"Electromagnetics" redirects here. For the academic journal, seeElectromagnetics (journal)."Electromagnetic force" redirects here. For the force exerted on particles by electromagnetic fields, seeLorentz force."Electromagnetic" redirects here. The term may also refer to the use of anelectromagnet.
Electromagnetic interactions are responsible for the glowing filaments in thisplasma globe.
In physics,electromagnetism is an interaction that occurs betweenparticles withelectric charge viaelectromagnetic fields. The electromagnetic force is one of the fourfundamental forces of nature. It is the dominant force in the interactions ofatoms andmolecules. Electromagnetism can be thought of as a combination ofelectrostatics andmagnetism, which are distinct but closely intertwined phenomena. Electromagnetic forces occur between any two charged particles. Electric forces cause an attraction between particles with opposite charges and repulsion between particles with the same charge, while magnetism is an interaction that occurs between charged particles in relative motion. These two forces are described in terms of electromagnetic fields. Macroscopic charged objects are described in terms ofCoulomb's law for electricity andAmpère's force law for magnetism; theLorentz force describes microscopic charged particles.
The electromagnetic force is responsible for many of thechemical and physical phenomena observed in daily life. The electrostatic attraction betweenatomic nuclei and theirelectrons holds atoms together. Electric forces also allow different atoms to combine into molecules, including themacromolecules such asproteins that form the basis oflife. Meanwhile, magnetic interactions between thespin andangular momentum magnetic moments of electrons also play a role in chemical reactivity; such relationships are studied inspin chemistry. Electromagnetism also plays several crucial roles in moderntechnology: electrical energy production, transformation and distribution; light, heat, and sound production and detection; fiber optic and wireless communication; sensors; computation; electrolysis; electroplating; and mechanical motors and actuators.
Electromagnetism has been studied since ancient times. Many ancient civilizations, including theGreeks and theMayans, created wide-ranging theories to explainlightning,static electricity, and the attraction between magnetized pieces ofiron ore. However, it was not until the late 18th century that scientists began to develop a mathematical basis for understanding the nature of electromagnetic interactions. In the 18th and 19th centuries, prominent scientists and mathematicians such asCoulomb,Gauss andFaraday developed namesake laws which helped to explain the formation and interaction of electromagnetic fields. This process culminated in the 1860s with the discovery ofMaxwell's equations, a set of fourpartial differential equations which provide a complete description of classical electromagnetic fields. Maxwell's equations provided a sound mathematical basis for the relationships between electricity and magnetism that scientists had been exploring for centuries, and predicted the existence of self-sustainingelectromagnetic waves. Maxwell postulated that such waves make upvisible light, which was later shown to be true. Gamma-rays, x-rays, ultraviolet, visible, infrared radiation, microwaves and radio waves were all determined to be electromagnetic radiation differing only in their range of frequencies.
In the modern era, scientists continue to refine the theory of electromagnetism to account for the effects ofmodern physics, includingquantum mechanics andrelativity. The theoretical implications of electromagnetism, particularly the requirement that observations remain consistent when viewed from various moving frames of reference (relativistic electromagnetism) and the establishment of the speed of light based on properties of the medium of propagation (permeability andpermittivity), helped inspireEinstein's theory ofspecial relativity in 1905.Quantum electrodynamics (QED) modifies Maxwell's equations to be consistent with thequantized nature of matter. In QED, changes in the electromagnetic field are expressed in terms of discrete excitations, particles known asphotons, thequanta of light.
Investigation into electromagnetic phenomena began about 5,000 years ago. There is evidence that the ancientChinese,[1]Mayan,[2][3] and potentially evenEgyptian civilizations knew that the naturally magnetic mineralmagnetite had attractive properties, and many incorporated it into their art and architecture.[4] Ancient people were also aware oflightning andstatic electricity, although they had no idea of the mechanisms behind these phenomena. TheGreek philosopherThales of Miletus discovered around 600 B.C.E. thatamber could acquire an electric charge when it was rubbed with cloth, which allowed it to pick up light objects such as pieces of straw. Thales also experimented with the ability of magnetic rocks to attract one other, and hypothesized that this phenomenon might be connected to the attractive power of amber, foreshadowing the deep connections between electricity and magnetism that would be discovered over 2,000 years later. Despite all this investigation, ancient civilizations had no understanding of the mathematical basis of electromagnetism, and often analyzed its impacts through the lens ofreligion rather than science (lightning, for instance, was considered to be a creation of the gods in many cultures).[5]
19th century
Cover ofA Treatise on Electricity and Magnetism
Electricity and magnetism were originally considered to be two separate forces. This view changed with the publication ofJames Clerk Maxwell's 1873A Treatise on Electricity and Magnetism[6] in which the interactions of positive and negative charges were shown to be mediated by one force. There are four main effects resulting from these interactions, all of which have been clearly demonstrated by experiments:
Electric chargesattract orrepel one another with a forceinversely proportional to the square of the distance between them: opposite charges attract, like charges repel.[7]
Magnetic poles (or states of polarization at individual points) attract or repel one another in a manner similar to positive and negative charges and always exist as pairs: every north pole is yoked to a south pole.[8]
An electric current inside a wire creates a corresponding circumferential magnetic field outside the wire. Its direction (clockwise or counter-clockwise) depends on the direction of the current in the wire.[9]
A current is induced in a loop of wire when it is moved toward or away from a magnetic field, or a magnet is moved towards or away from it; the direction of current depends on that of the movement.[9]
In April 1820,Hans Christian Ørsted observed that an electrical current in a wire caused a nearby compass needle to move. At the time of discovery, Ørsted did not suggest any satisfactory explanation of the phenomenon, nor did he try to represent the phenomenon in a mathematical framework. However, three months later he began more intensive investigations.[10][11] Soon thereafter he published his findings, proving that an electric current produces a magnetic field as it flows through a wire. TheCGS unit ofmagnetic induction (oersted) is named in honor of his contributions to the field of electromagnetism.[12]
His findings resulted in intensive research throughout the scientific community in electrodynamics. They influenced French physicistAndré-Marie Ampère's developments of a single mathematical form to represent the magnetic forces between current-carrying conductors. Ørsted's discovery also represented a major step toward a unified concept of energy.
Ørsted was not the only person to examine the relationship between electricity and magnetism. In 1802,Gian Domenico Romagnosi, an Italian legal scholar, deflected a magnetic needle using a Voltaic pile. The factual setup of the experiment is not completely clear, nor if current flowed across the needle or not. An account of the discovery was published in 1802 in an Italian newspaper, but it was largely overlooked by the contemporary scientific community, because Romagnosi seemingly did not belong to this community.[14]
An earlier (1735), and often neglected, connection between electricity and magnetism was reported by a Dr. Cookson.[15] The account stated:
A tradesman at Wakefield in Yorkshire, having put up a great number of knives and forks in a large box ... and having placed the box in the corner of a large room, there happened a sudden storm of thunder, lightning, &c. ... The owner emptying the box on a counter where some nails lay, the persons who took up the knives, that lay on the nails, observed that the knives took up the nails. On this the whole number was tried, and found to do the same, and that, to such a degree as to take up large nails, packing needles, and other iron things of considerable weight ...
E. T. Whittaker suggested in 1910 that this particular event was responsible for lightning to be "credited with the power of magnetizing steel; and it was doubtless this which led Franklin in 1751 to attempt to magnetize a sewing-needle by means of the discharge of Leyden jars."[16]
A fundamental force
Representation of the electric field vector of a wave of circularly polarized electromagnetic radiation
The electromagnetic force is the second strongest of the four knownfundamental forces and has unlimited range.[17]All other forces, known asnon-fundamental forces.[18] (e.g.,friction, contact forces) are derived from the four fundamental forces. At high energy, theweak force and electromagnetic force are unified as a single interaction called theelectroweak interaction.[19]
Most of the forces involved in interactions betweenatoms are explained by electromagnetic forces between electrically chargedatomic nuclei andelectrons. The electromagnetic force is also involved in all forms ofchemical phenomena.
Electromagnetism explains how materials carry momentum despite being composed of individual particles and empty space. The forces we experience when "pushing" or "pulling" ordinary material objects result fromintermolecular forces between individualmolecules in our bodies and in the objects.
The effective forces generated by the momentum of electrons' movement is a necessary part of understanding atomic and intermolecular interactions. As electrons move between interacting atoms, they carry momentum with them. As a collection of electrons becomes more confined, their minimum momentum necessarily increases due to thePauli exclusion principle. The behavior of matter at the molecular scale, including its density, is determined by the balance between the electromagnetic force and the force generated by the exchange of momentum carried by the electrons themselves.[20]
In 1600,William Gilbert proposed, in hisDe Magnete, that electricity and magnetism, while both capable of causing attraction and repulsion of objects, were distinct effects.[21] Mariners had noticed that lightning strikes had the ability to disturb a compass needle. The link between lightning and electricity was not confirmed untilBenjamin Franklin's proposed experiments in 1752 were conducted on 10May 1752 byThomas-François Dalibard of France using a 40-foot-tall (12 m) iron rod instead of a kite and he successfully extracted electrical sparks from a cloud.[22][23]
One of the first to discover and publish a link between human-made electric current and magnetism wasGian Romagnosi, who in 1802 noticed that connecting a wire across avoltaic pile deflected a nearbycompass needle. However, the effect did not become widely known until 1820, when Ørsted performed a similar experiment.[24] Ørsted's work influenced Ampère to conduct further experiments, which eventually gave rise to a new area of physics: electrodynamics. By determining a force law for the interaction between elements of electric current, Ampère placed the subject on a solid mathematical foundation.[25]
A theory of electromagnetism, known asclassical electromagnetism, was developed by several physicists during the period between 1820 and 1873, whenJames Clerk Maxwell'streatise was published, which unified previous developments into a single theory, proposing that light was an electromagnetic wave propagating in theluminiferous ether.[26] In classical electromagnetism, the behavior of the electromagnetic field is described by a set of equations known asMaxwell's equations, and the electromagnetic force is given by theLorentz force law.[27]
One of the peculiarities of classical electromagnetism is that it is difficult to reconcile withclassical mechanics, but it is compatible with special relativity. According to Maxwell's equations, thespeed of light in vacuum is a universal constant that is dependent only on theelectrical permittivity andmagnetic permeability offree space. This violatesGalilean invariance, a long-standing cornerstone of classical mechanics. One way to reconcile the two theories (electromagnetism and classical mechanics) is to assume the existence of aluminiferous aether through which the light propagates. However, subsequent experimental efforts failed to detect the presence of the aether. After important contributions ofHendrik Lorentz andHenri Poincaré, in 1905,Albert Einstein solved the problem with the introduction of special relativity, which replaced classical kinematics with a new theory of kinematics compatible with classical electromagnetism. (For more information, seeHistory of special relativity.)
In addition, relativity theory implies that in moving frames of reference, a magnetic field transforms to a field with a nonzero electric component and conversely, a moving electric field transforms to a nonzero magnetic component, thus firmly showing that the phenomena are two sides of the same coin. Hence the term "electromagnetism". (For more information, seeClassical electromagnetism and special relativity andCovariant formulation of classical electromagnetism.)
Today few problems in electromagnetism remain unsolved. These include: the lack ofmagnetic monopoles,Abraham–Minkowski controversy, the location in space of the electromagnetic field energy,[28] and the mechanism by which some organisms can senseelectric andmagnetic fields.
Extension to nonlinear phenomena
The Maxwell equations arelinear, in that a change in the sources (the charges and currents) results in a proportional change of the fields.Nonlinear dynamics can occur when electromagnetic fields couple to matter that follows nonlinear dynamical laws.[29] This is studied, for example, in the subject ofmagnetohydrodynamics, which combines Maxwell theory with theNavier–Stokes equations.[30] Another branch of electromagnetism dealing with nonlinearity isnonlinear optics.
In the electromagneticCGS system, electric current is a fundamental quantity defined viaAmpère's law and takes thepermeability as a dimensionless quantity (relative permeability) whose value in vacuum isunity.[32] As a consequence, the square of the speed of light appears explicitly in some of the equations interrelating quantities in this system.
Formulas for physical laws of electromagnetism (such asMaxwell's equations) need to be adjusted depending on what system of units one uses. This is because there is noone-to-one correspondence between electromagnetic units in SI and those in CGS, as is the case for mechanical units. Furthermore, within CGS, there are several plausible choices of electromagnetic units, leading to different unit "sub-systems", includingGaussian, "ESU", "EMU", andHeaviside–Lorentz. Among these choices, Gaussian units are the most common today, and in fact the phrase "CGS units" is often used to refer specifically toCGS-Gaussian units.[34]
^VIII. An account of an extraordinary effect of lightning in communicating magnetism. Communicated by Pierce Dod, M.D. F.R.S. from Dr. Cookson of Wakefield in Yorkshire.Phil. Trans. 1735 39, 74-75, published 1 January 1735
^Browne, "Physics for Engineering and Science", p. 160: "Gravity is one of the fundamental forces of nature. The other forces such as friction, tension, and the normal force are derived from the electric force, another of the fundamental forces. Gravity is a rather weak force... The electric force between two protons is much stronger than the gravitational force between them."
^Stern, Dr. David P.; Peredo, Mauricio (2001-11-25)."Magnetic Fields – History". NASA Goddard Space Flight Center.Archived from the original on 2015-11-16. Retrieved2009-11-27.
^Purcell, p. 436. Chapter 9.3, "Maxwell's description of the electromagnetic field was essentially complete."
^Purcell: p. 278: Chapter 6.1, "Definition of the Magnetic Field." Lorentz force and force equation.
^Feynman, Richard P. (2011)."27–4 The ambiguity of the field energy".The Feynman lectures on physics. Volume 1: Mainly mechanics, radiation, and heat (The new millennium edition, paperback first published ed.). New York: Basic Books.ISBN978-0-465-04085-8.Archived from the original on 2024-10-03. Retrieved2024-09-05.
Purcell, Edward M and Morin, David. (2013).Electricity and Magnetism, 820p (3rd ed.). Cambridge University Press, New York.ISBN978-1-107-01402-2.{{cite book}}: CS1 maint: multiple names: authors list (link)
Rao, Nannapaneni N. (1994).Elements of engineering electromagnetics (4th ed.). Prentice Hall.ISBN978-0-13-948746-0.
Rothwell, Edward J.; Cloud, Michael J. (2001).Electromagnetics. CRC Press.ISBN978-0-8493-1397-4.
Tipler, Paul (1998).Physics for Scientists and Engineers: Vol. 2: Light, Electricity and Magnetism (4th ed.). W.H. Freeman.ISBN978-1-57259-492-0.
Wangsness, Roald K.; Cloud, Michael J. (1986).Electromagnetic Fields (2nd ed.). Wiley.ISBN978-0-471-81186-2.
General coverage
A. Beiser (1987).Concepts of Modern Physics (4th ed.). McGraw-Hill (International).ISBN978-0-07-100144-1.
