|Electricity · Magnetism
Electromagnetism is one of the four fundamental interactions of nature, along with strong interaction, weak interaction and gravitation. It is the force that causes the interaction between electrically charged particles; the areas in which this happens are called electromagnetic fields.
Electromagnetism is the force responsible for practically all the phenomena encountered in daily life (with the exception of gravity). This includes the forces we experience in "pushing" or "pulling" ordinary material objects (such as coffee cups), which come from the intermolecular forces between the individual molecules in our bodies and those in the objects. These in turn result from the forces involved in interactions between atoms, which themselves can be traced to electromagnetism acting on the electrically charged protons and electrons inside the atoms.
Electromagnetism is also the force which holds electrons and protons together in atoms, and which holds atoms together to make molecules, thus governing of the processes involved in chemistry, which arise from interactions between the electrons orbiting atoms.
The force of electromagnetism is manifested both in electric fields and magnetic fields; both are simply different aspects of electromagnetism, and hence are intrinsically related to each other. Thus, a changing electric field generates a magnetic field; coversely a changing magnetic field generates an electric field. This effect is called electromagnetic induction, and is the basis of operation for electrical generators, induction motors, and transformers. Mathematically speaking, magnetic fields and electric fields are convertible with relative motion as a four vector.
Electric fields are the cause of several common phenomena, such as electric potential (such as the voltage of a battery) and electric current (such as the flow of electricity through a flashlight). Magnetic fields are the cause of the force associated with magnets.
The electromagnetic force operates via the exchange of messenger particles called photons and virtual photons. The exchange of messenger particles between bodies acts to create the perceptual force whereby instead of just pushing or pulling particles apart, the exchange changes the character of the particles that swap them.
Originally electricity and magnetism were thought of as two separate forces. This view changed, however, with the publication of James Clerk Maxwell's 1873 Treatise on Electricity and Magnetism in which the interactions of positive and negative charges were shown to be regulated by one force. There are four main effects resulting from these interactions, which have been clearly demonstrated by experiment:
While preparing for an evening lecture on 21 April 1820, Hans Christian Ørsted made a surprising observation. As he was setting up his materials, he noticed a compass needle deflected from magnetic north when the electric current from the battery he was using was switched on and off. This deflection convinced him that magnetic fields radiate from all sides off of a wire carrying an electric current, just as light and heat do, and that it confirmed a direct relationship between electricity and magnetism.
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. Soon thereafter he published his findings, proving that an electric current produces a magnetic field as it flows through a wire. The CGS unit of magnetic induction (oersted) is named in honor of his contributions to the field of electromagnetism.
His findings resulted in intensive research throughout the scientific community in electrodynamics. They influenced French physicist André-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.
This unification, which was observed by Michael Faraday, extended by James Clerk Maxwell, and partially reformulated by Oliver Heaviside and Heinrich Hertz, is one of the accomplishments of 19th century Mathematical Physics. It had far-reaching consequences, one of which was the understanding of the nature of light. Light and other electromagnetic waves take the form of quantized, self-propagating oscillatory electromagnetic field disturbances called photons. Different frequencies of oscillation give rise to the different forms of electromagnetic radiation, from radio waves at the lowest frequencies, to visible light at intermediate frequencies, to gamma rays at the highest frequencies.
Ørsted was not the only person to examine the relation between electricity and magnetism. In 1802 Gian Domenico Romagnosi, an Italian legal scholar, deflected a magnetic needle by electrostatic charges. Actually, no galvanic current existed in the setup and hence no electromagnetism was present. An account of the discovery was published in 1802 in an Italian newspaper, but it was largely overlooked by the contemporary scientific community.
The electromagnetic force is one of the four fundamental forces. The other fundamental forces are: the strong nuclear force (which holds quarks together, along with its residual strong force effect that holds atomic nuclei together, to form the nucleus), the weak nuclear force (which causes certain forms of radioactive decay), and the gravitational force. All other forces (e.g. friction) are ultimately derived from these fundamental forces.
The electromagnetic force is the one responsible for practically all the phenomena one encounters in daily life, with the exception of gravity. Roughly speaking, all the forces involved in interactions between atoms can be traced to the electromagnetic force acting on the electrically charged protons and electrons inside the atoms. This includes the forces we experience in "pushing" or "pulling" ordinary material objects, which come from the intermolecular forces between the individual molecules in our bodies and those in the objects. It also includes all forms of chemical phenomena, which arise from interactions between electron orbitals.
The scientist William Gilbert proposed, in his De Magnete (1600), 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, but the link between lightning and electricity was not confirmed until Benjamin Franklin's proposed experiments in 1752. One of the first to discover and publish a link between man-made electric current and magnetism was 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 produce a theory of electromagnetism that set the subject on a mathematical foundation.
An accurate theory of electromagnetism, known as classical electromagnetism, was developed by various physicists over the course of the 19th century, culminating in the work of James Clerk Maxwell, who unified the preceding developments into a single theory and discovered the electromagnetic nature of light. In classical electromagnetism, the electromagnetic field obeys a set of equations known as Maxwell's equations, and the electromagnetic force is given by the Lorentz force law.
One of the peculiarities of classical electromagnetism is that it is difficult to reconcile with classical mechanics, but it is compatible with special relativity. According to Maxwell's equations, the speed of light in a vacuum is a universal constant, dependent only on the electrical permittivity and magnetic permeability of free space. This violates Galilean invariance, a long-standing cornerstone of classical mechanics. One way to reconcile the two theories is to assume the existence of a luminiferous aether through which the light propagates. However, subsequent experimental efforts failed to detect the presence of the aether. After important contributions of Hendrik Lorentz and Henri Poincaré, in 1905, Albert Einstein solved the problem with the introduction of special relativity, which replaces classical kinematics with a new theory of kinematics that is compatible with classical electromagnetism. (For more information, see History of special relativity.)
In addition, relativity theory shows that in moving frames of reference a magnetic field transforms to a field with a nonzero electric component and vice versa; thus firmly showing that they are two sides of the same coin, and thus the term "electromagnetism". (For more information, see Classical electromagnetism and special relativity.)
In another paper published in that same year, Albert Einstein undermined the very foundations of classical electromagnetism. His theory of the photoelectric effect (for which he won the Nobel prize for physics) posited that light could exist in discrete particle-like quantities, which later came to be known as photons. Einstein's theory of the photoelectric effect extended the insights that appeared in the solution of the ultraviolet catastrophe presented by Max Planck in 1900. In his work, Planck showed that hot objects emit electromagnetic radiation in discrete packets, which leads to a finite total energy emitted as black body radiation. Both of these results were in direct contradiction with the classical view of light as a continuous wave, although it is now known that the photoelectric effect does not, in fact, compel one to any conclusion about light being made of "photons", as discussed in the photoelectric effect article. Planck's and Einstein's theories were progenitors of quantum mechanics, which, when formulated in 1925, necessitated the invention of a quantum theory of electromagnetism. This theory, completed in the 1940s, is known as quantum electrodynamics (or "QED"), and, in situations where perturbation theory is applicable, is one of the most accurate theories known to physics.
Electromagnetic units are part of a system of electrical units based primarily upon the magnetic properties of electric currents, the fundamental SI unit being the ampere. The units are:
In the electromagnetic cgs system, electrical current is a fundamental quantity defined via Ampère's law and takes the permeability as a dimensionless quantity (relative permeability) whose value in a vacuum is unity. As a consequence, the square of the speed of light appears explicitly in some of the equations interrelating quantities in this system.
SI electromagnetism units
|Symbol||Name of Quantity||Derived Units||Unit||Base Units|
|I||Electric current||ampere (SI base unit)||A||A (= W/V = C/s)|
|U, ΔV, Δφ; E||Potential difference; Electromotive force||volt||V||J/C = kg·m2·s−3·A−1|
|R; Z; X||Electric resistance; Impedance; Reactance||ohm||Ω||V/A = kg·m2·s−3·A−2|
|P||Electric power||watt||W||V·A = kg·m2·s−3|
|C||Capacitance||farad||F||C/V = kg−1·m−2·A2·s4|
|E||Electric field strength||volt per metre||V/m||N/C = kg·m·A−1·s−3|
|D||Electric displacement field||Coulomb per square metre||C/m2||A·s·m−2|
|ε||Permittivity||farad per metre||F/m||kg−1·m−3·A2·s4|
|G; Y; B||Conductance; Admittance; Susceptance||siemens||S||Ω−1 = kg−1·m−2·s3·A2|
|κ, γ, σ||Conductivity||siemens per metre||S/m||kg−1·m−3·s3·A2|
|B||Magnetic flux density, Magnetic induction||tesla||T||Wb/m2 = kg·s−2·A−1 = N·A−1·m−1|
|Φ||Magnetic flux||weber||Wb||V·s = kg·m2·s−2·A−1|
|H||Magnetic field strength||ampere per metre||A/m||A·m−1|
|L, M||Inductance||henry||H||Wb/A = V·s/A = kg·m2·s−2·A−2|
|μ||Permeability||henry per metre||H/m||kg·m·s−2·A−2|
With the exception of gravitation, electromagnetic phenomena as described by quantum electrodynamics (which includes as a limiting case classical electrodynamics) account for almost all physical phenomena observable to the unaided human senses, including light and other electromagnetic radiation, all of chemistry, most of mechanics (excepting gravitation), and of course magnetism and electricity. Magnetic monopoles (and "Gilbert" dipoles) are not strictly electromagnetic phenomena, since in standard electromagnetism, magnetic fields are generated not by true "magnetic charge" but by currents. There are, however, condensed matter analogs of magnetic monopoles in exotic materials(spin ice) created in the laboratory.
It is necessary to revise the following mathematical concepts as they are used throughout this course:
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An electric field is an area where charged particles will feel an electric force. The units used to measure electric fields are newtons per coulomb. Electromagnetism is closely related to both electricity and magnetism because both involve movement of electrons.
Electric fields can be drawn as arrows. The arrows point which way a positive particle, like a proton, will be pushed if it's in the field. Negative particles, like electrons, will go in the opposite direction as the arrows. In an electric field, arrows will point away from positive particles, and towards negative ones. So, a proton in an electric field would move away from another proton, or towards an electron.
Through electromagnetic induction, a changing magnetic field can produce an electric field. This concept is used to make electric generators, induction motors, and transformers work. Since the two types of fields were dependent on each other, the two are thought to be one. Together they are called the electromagnetic field.
The electromagnetic force is one of the fundamental forces of nature. The electromagnetic force is the force that causes an attraction between electrons and the positive nucleus. All forces between atoms are caused by the electromagnetic force.
The electromagnetic radiation is thought to be both a particle and a wave. This is because it sometimes acts like a particle and sometimes acts like a wave. To make things easier we can think of an electromagnetic wave as a stream of photons (symbol γ).
A photon is an elementary particle. It is the particle that light is made up of. Photons also make up all other types of electromagnetic radiation such as gamma rays, X-rays, and UV rays. The idea of photons was thought up by Einstein. Using his theory for the photoelectric effect, Einstein said that light existed in small "packets" or parcels which he called photons.
Photons have energy and momentum. When two electromagnetic fields act on each other, they switch photons. So photons carry the electromagnetic force between charged objects. Photons are also known as messenger particles in physics because these particles often carry messages between objects. Photons send messages saying "come closer" or "go away" depending on the charges of the objects that are being looked at. If a force exists while time passes, then photons are being exchanged during that time.
Fundamental electromagnetic interactions occur between any two particles that have electric charge. These interactions involve the exchange or production of photons. Thus, photons are the carrier particles of electromagnetic interactions.
Electromagnetic decay processes can often be recognized by the fact that they produce one or more photons (also known as gamma particles). They proceed less rapidly than strong decay processes with comparable mass differences, but more rapidly than comparable weak decays.
In 1600, William Gilbert said that electricity and magnetism were two different effects in his book De Magnete. The link between electricity and magnetism was found through the work of Benjamin Franklin, Romagnosi, and Ørsted. A scientist named Ampère then used mathematics in electromagnetism. Many physicists then developed a theory of electromagnetism now known as classical electromagnetism. James Clerk Maxwell then brought everything together into one theory of electromagnetism. This type of electromagnetism was based on Maxwell's equations and the Lorentz force law. Maxwell's studies showed what light actually was. Maxwell's work did not work with classical mechanics because he said that the speed of light was always constant. It only depended on the permeability of the substance it was travelling through. This led to the development of the theory of special relativity by Einstein.
Albert Einstein's work with the photoelectric effect and Max Planck's work with black body radiation did not work with the traditional view of light as a continuous wave. This problem would be solved after the development of quantum mechanics in 1925. This development led to the development of quantum electrodynamics which was developed by Richard Feynman and Julian Schwinger. Quantum electrodynamics was able to describe the interactions particles in detail.