Electric charge

• 1. Introduction

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  Electric charge is so basic to matter that in 1838 Michael Faraday found he could not charge a single body without an opposite charge appearing in another. That stubborn pairing is one of the strangest facts about a property nobody can see directly. Electric charge, written with the symbol q, is what makes matter feel a force when it sits in an electromagnetic field. It decides whether objects pull together or push apart. How did people first notice it? Why is the choice of which kind is called positive really a matter of convention? And what is the smallest unit that every freely existing charge must be a multiple of? This documentary follows those questions from a rubbed piece of amber to the quarks of modern physics.

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• 2. Amber and Ancient Sparks

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  Around 600 BC, according to a report tied to the Greek thinker Thales of Miletus, amber rubbed with fur could draw small, light objects toward itself. Thales lived from about 624 to about 546 BC. Historians doubt that he left any writings, and the report of his view survives only in an account from the early 200s. Thales read the effect as proof that lifeless objects have a soul, so there was still no notion of charge.

  The ancient Greeks knew four phenomena that we now explain through charge. They were lightning, the torpedo fish or electric ray, St Elmo's Fire, and the pull of rubbed amber. Nobody connected them. The Greeks saw amber buttons attract hair, and they found that rubbing long enough could make a spark jump. One claim, however, says sparks went unmentioned until the late 17th century. The underlying cause is called the triboelectric effect.

  In the late 1100s, jet, a compacted form of coal, was noted to behave like amber. In the middle of the 1500s, Girolamo Fracastoro discovered that diamond showed the same effect. He and others, especially Gerolamo Cardano, tried to explain why.

  Steady research began in 1600, when the English scientist William Gilbert published De Magnete. Astronomy, mechanics and optics had been studied with measurement since antiquity, but electrical phenomena had no such tradition. Gilbert called the pull the amber effect. He coined the Neo-Latin word electrica from the Greek word for amber, ēlektron, and English turned it into electrics. He is also credited with the term electrical. The word electricity came later, first attributed to Sir Thomas Browne in his Pseudodoxia Epidemica of 1646.

  Gilbert proposed an effluvium, a small stream of particles flowing from the rubbed object without reducing its bulk or weight. That idea stayed influential through the 17th and 18th centuries. It was a forerunner of the later talk of electric fluid by Dufay, Nollet and Franklin.

  Around 1663, Otto von Guericke built what was probably the first electrostatic generator, yet he barely treated it as an electrical device. In 1675, Robert Boyle published the first English book devoted entirely to electrical phenomena. It largely repeated Gilbert, but Boyle identified more electrics and noticed that two bodies attract each other mutually.

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• 3. Conductors and Two Fluids

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  In 1729, Stephen Gray rubbed a glass tube and noticed that the cork protecting it from dust and moisture became electrified as well. He then lengthened the cork by pushing thin sticks into it. This showed for the first time that what he called electrical effluvia could travel across a distance. Gray sent charge along twine for 765 feet and along wire for 865 feet.

  Gray also saw that different materials helped or hindered the travel of effluvia. John Theophilus Desaguliers repeated many of the experiments and is credited with the terms conductors and insulators. Gray found induction too, where charge reaches another object with no direct contact. He held a charged glass tube near a lump of lead hanging from a thread, and the lead began to attract and repel brass filings.

  The discovery that charge could be passed along opened a theoretical possibility. Perhaps the property was not inseparably tied to the bodies that were rubbed. In 1733, Charles François de Cisternay du Fay, inspired by Gray, reported a series of experiments in the Mémoires de l'Académie Royale des Sciences. He showed that nearly all substances could be electrified by rubbing, except metals and fluids.

  Du Fay proposed that electricity comes in two varieties that cancel each other, a two-fluid theory. Glass rubbed with silk carried what he called vitreous electricity. Amber rubbed with fur carried resinous electricity. Today positive charge is defined as that of glass rubbed with silk, but which kind gets which name is arbitrary. In 1745, Jean-Antoine Nollet put forward another important two-fluid theory.

  Until about 1745, the main explanation for electrical attraction and repulsion was that electrified bodies gave off an effluvium. Benjamin Franklin would begin his own electrical experiments in late 1746.

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• 4. Franklin's Single Fluid

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  By 1750, Benjamin Franklin had built a one-fluid theory of electricity. It rested on an experiment showing that rubbed glass gained the same strength of charge, but opposite in kind, as the cloth that rubbed it. Franklin pictured electricity as an invisible fluid present in all matter. He coined the term charge itself, along with battery and some others.

  In his picture, rubbing insulating surfaces together made the fluid change location, and a flow of the fluid made an electric current. Matter with an excess of fluid was positively charged, and matter with a deficit was negatively charged. He believed the glass of a Leyden jar held its accumulated charge.

  Franklin tied the words positive and negative to vitreous and resinous electricity after a test with a glass tube from his overseas colleague Peter Collinson. One participant charged the tube, and a second participant received a shock to the knuckle. Franklin judged the shocked person to be positively charged.

  The credit is tangled with William Watson. After seeing Franklin's letter to Collinson, Watson claimed he had given the same explanation in spring 1747. Franklin had studied some of Watson's works beforehand, which was probably significant for his theorizing. One physicist suggests Watson proposed a one-fluid theory first and Franklin elaborated it more influentially. A historian of science argues Watson missed a subtle difference between the two sets of ideas. There was no animosity between the men.

  Franklin's model, formulated in early 1747, eventually became widely accepted, and effluvia-based explanations were rarely offered afterward. We now know it was fundamentally correct. There is only one kind of electrical charge, and one variable is enough to track the amount.

  One limit remained. Until 1800, conduction could be studied only through electrostatic discharge.

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• 5. From Fluid to Field

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  In 1800, Alessandro Volta became the first to show that charge could be kept in continuous motion around a closed path. Electricity was no longer limited to a single discharge.

  In 1833, Michael Faraday set out to remove any doubt that electricity is identical whatever its source. He sorted the known forms into common electricity, such as static electricity, piezoelectricity and magnetic induction. The second group was voltaic electricity, such as current from a voltaic pile. The third was animal electricity, meaning bioelectricity.

  Faraday's thinking kept moving in 1838. He asked whether electricity was a fluid, several fluids, or a property of matter like gravity. He also offered a theoretical account of electric force while staying neutral on whether it comes from one fluid, two, or none. He focused on the idea that particles normally sit in a nonpolarized state and, when polarized, seek to return to it.

  Starting in the mid-1850s, James Clerk Maxwell developed a field theory approach to electrodynamics. He stopped treating charge as a special substance that piles up in objects. Instead he understood it as a consequence of energy transformation in the field. This pre-quantum view still took the size of a charge to be a continuous quantity, even at the microscopic level.

  The modern picture keeps the field at its center. Electric charges produce electric fields, and a moving charge also produces a magnetic field. The interaction of charges with an electromagnetic field gives rise to the Lorentz force, one of the four fundamental interactions in physics. The study of photon-mediated interactions among charged particles is called quantum electrodynamics.

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• 6. The Smallest Unit

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  Faraday's electrolysis experiments were the first to reveal that electric charge comes in discrete amounts. Robert Millikan's oil drop experiment later demonstrated this directly and measured the elementary charge. That charge, written e, is the smallest that can exist freely, and the charge of any free-standing particle is an integer multiple of it.

  In 1891, after the discrete nature of charge was recognized, George Stoney proposed the name electron for this fundamental unit. J. J. Thomson then discovered the particle we now call the electron in 1897. The unit today goes by elementary charge, fundamental unit of charge, or simply e. An electron carries negative e, and a proton carries positive e.

  Quarks complicate the picture. Their charges are either negative one third or positive two thirds of e. Yet free-standing quarks have never been observed, and they are believed to occur only in combinations with a whole-number multiple of e. In the Standard Model, charge is an absolutely conserved quantum number. Fractions of e can still be meaningful in some contexts, such as the fractional quantum Hall effect, even though at large scales charge seems continuous.

  The SI unit for quantity of charge is the coulomb, symbol C, named after the French physicist Charles-Augustin de Coulomb. It is the charge passing through the cross section of a conductor carrying one ampere for one second. The unit was proposed in 1946 and ratified in 1948.

  Other fields prefer other units. Electrical engineers often use the ampere-hour, and physicists and chemists often use the elementary charge itself. Electrochemistry uses the faraday, the charge of one mole of elementary charges. Chemists also use the Faraday constant for the same quantity. An electrometer measures charge directly, while a ballistic galvanometer measures it indirectly.

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• 7. Matter, Ions and Static

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  Most everyday objects carry almost no net charge because atoms typically hold equal numbers of protons and electrons. Their charges cancel, so each atom is neutral. The charge of a large object is simply the sum of the charges of its particles.

  An ion is an atom or group of atoms that has lost or gained electrons. Losing one or more gives a net positive charge and a cation. Gaining one or more gives a net negative charge and an anion. Monatomic ions form from single atoms, while polyatomic ions form from two or more bonded atoms. When large objects form, atoms and ions usually combine into neutral ionic compounds, but they are rarely perfectly neutral overall.

  Rubbing amber with fur or glass with silk produces static electricity, the name for a non-zero, motionless net charge. Charge taken from one material moves to the other, leaving an equal and opposite charge behind. Conservation of charge always applies.

  A classic demonstration uses a piece of glass and a piece of resin. Pressed together after rubbing, they show no electrical properties, but once separated they attract each other. A second rubbed pair reveals more. The two glass pieces repel each other, the two resin pieces repel each other, and each glass piece attracts each resin piece.

  Coulomb's law puts numbers on these pushes and pulls. The force between two particles is proportional to the product of their charges and inversely proportional to the square of the distance between them. An antiparticle has the same charge as its particle but with opposite sign.

  Even an object with zero net charge can have its charge spread unevenly, for example under an external electromagnetic field or because of bound polar molecules. Such an object is polarized. Charge from polarization is called bound charge, and charge from electrons gained or lost from outside is called free charge.

  Current is the flow of charge. In conductive metals, electrons moving in one direction make a current. By convention, the direction of conventional current in most metallic wires is opposite to the drift of the electrons. Other carriers exist too, including electron holes that act like positive particles, and ions flowing both ways in an electrolytic solution or a plasma.

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• 8. Conserved and Invariant

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  The total charge of an isolated system stays constant no matter what changes inside it. This law holds in every process known to physics. It can be derived in a local form from the gauge invariance of the wave function, and it leads to the charge-current continuity equation.

  The continuity equation says that the net current flowing out through a closed surface equals the rate at which charge inside the enclosed volume falls. Integrating that current over a span of time gives the amount of charge transferred between two moments. The same logic means that if one object loses negative charge, another gains exactly as much.

  Charge has one more distinctive property. It is a relativistic invariant, so a particle with charge q has the same charge however fast it travels. Experiments have verified this with nuclei of helium and deuterium. A helium nucleus holds two protons and two neutrons bound together and moves at high speed. Two deuterium nuclei, each a proton and a neutron, move much more slowly. Their combined charge matches that of the single helium nucleus.

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