitational forces. The weak force has now been unified with electromagnetism, called the electroweak force. There are proposals to include the strong force in a grand unified theory, but the inclusion of gravity remains an open problem.

The fundamental role of special relativity in electromagnetism

Maxwell’s theory is in fact in contradiction with Newtonian mechanics, and in trying to find the resolution to this conflict, Einstein was led to his theory of special relativity. Maxwell’s equations withstood the conflict, but it was Newtonian mechanics that were corrected by relativistic mechanics. These corrections are most necessary at velocities, close to the speed of light. The many strange predictions about space and time that follow from special relativity are found to be a part of the real world.

Paradoxically, magnetism is a counter example to the frequent claims that relativistic effects are not noticeable for low velocities. The moving charges that compose an electric current in a wire might typically only be traveling at several feet per second (walking speed), and the resulting Lorentz contraction of special relativity is indeed minute. However, the electrostatic forces at balance in the wire are of such great magnitude, that this small contraction of the moving (negative) charges exposes a residue force of real world magnitude, namely the magnetic force. It is in exactly this way that the magnetic force derives from the electric. Special relativity is indeed hidden in Maxwell’s equations, which were known before special relativity was understood or separately formulated by Einstein.

Technological uses of electromagnetism

Before the advent of technology, electromagnetism was perhaps most strongly experienced in the form of lightning, and electromagnetic radiation in the form of light. Ancient man kindled fires which he thought were kept alive in trees struck by lightning.

Much of the magic of nature has been put to work by man, but not always for his betterment or that of his surroundings. Electricity at high voltages can carry energy across extended distances with little loss. Magnetism derived from that electricity can then power vast motors. But electromagnetism can also be employed in a more delicate fashion as a means of communication, either with wires (as in the telephone), or without them (as in radio communication). It also drives our electronics devices (as in computers).

Magnetism has long been employed for navigation in the compass. This works because Earth is itself a huge magnet, thought to have arisen from the great heat driven convection currents of molten iron in its center. In fact, it is known that Earth’s magnetic poles have exchanged positions in the past.