What is Electromagnetism?
Electromagnetism is the branch of classical physics that studies electromagnetic interaction and constitutes a fundamental theory that allowed to explain natural phenomena such as electricity, magnetism, and light; it is the first example of unification of two different forces, the electric and the magnetic one.
In applied engineering, electromagnetics is the study of those aspects of electrical engineering in situations in which the electromagnetic properties of materials and the geometry in which those materials are arranged is important. This requires an understanding of electromagnetic fields and waves, which are of primary interest in some applications.
Electromagnetic phenomena are defined in terms of the electromagnetic force, sometimes called the Lorentz force, which includes both electricity and magnetism as different manifestations of the same phenomenon. The electromagnetic force plays a major role in determining the internal properties of most objects encountered in daily life:
- the electromagnetic attraction between atomic nuclei and their orbital electrons holds atoms together;
- electromagnetic forces are responsible for the chemical bonds between atoms that create molecules, and intermolecular forces;
- the electromagnetic force governs all chemical processes, which arise from interactions between the electrons of neighboring atoms.
- other forces (e.g., friction, tension, and contact forces) are derived from electromagnetic forces (and the other fundamental forces).
From electromagnetic theory originate important theoretical and applicative branches concerning electric current through the circuit theory, electrical engineering, and electronics.
Usually, electromagnetism refers to classical theory, summarized in Maxwellâs equations. This theory accurately describes physical reality down to quantum dimensions: the extension of the theory also on a quantum scale is called quantum electrodynamics; the study of electromagnetism combined with special relativity leads instead to classical electrodynamics.
Historical notes
The foundations of electromagnetism are laid at the beginning of the nineteenth century. Its initiator is the Danish physicist H. C. Oersted that on July 20, 1820 announced in a memory the fundamental experiment that showed the deviation of the magnetic needle in the presence of a straight stretch of electric circuit. The result made a sensation and subsequent studies of J. B. Biot and F. Savart showed, among other things, that the acting force is not Newtonian and depends on the direction of the current. At the same time applied to these studies A.. M. AmpĂšre, who discovered the electrodynamic actions between electric currents (to him we owe the term electrodynamics to indicate this area of physics), rejected the theory that tried to explain Oerstedâs result, admitting that the circuit crossed by current turns into a magnet, and proposed instead to consider the magnet as composed of a multitude of small circuits all parallel to each other and whose currents move in the same direction.
The verification of this brilliant theory led AmpĂšre to study various types of circuits including solenoids. We owe to AmpĂšre also the mathematical formulation of the laws relating to the force that is exerted between two elements of current as a function of their intensity, distance and mutual position. The relation electricity-magnetism constitutes the fundamental problem in the work of M. Faraday, who laid the theoretical foundations and elaborated the fundamental laws of electromagnetism: in 1831 he came, in fact, to discover the phenomenon of electromagnetic induction by producing electric currents through variations of magnetic fields.
Faraday explained the phenomenon by introducing the fundamental concept of âlines of forceâ, lines of magnetic induction that are generated as concentric circles around a metal wire when it is crossed by electric currents and that are similar to those existing between the two poles of a natural magnet. From this he derived the fundamental law on the direction of the induced current depending on the variation of the lines of force concatenated with the circuit, law revised and made more rigorous by E. Lenz. From Faradayâs work started J. C. Maxwell.
In the fundamental Treatise on Electricity and Magnetism (1873) mathematically formulated the concept of line of force, introduced the concepts of electromagnetic field and displacement current, founded the electrodynamics of dielectrics, which allowed him then a general treatment of polarization, and finally condensed the whole theory in six equations that connected in a single building electricity, magnetism and optics. The mechanistic conception that dominated physics suffered a decisive blow. H. Hertz, later, verified Maxwellâs hypothesis and was able to produce electromagnetic waves that, like light, could be reflected, refracted and polarized, opening the way to the development of radiotelecommunications. Subsequent developments are related to the names of H. PoincarĂ©, H. A. Lorentz, P. and M. Curie, M. Planck, A. Einstein, in whose works electromagnetism is totally merged in the new atomic and nuclear physics.
Electrostatics
In physics, electrostatics is a branch of electromagnetism that studies electric charges âat restâ, that is those charges stationary in time, which in turn generate an electrostatic field. Phenomena of moving charges, which produce currents, are instead studied by electrodynamics.
Electrostatic phenomena derive from the forces that electric charges exert on each other. These forces are described by Coulombâs law. Although electrostatically induced forces appear to be quite weak, some electrostatic forces such as that between an electron and a proton, which together form a hydrogen atom, are about 36 orders of magnitude stronger than the gravitational force acting between them.
In other words, electrostatics studies electromagnetic phenomena that occur when there are no moving charges, that is, after a static equilibrium has been established. Mathematical methods applied to electrostatics allow the calculation of electric field and electric potential distributions from a known configuration of charges.
Electrostatics involves the accumulation of charge on the surface of objects due to contact with other surfaces. Although charge exchange occurs whenever any two surfaces come into contact and then separate, the effects of charge exchange are usually only noticed when at least one of the surfaces has a high resistance to electric flow. This is because the transferred charges remain trapped long enough to be observed. These charges then remain on the object until they discharge to the ground or are rapidly neutralized by a discharge: for example, the familiar phenomenon of a static âshockâ is caused by neutralization of the charge accumulated in the body by contact with isolated surfaces. Electrization of a body can generally be achieved by three different methods:
- by rubbing;
- by contact (permanent electrization)
- by induction (temporary electrization).
The ability of some materials such as amber (natural resin), or glass, when rubbed on wool, to attract small pieces of paper was known since ancient times. Plato, in the fourth century BC considered the origin of these effects similar to that of magnetic phenomena. Plutarch, in the 1st century AD, observed that the nature of these effects must be dissimilar to that of magnetic phenomena since, while magnetite seemed to attract only iron, rubbed amber attracts different objects as long as they are light.
In the sixteenth century, W. Gilbert discovered that other substances when rubbed acquire the same properties, while others do not. He introduced the adjective electric to indicate this class of phenomena, from the Greek name of amber, ηλΔ xÏÏÎżÎœ. He formulated a theory that justified this phenomenon, called electrization by rubbing, assuming that, due to the heating of the bodies by rubbing, was emitted from the body a fluid that had the ability to attract light objects placed nearby. In the first half of the 18th century, the French scientist C. Du Fay began a methodical research activity around electrical phenomenology and verified that:
all materials, except metals, could be electrified by rubbing deducing that electricity is a property of matter;
rubbed objects did not always attract small bodies but, in some cases repelled them;
there had to exist two types of electricity that he called resinous electricity and glassy electricity and he proposed a theory that non-electrified bodies have the two types of electricity in equal measure.
Electrostatic phenomena
Are called electrostatic phenomena all those electrical phenomena that are produced in the space (and in the bodies that are immersed in it) by free electric charges, positive or negative, that are in static equilibrium on the electrified bodies. In 1887, J. Thomson discovered electron and identified atom as the fundamental component of matter. It was then possible to provide an explanation of the phenomenology of electrization by rubbing: the state of electric equilibrium of atoms of rubbed bodies has been changed, because the energy developed in the rubbing operation has literally âtornâ some electrons. In correspondence to the rubbing of the glass rod with a wool cloth, some electrons from the rod are torn off by the abrasive action and are transferred to the cloth. Therefore the glass rod acquires a non-zero net charge: it becomes electrified.
There are many examples of electrostatic phenomena, such as when you rub a glass rod with a woolen cloth, some electrons free to move, leave the rod and transfer to the woolen cloth. The glass rod loses electrons and has more protons than electrons, so it is said to be positively charged; the cloth, instead, gains electrons and has more electrons than protons, so it is said to be negatively charged. If instead of a glass wand is used an amber wand, electrons pass from the wool cloth to the wand. In this case the amber wand, which gains electrons, is negatively charged and the cloth, which loses electrons, is positively charged. So when two bodies are rubbed, one is positively electrified and one negatively.
They were called insulators, material bodies that are charged by rubbing, that do not easily carry charges (for example rubber in the coating of electric wires). They were called conductors, material bodies in which electrical charges move freely. They were called semiconductors, the material bodies that have intermediate behavior between conductors and insulators (such as silicon and germanium used in integrated circuits of electronic computers). They were called superconductors, the material bodies perfectly conducting, that allow charges to move inside them without any obstacle (absence of electrical resistance).
If we remember that, according to Coulombâs law, electric charges act mutually on each other with mutual attractions and repulsions, which are exerted in all directions radiating from each of them, we understand that electric actions are not manifested only within the bodies in which they are contained, but extend and invest the entire surrounding space: experience proves that all electric actions are exerted at a distance even through empty space without the intervention of any material continuity that must transmit them.
A point electric charge, positive or negative, acts radially in all directions on all other charges of opposite sign. We express this fact by saying that every positive or negative charge, is always subject to a force that is the resultant of attractions and repulsions that it feels from the surrounding elementary charges. This fact can be expressed by saying that every electric charge undergoes the action of the electric field resulting from the action of the proper fields of all the remaining electric charges.
An alternative procedure to electrization by rubbing, is called electrization by induction. Approaching a charged body, to a neutral isolated conducting sphere, the region of the sphere closest to the charged body is charged with the opposite sign, while the farthest one is charged with the same sign (in fact the electrons of the neutral sphere move, leaving uncovered the positive charge). If the sphere, instead of being isolated, is connected to the ground, some electrons flow towards the ground (the sphere and the ground are a single conductor, the electrons move away). Interrupting the connection the sphere remains positively charged. By subsequently moving the charged body away, the charge of the sphere is distributed uniformly due to the mutual repulsion of the equal charges. Also in this case, the principle of conservation of charge continues to apply.
Historical notes
The first documented experiments and researches on electrostatics date back to ancient Greece of 600 BC with Thales of Miletus and Theophrastus, discovered that rubbing of amber (which in ancient greek was called ጀλΔÎșÏÏÎżÎœ, Ă©lektron) with a woolen cloth, allowed to attract towards itself straws, feathers, wires and the like. The different forms of this device were later called in 1600 âelectrical phenomenaâ thanks to a second scientific contribution regarding the study of these phenomena, by William Gilbert, who distinguished these phenomena from magnetic ones, as he had observed that electrical phenomena had a finite energy (which was called electric fluid) and the force of attraction lasted as long as there was enough energy, coining also the term electric force.
The third contribution was made by Otto Von Guericke in the middle of XVII century, both for the realization of the first electrostatic generator (Guerickeâs electrostatic rubbing machine), with which he observed the electric discharges generated during its loading and the relative luminescence and crackling (this phenomenon was called âelectric fireâ), but he also demonstrated how the electric force generated by a charged body could be transported, applying to this charged body a wire, which has the same properties of the charged body. Guericke made another important discovery: studying amber he noticed that objects that were initially attracted by it, once in contact with the charged amber, were repelled by it, thus demonstrating that the electric force can be both attractive and repulsive.
Subsequently Charles François de Cisternay du Fay, determined the existence of a positive electric charge and a negative one, which were generated by different substances, which were called âresinousâ (amber, hard rubber, wax, resinous substances) and âglassyâ (glass and similar), discovering also that bodies charged in the same way and therefore with the same electrical charges repelled each other (amber-amber or glass-glass), while bodies charged with different charges attracted each other (amber-glass), at the same time assumed that neutral bodies contained equal amounts of the two electrical fluids, while charged bodies had an excess of one fluid compared to the other.
Later it was defined with more precision how the electrization occurs by electron transfer (so positively charging) or by electron acquisition (negatively charging), because the electron movement requires very small energies compared to protons.
Voltage
The potential energy, stored in the form of an electric charge imbalance and capable of provoking electrons to flow through a conductor, can be expressed as a term called voltage (or electric potential difference, electric pressure, and electric tension), which technically is a measure of potential energy per unit charge of electrons or something a physicist would call specific potential energy. Defined in the context of static electricity, voltage is the measure of work required to move a unit charge from one location to another, against the force which tries to keep electric charges balanced.
In the context of electrical power sources, voltage is the amount of potential energy available (work to be done) per unit charge, to move electrons through a conductor. When a charge q is immersed in an electric field generated by other charges, it is endowed with a certain aptitude for doing work, simply because it is immersed in this field. This attitude is called potential energy. In stationary conditions the potential difference is equal to the work done to move a unitary charge across the field from one point to another, changed in sign.
In metrology, the International System establishes that the unit of measurement of the electrical potential difference is the volt [V]. The measuring instrument for making the measurement is the voltmeter, generally integrated in an electrical âtesterâ.
Conventions and properties of potential differences
The potential difference existing between two points A and B of an electrical system can be represented with the symbol \(\Delta V_ {AB}\), where the lower indices (A and B) indicate between which points of the system is meant to refer the potential difference.
\[V_{AB} = V_A â V_B\]
remember that:
- if \(V_A > V_B\) â \(V_{AB} > 0\)
- if \(V_B > V_A\) â \(V_{AB} < 0\)
Additivity principle of potential differences
Consider two terminal circuit elements (dipoles) connected in series. The potential difference between the two extremes A and C (the point B is the one of conjunction between the two elements in series) is obtained by making the sum of the potential differences at the ends of each element.
\[V_{AC} =V_A â V_C =V_A -V_B +V_B -V_C =V_{AB} +V_{BC}\]
Things do not change if there are three or more elements connected in series. In general the following principle applies, called the principle of additivity of potential differences:
the potential difference at the ends of \(n\) elements connected in series is equal to the sum of the potential differences at the ends of each electric element.
No-load voltage
âNo load voltageâ is a common term used for unregulated power supplies, generators, and batteries. We define the no-load voltage \(V_0\), the electric potential difference present at the ends of a dipole, or a port, when the current flowing through it is zero, or when the two terminals of the port are disconnected from the electric circuit. It is the output voltage when nothing is connected to the output.
The higher the current, the lower the voltage. This is due to the supplyâs internal series impedance. When nothing is connected to the output of the power supply (âno-loadâ), the current is 0 A, and the voltage will be at its highest. So the âno-load voltageâ is the highest voltage the power supply will produce.