In physics, electromagnetism is an interaction that occurs between particles with electric charge via electromagnetic fields. The electromagnetic force is one of the four fundamental forces of nature. It is the dominant force in the interactions of atoms and molecules. Electromagnetism describes and relates the three distinct but closely intertwined phenomena of electricity, magnetism, and optics. In the study of electromagnetism these phenomena are described by the 3 sub-disciplines: electrostatics, magnetostatics, and electrodynamics.
The electromagnetic force is responsible for many chemical and physical phenomena observed in daily life. The electrostatic attraction between atomic nuclei and their electrons holds atoms together. Electric forces also allow different atoms to combine into molecules. Meanwhile, magnetic interactions between the spin and angular momentum magnetic moments of electrons also play a role in chemical reactivity; such relationships are studied in spin chemistry. Electromagnetism also plays several crucial roles in modern technology: 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 to explain the electric phenomena of lightning and static electricity and the magnetic phenomenon of the attraction between magnetized pieces of iron ore and of compasses. In the 18th and 19th centuries scientists developed the mathematical relationships between these phenomena and electric charges and currents. They also showed that electrical and magnetic phenomena were related and could create self-sustaining electromagnetic waves which forms visible light and other forms of electromagnetic radiation including Gamma-rays, x-rays, ultraviolet, visible, infrared radiation, microwaves and radio waves.
In the modern era, scientists continue to refine the theory of electromagnetism to account for the effects of modern physics, including quantum mechanics and relativity. 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 and permittivity), helped inspire Einstein's theory of special relativity in 1905. Quantum electrodynamics (QED) modifies Maxwell's equations to be consistent with the quantized nature of matter. In QED, changes in the electromagnetic field are expressed in terms of discrete excitations, particles known as photons, the quanta of light.
Mathematically, electromagnetism is described by electromagnetic fields. These are quantities that describe the electrical (electric field) and magnetic (magnetic field) influence at a given location and time.
A fundamental force
The electromagnetic force is the second strongest of the four known fundamental forces and has unlimited range. All other forces, known as non-fundamental forcesBrowne, "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." (e.g., friction, contact forces) are derived from the four fundamental forces. At high energy, the weak force and electromagnetic force are unified as a single interaction called the electroweak interaction.
Most of the forces involved in interactions between atoms are explained by electromagnetic forces between electrically charged atomic nuclei and electrons. The electromagnetic force is also involved in all forms of chemical 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 from intermolecular forces between individual molecules 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 the Pauli 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.Purcell, "Electricity and Magnetism, 3rd Edition", p. 546: Ch 11 Section 6, "Electron Spin and Magnetic Moment."
Electromagnetic fields
The phenomena of electromagnetism is described mathematically using electromagnetic fields. An electromagnetic field (also EM field) is a physical field, varying in space and time, that represents the electric and magnetic influences generated by and acting upon electric charges. The field at any point in space and time can be regarded as a combination of an electric field and a magnetic field. Because of the interrelationship between the fields, a disturbance in the electric field can create a disturbance in the magnetic field which in turn affects the electric field, leading to an oscillation that propagates through space, known as an electromagnetic wave.
There are 6 mathematical quantities which are used to describe electromagnetic fields in 3 distinct cases. The electric field, , and magnetic flux density, are used to describe the electromagnetic fields in cases where all the charges are known directly such as the electromagnetic field in a vacuum. To describe the case where the charges inside of a material respond to applied electric and magnetic fields, it is useful to define the electric displacement field, and magnetic field strength, , in addition to and . Finally, it is sometimes useful to describe electromagnetic fields not in terms of force but in terms of potential energy and momentum. In this case, the electromagnetic field is represented by the electric potential, , and the magnetic vector potential, , respectively.
Force on an electric charge due to electromagnetic fields
The force, . on a charge particle of charge moving at speed in an electromagnetic field is given by the Lorentz force(SI definition of quantities):
Here, is the vector cross product, and all quantities in bold are vectors.
The Lorentz force can be used to define both the electric field and the magnetic flux density .
Electromagnetic fields in matter
When dealing with electromagnetic fields in matter it is often useful to introduce 2 other fields, in addition to and . Since matter is made of particles with both electric charge and intrinsic magnetic moment it responds to an applied electromagnetic field to produce its own electromagnetic field. The details are complicated but often can be described by introducing an electric displacement field, to complement and a magnetic field strength, , to complement .
Electric potential formulation
In advanced formulations of electromagnetism and to simplify certain calculations it is often useful to reformulate the electromagnetic field in terms of electric potential energy and momentum instead of force. Here, the electromagnetic fields of interest are the electric potential, , and the magnetic vector potential . These are related to and by:
where is the gradient, is the divergence, and is the curl.
Electrostatics
Electrostatics is a branch of physics that studies slow-moving or stationary electric charges on macroscopic objects where quantum effects can be neglected. Under these circumstances, the electric field, electric potential, and the charge density are related without complications from magnetic effects.
Since classical antiquity, it has been known that some materials, such as amber, attract lightweight particles after rubbing. The Greek word (), meaning 'amber', was thus the root of the word electricity. Electrostatic phenomena arise from the forces that electric charges exert on each other. Such forces are described by Coulomb's law.
There are many examples of electrostatic phenomena, from those as simple as the attraction of plastic wrap to one's hand after it is removed from a package, to the apparently spontaneous explosion of grain silos, the damage of electronic components during manufacturing, and photocopier and laser printer operation.
Magnetostatics
Magnetostatics is the study of magnetic fields in systems where the currents are steady (not changing with time). It is the magnetic analogue of electrostatics, where the charges are stationary. The magnetization need not be static; the equations of magnetostatics can be used to predict fast magnetic switching events that occur on time scales of nanoseconds or less. Magnetostatics is even a good approximation when the currents are not static – as long as the currents do not alternate rapidly. Magnetostatics is widely used in applications of micromagnetics such as models of magnetic storage devices as in computer memory.
Electrodynamics
In 1600, William Gilbert proposed, in his De Magnete, that electricity and magnetism, while both capable of causing attraction and repulsion of objects, were distinct effects. Mariners had noticed that lightning strikes had the ability to disturb a compass needle. The link between lightning and electricity was not confirmed until Benjamin Franklin's proposed experiments in 1752 were conducted on 10May 1752 by Thomas-François Dalibard of France using a iron rod instead of a kite and he successfully extracted electrical sparks from a cloud.
One of the first to discover and publish a link between human-made electric currents and magnetism was Gian Romagnosi, who in 1802 noticed that connecting a wire across a voltaic pile deflected a nearby compass needle. However, the effect did not become widely known until 1820, when Ørsted performed a similar experiment. 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.
A theory of electromagnetism, known as classical electromagnetism, was developed by several physicists during the period between 1820 and 1873, when James Clerk Maxwell's treatise was published, which unified previous developments into a single theory, proposing that light was an electromagnetic wave propagating in the luminiferous ether.Purcell, p. 436. Chapter 9.3, "Maxwell's description of the electromagnetic field was essentially complete." In classical electromagnetism, the behavior of the electromagnetic field is described by a set of equations known as Maxwell's equations, and the electromagnetic force is given by the Lorentz force law.Purcell: p. 278: Chapter 6.1, "Definition of the Magnetic Field." Lorentz force and force equation.