By Amna Kanwal
This article will take you through the history of Physics, focusing on significant events in electrodynamics. It is written especially for those interested and curious about how things in Physics have evolved, and how the world of electrodynamics reached the point where it is now.
Starting from the very beginning, where Newtonian Mechanics proposed that the speed of light depended on the observer’s motion and was relative, a position which contradicted the laws of Electrodynamics. This conflict was later resolved in the 2000s by Einstein’s Special Theory of Relativity, which proposed that the speed of light is constant in all frames of reference. In between, five major events took place in the history of electrodynamics. This article will primarily focus on Benjamin Franklin’s contributions, which formed the basis for electrodynamics.
#1: Benjamin Franklin’s Work on Electromagnetic Fields
Franklin spoke from experience, having had his own share of electric shocks. Writing in 1750 to a friend, he said, “I have lately made an experiment in electricity that I desire never to repeat.” Describing the experiment, he stated, “I inadvertently took the whole [fire] through my own arms and body, by receiving the fire from the united top wires … I felt what I know not how to describe—a universal blow throughout my whole body from head to foot, which seemed within as well as without; after which the first thing I noticed was a violent, quick shaking of my body”, announcing “the quickness of the electrical fire seems to be greater than that of sound, light, or animal sensation.”
So, What are Franklin’s Contributions to Electricity?
He made three major contributions that helped with the understanding of this force of nature:
1. There is only one type of electricity, the single-fluid theory.
2. Charge is always conserved.
3. Lightning is nothing other than electricity in the air.
In the process, he coined terms still being used today: positive (plus) and negative (minus) charges, battery, conventional current, and condensers. When reporting his results to his friend Peter Collinson, he asked for pardon for coining the new terms positive and negative, saying “these terms we may use until your philosophers give us better.” No one ever did!
His serious work on electricity began in 1747 when he received a Leyden jar from Europe. Tinkering with it, he noticed charges, otherwise known as electric fluid, didn’t reside within the water but rather collected on the glass. With this in mind, he set out to improve how charges collect on the jar’s metal foil.
Franklin had similar Leyden jars made for his experiments and connected them in a series to increase the amount of stored charge when charged simultaneously—calling the new device a “battery”.
Connecting Leyden Jars in a Series
Franklin’s experiments connecting several Leyden jars through wire produced the first electric circuit. Franklin studied the charging process using “two or more vials on the prime conductor, one hanging on the tail of the other, and a wire from the last to the floor.” With this, Franklin gave the world the first series-connected electric source.
As Park Benjamin said, Franklin was “forging the link between the Leyden jar and the voltaic cell”. He fell short of discovering the electron as the basic charge, but in him, the concept of current flowing in a wire or conductors was born.
At the same period across the Atlantic, the British Watson commented, “over any of the distances yet experienced, [electrical transmission] is nearly instantaneous”. Watson’s wire was an early form of the telegraph—sending painful shocks instead of messages.
The idea of connecting several Leyden jars to produce short-term sparks served as a stepping stone to Volta’s invention of long-term or continuous current flow 50 years later.
Where Does the Charge Reside in the Leyden Jar?
The world has theorized about it. “In the man who holds it,” said Von Kleist. “In the water”, said Musschenbroek. “In the inner conducting coating”, said Watson. But Franklin corrected all when he declared it “is in the glass itself.”
- One-Fluid Theory
After several repeated experiments with different electric bodies and occasional human physical participation, Franklin concluded there is only one kind of electric fluid. He stated that “electric fire (virtue) is a common element”. He declared “electricity cannot be made, it is always there” and that electrical effects are simply charges induced to move from one place to another. If one body is missing the fluid then there is a deficit. He then describes how charges come from the same ‘fluid’ or ‘fire’ as two types of charges, positive and negative, saying certain bodies will “[be] electrified positively (or plus)” and others “negatively (or minus)”, depending on the body passing (giving) the fire or receiving (accepting) it.
Bodies became either positively or negatively charged because of an excess or deficit of charges. He then observed an electrified body with an excess of electrical fluid will attract another body that has an electric fluid deficiency. By the same token, each with an excess of electrical fluid will repel each other.
With the one-fluid theory comes the first contribution from the new world. Unlike many of the European natural philosophers like DuFay, his student Nollet, and Coulomb who believed in the two-fluid theory, Franklin disrupted the state of knowledge in a new field of science while having no mathematical and scientific foundation.
The single-fluid theory was his most important and highly insightful scientific breakthrough in the 1700s.
- Conservation of Charge
Furthermore, with the single-fluid concept, he argued that the amount of charge is always the same. He became the first to report the concept of strength of charge, neutral point, and the conservation of charge.
When two bodies are electrified, one positively and the other negatively, he said, a third body holds a “middle quantity of electrical fire”. With this, he defines the meaning of neutral, or ground, by referring to the middle quantity—the normal state of bodies is equilibrium. Then he added that the strength of the spark is highest when electric fire moves from two simultaneously positively or negatively electrified bodies rather than when only one is electrified, with a third unelectrified (common stock or neutral) body.
Franklin explained, “When a body is electrified plus, it will repel a positively electrified feather or small cork ball. When minus (or when in the common state), it will attract them, but stronger when minus than when in the common state, the difference being greater”. This statement provided us with a quantitative discussion of Coulomb’s Law. In the same letter, “If these electrified bodies touch while electrizing [sic], the equality is never destroyed, the fire only circulating”. Hence, the concept of conservation of charge was born.
This is as ground-breaking of a concept as Newton’s conservation of momentum. With Franklin’s new breakthrough concepts, the road to the quantitative analysis of electricity was opened wide in the 1750s.
- Lightning
Franklin then turned his experimentation to lightning. For centuries, man feared the devastating effects of lightning and accepted it as a supernatural phenomenon, God’s will, or God’s vengeance on mankind. Churches throughout Europe were devastated by lightning, especially when church bells rung, even though there remained a belief that the sound of these bells would protect the church and the worshippers inside. “The tones of the consecrated metal repel the demon and avert storm and lightning”, declared St. Thomas Aquinas.
But Franklin insightfully speculated that this devastating lighting force from the heavens is the same fire observed in the Leyden jar. Until then, literature on the subject had only eluded that lighting produces a cracking noise and accompanying smell similar to sparks produced with a sudden discharge of electricity.
In 1749, in a letter to Peter Collinson, he documented 12 reasons why he thought “electrical fluid agrees with lightning:
1. Giving light,
2. The color of the light,
3. The crooked direction of the flame,
4. The swift motion,
5. Conducted by metals,
6. The crack or noise of the explosion,
7. Subsisting in water or ice,
8. The rending of bodies it passes through,
9. Destroying animals,
10. Melting metals,
11. Firing inflammable substances,
12. The sulphurous smell”.
He went on questioning whether lighting can be attracted by pointed metal as the electric fire and proclaimed “Let the experiment be made!”
Franklin first theorized that water vapor of clouds caused positive and negative electrical charges. When these electrified clouds pass over high structures like trees and towers, the clouds unleash their fiery wrath. To know if this is the case, he wanted to capture that fire and prove it the same fire in his Leyden jar. With a tall metal rod “pointed very sharp at the end”, he believed that when clouds “passing low might be electrified and afford sparks … then the electrical fire would, I think, be drawn out of the cloud silently.”
He set out to pacify one great danger of nature by designing a lightning rod, declaring pointed metal objects more attractive of electric charge than other shapes— and in doing this, winning points for science against the supernatural view of the phenomenon.
Details on its experimental verification show in his correspondence to Peter Collinson, published as an 86-page pamphlet in 1752 and soon translated into French. Word reached Paris in the same year, and Messieurs Jean Dalibard and Delor, under the direction of Louis V, drew sparks from a thunderstorm cloud through a lightning rod via Franklin’s instructions.
Then comes his most famous experiment: cloud, lightning, kite, and key.
In June of 1752, accompanied by his 21-year-old son, he flew his kite into a thunderstorm with a key tied on by silk thread. His experiment proved lightning is nothing but the same electricity found in the pop-star Leyden jar.
It was his luckiest experiment ever. If he had been struck directly, he would have died, as Russian professor George Wilhelm Richmann did when repeating his experiment.
The kite experiment faced an uphill battle for acceptance. Instead of instant applause, it met with incredulous skepticism. The only confirmation came from Joseph Priestly’s assertion that Franklin showed lightning is of the “same quality of electricity produced in the lab”.
Final Remarks
Benjamin Franklin became immortalized in the story of the electron.
He was a practical experimentalist with no mathematical grounding, no theoretical experience, and limited bookkeeping skills. It was intellectual and imaginative might behind the breakthroughs he accomplished in just a few years. Franklin presented two new concepts to guide scientists to come: the single-fluid theory and the conservation of charge.
According to Sir Joseph J. Thomson, who discovered the electron in 1896, the service of Franklin’s single-fluid theory “has rendered to the science of electricity can hardly be overestimated”.
He was the most important experimental natural philosopher of his era. The single-fluid theory was the greatest contribution of the 1700s and ushered in a scientific revolution in our understanding of the source of electron flow. “We shall, I am sure, be struck by the similarity between some of the views which we are led to take by the results of the most recent researches, with those enunciated by Franklin in the very infancy of the subject”.
In 1784, King Louis XV appointed Franklin a member of the commission investigating claims by the German Mesmer that electricity has the power to heal. It was left to Franklin to draft the final report exposing Mesmer to embarrassment. French medical expert Dr. Gilles de la Tourette of the Faculty of Medicine in Paris declared the report “a scientific work of the first order which is worth of being consulted today by all those who are interested in hypnotism and the diseases of the nervous system.”
After Franklin, excitement over the single-fluid concept energized the effort for electrical discovery. The scene of the day shifts back to Europe, where it would remain until Joseph Henry seventy-five years later.
#2: Weber Electrodynamics
Weber’s electrodynamic force is introduced to provide a mathematical overview and to familiarize the reader with Weber’s direct-action approach as an explanation for electricity and magnetism. Weber’s force describes the interaction of two point charges and was postulated before the electron was even discovered.
Hence, it was originally based on Fechner’s hypothesis that a current consists of equal amounts of positive and negative charges moving in opposite directions, which was the conventional wisdom at the time, as scientists imagined so-called “electrical fluidae” moving through wires and circuits when subjected to electromotive forces. It shall be noted that Weber’s theory can still be used when we assume that only electrons are charge carriers in motion responsible for conduction currents in circuits.
Limitations and Applications of Weber’s Theory
One can conclude that Weber’s theory is not without limitations and is only valid within the low-velocity regime, with standing problems in relativistic physics (special as well as general), radiation and plasma applications as well as quantum electrodynamics.
However, if the force law is considered as an addition to Maxwellian field theory, it can enrich one’s perspective on electromagnetic phenomena and beyond. It offers an explanation of observed phenomena from a particle perspective, as well as the prediction thereof. In this sense, Weber’s theory can be regarded as an important component of electrodynamic theory, even though it has not been developed anywhere near to the same level as field theory.
While experimental evidence in relativistic electrodynamics and QED supports Maxwell’s field theory, it has been found that Weber agrees with experiments in the near field and low-velocity limit. This suggests, along with recent studies, that Weber’s force is a low-velocity approximation up to second order in v/c of a more fundamental underlying force, and needs further development.
Advantages and Further Possibilities
A benefit of Weber’s force is that it follows Newton’s third law of motion, thus conserving linear momentum, and additionally conserving angular momentum as well as energy, so it does not violate conservation laws. In the field approach, it is usually argued that conservation laws are not violated when the energy content of the field is taken into account, i.e., the field can obtain energy or momentum from a system and store it as well as release energy and momentum.
Further, Weber’s force accounts for longitudinal forces intrinsically. However, the absence of longitudinal forces seems to be desired in radar and plasma applications where the field approach benefits from this quality instead and Weber seems to fail to predict the expected results. Nonetheless, Weber’s force can be calculated directly from the movement of the charges involved in an interaction and does not necessitate the calculation of one or more fields of those charges from which the force is calculated, which offers clear force and particle-based explanations, which avoids problems such as the self-energy divergence.
Another benefit in Weber’s theory is that charge velocities are clearly defined, whereas in field theory there may be some ambiguity left as to what velocities are to be used in the Lorentz force equation. Lastly, Weber’s force offers the possibility to unify gravitational forces with those of electromagnetism. It is thus considered constructive to use both theories in cooperation with each other, as each can compensate for the other’s weakness and regarding a specific problem in question from both perspectives can potentially lead to new insight.
#4: Tesla’s Contributions
Life and Achievements
Tesla is often described as the most important scientist and inventor of the modern age. He is best known for many revolutionary contributions and inventions in the field of electricity and magnetism in the late 19th and early 20th centuries. Tesla’s patents and theoretical work formed the basis of modern alternating current electric power (AC) systems, including the polyphase power distribution systems and the AC motor, with which he helped usher in the Second Industrial Revolution.
After his demonstration of wireless communication (radio) in 1894 and after being the victor in the “War of Currents” (in the late 1880s, George Westinghouse and Thomas Edison became adversaries due to Edison’s promotion of direct current (DC) for electric power distribution over alternating current (AC) advocated by Westinghouse and Nikola Tesla), he was widely respected as one of the greatest electrical engineers who worked in America.
Much of his early work pioneered modern electrical engineering and many of his discoveries were of groundbreaking importance. However, due to his eccentric personality and his seemingly unbelievable and sometimes bizarre claims about possible scientific and technological developments, Tesla was ultimately ostracized and regarded as a mad scientist. Never having put much focus on his finances, Tesla died impoverished at the age of 86.
Extended Influence
Aside from his work on electromagnetism and electromechanical engineering, Tesla has contributed in varying degrees to the establishment of robotics, remote control, radar and computer science, and to the expansion of ballistics, nuclear physics, and theoretical physics.
Many interpret the 1943 Supreme Court of the United States decision as crediting Tesla as being the inventor of the radio. Many of his achievements have been used, with some controversy, to support various pseudosciences, UFO theories, and early New Age occultism.
#5: The Theory of Magnetic Action upon Light
A satisfactory theory of magnetic action upon light can be constructed by means of a modification of Maxwell’s theory which was proposed by Prof. Fitzgerald in 1879; he alleges, with special emphasis, that his theory furnishes “a consistent scheme of equations of reflection and refraction, without the necessity of condoning any dynamical difficulties in the process.”
And on p. 359, after raising objections against a theory originally suggested by Prof. Rowland, and afterwards fully developed by myself, he says:- “But against this procedure,” that is my own, “there stands the pure assumption as regards discontinuity of electric force at an interface.” To fully discuss the defects of Larmor’s resuscitation of Fitzgerald’s theory would occupy too much space, and would necessitate the introduction of a considerable amount of mathematical analysis. I shall, therefore, confine myself to pointing out that his theory is open to exactly the same objections as my own, viz. discontinuity of the tangential component if electromotive force at an interface.
One of Larmor’s boundary conditions is equivalent to the condition that the expression
should be continuous. Now 4πg/K = Q, where Q is one of the tangential components of the E.M.F. at an interface; also in unmagnetized media C = 0. Consequently, if accented letters refer to the latter medium, the condition becomes
in other words, the tangential component of the E.M.F. is discontinuous.
Quantum Electrodynamics
Overview
The modern Quantum Electrodynamics (QED) developed mainly by Schwinger, Feynman, Tomogana and Dyson was connected from the very beginning with atomic theory. One of the first great successes of QED was the explanation of the Lamb Shift in atomic hydrogen, made by Bethe, Kroll and Lamb, French and Weisskopf. Due to the excellent convergence of the perturbation theory expansions in QED coupling constant α =e²/ħc = 1/137.035999… (e is the electron charge, h is the Planck’s constant and c is the speed of light), QED theory of the light H-like atoms is elaborated now up to very high orders, demonstrating an excellent agreement with experiment. The most recent review on the subject one can find in.
Further Developments and Challenges
The QED theory of the light atoms, apart from the α-expansion, also exploits the expansion in parameters αZ where Z is the charge of the nucleus. Thus it is valid only for αZ≪ 1. This condition does not hold for the inner electron shells in heavy atoms. Moreover, it does not hold even for the valence electrons in heavy atoms due to the singularity of QED operators.
For the evaluation of the matrix elements of such operators the small distances of the electron from the nucleus become important. At such distances the effective electron charge Zeff for the valence electrons may not be small. Therefore the QED theory without αZ expansion appears to be necessary. Such theory was first introduced in and applied to the K-shell electrons in the Hg atom. The further development of this theory can be found in.
Future Applications and Considerations
Another important application of all-orders in αZ atomic QED is the theory of the multicharged ions. Nowadays all elements of the Periodic Table up to Uranium (Z=92) can be observed in the laboratory as H-like, He-like etc ions. In principle, the QED theory of atoms includes the evaluation of the QED corrections to the energy levels and corrections to the hyperfine structure intervals, as well as the QED corrections to the transition probabilities and cross-sections of the different atomic processes: photon and electron scattering, photo-ionization, electron capture etc.
QED corrections can be evaluated also to the different atomic properties in the external fields: bound electron g-factors and polarizabilities. In this review we will concentrate mainly on the corrections to the energy levels which are usually called the “Lamb Shift” (here the “Lamb Shift” should be understood in a more broad sense than the 2s, 2p level shift in a hydrogen). A question may arise whether there are non-QED corrections comparable in size with QED ones and thus preventing the direct tests of QED in atomic experiments. The most important among these corrections are nuclear ones.
The nuclear recoil is usually included in QED theory of atoms. The nuclear size correction even dominates over the QED corrections in HCI and heavy atoms. The uncertainty in the determination of the nuclear radius sets the principal limit to the ab initio calculations of the atomic energy levels. For HCI this uncertainty is still smaller than the second-order in α QED corrections, but the nuclear polarization corrections are approaching this level. Thus the evaluation of the higher than α² QED corrections is unreasonable without the detailization of the nuclear structure theory. In heavy atoms the nuclear uncertainty is still well below the electron correlation uncertainty, allowing for the evaluation of only the first-order in α QED corrections.
Another possible source of non-QED corrections to the energy levels is the parity–conserving weak interaction between the electron and the nucleus. The estimates show that these corrections are too small to be considered seriously both in light and heavy atoms and in HCI. The parity nonconserving weak interaction can influence atomic transition probabilities. This leads to the observable asymmetry effects in radiation. We should mention that QED effects appear to be observable also in molecules.
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