Electricity

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Lightning is one of the most prominent effects of electricity
Lightning is one of the most prominent effects of electricity

Electricity (from New Latin ēlectricus, "amber-like") is a general term that encompasses a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognisable phenomena such as lightning and static electricity, but in addition, less familiar concepts such as the electromagnetic field and electromagnetic induction.

In general usage, the word 'electricity' is adequate to refer to a number of physical effects. However, in scientific usage, the term is vague, and these related, but distinct, concepts are better identified by more precise terms:

Electricity has been studied since antiquity, though scientific advances were not forthcoming until the seventeenth and eighteenth centuries. It would remain however until the late nineteenth century that engineers were able to put electricity to industrial and residential use, a time which witnessed a rapid expansion in the development of electrical technology. Electricity's extraordinary versatility as a source of energy means it can be put to an almost limitless set of applications which include transport, heating, lighting, communications, and computation. The backbone of 21st-century industrial society is, and for the foreseeable future can be expected to remain, the use of electrical power.

Contents

History of electricity

Main article: History of electricity
See also: Etymology of electricity

That certain objects such as rods of amber could be rubbed with cat's fur and attract light objects like feathers was known to the ancient Greeks, Phoenicians, Parthians and Mesopotamians. Thales of Miletos conducted a series of experiments in 600 BC, from which he believed that friction rendered amber magnetic, in contrast to minerals such as magnetite, which needed no rubbing.[1][2] Thales was incorrect in believing the attraction was due to a magnetic effect, but later science would prove a link between magnetism and electricity.

A controversial claim is made that the Mesopotamians had some knowledge of electroplating, based on the 1936 discovery of the Baghdad Battery, which resembles a galvanic cell, though this claims lacks evidence supporting the exact nature of the artefact, and whether it was electrical in nature.[3]

Bejamin Franklin conducted extensive research on electricity in the 18th century
Bejamin Franklin conducted extensive research on electricity in the 18th century

Electricity would remain little more than an intellectual curiosity for over two millennia until 1600, when the English physician William Gilbert made a careful study of magnetism, distinguishing the lodestone effect from the static electricity produced by rubbing amber.[1] He coined the New Latin word electricus ("of amber" or "like amber", from ηλεκτρον [elektron], the Greek word for "amber") to refer to the property of attracting small objects after being rubbed.[4] This association gave rise to the English words "electric" and "electricity", which made their first appearance in print in Sir Thomas Browne's Pseudodoxia Epidemica of 1646.[5]

Further work was conducted by Otto von Guericke, Robert Boyle, Stephen Gray and C. F. du Fay. In the 18th century, Benjamin Franklin conducted extensive research in electricity to develop his theories on the relationship between lightning and static electricity. In an experiment of June 1752, he attached a metal key to the bottom of a dampened kite string and flew the kite in a storm-threatened sky.[6] He observed a succession of sparks jumping from the key to the back of his hand that showed him that lightning was indeed electrical in nature.[7] This famous experiment lit the interest of later scientists whose work provided the basis for modern electrical technology. In 1783 Luigi Galvani discovered bioelectricity, demonstrating that electricity was the medium by which nerve cells passed signals to the muscles. Alessandro Volta's battery, or voltaic pile, of 1800, made from alternating layers of zinc and copper, provided scientists with a reliable source of electrical energy. André-Marie Ampère discovered the relationship between electricity and magnetism in 1820; Michael Faraday invented the electric motor in 1821, and Georg Ohm mathematically analysed the electrical circuit in 1827.

While it had been the early nineteenth century that had seen rapid progress in electrical science, the late nineteenth century would see the greatest progress in electrical engineering. Through such giants as Nikola Tesla, Thomas Edison, George Westinghouse, Werner von Siemens, Alexander Graham Bell and Lord Kelvin, electricity was turned from a scientific curiosity into an essential tool for modern life, becoming a driving force for the Second Industrial Revolution.

Concepts in electricity

Electric charge

Main article: Electric charge
See also: electron, proton, and ion

Electric charge is a property of certain subatomic particles, which gives rise to and interacts with, the electromagnetic force, one of the four fundamental forces of nature. Charge originates in the atom, in which its most familiar carriers are the electron and proton. It is a conserved quantity, that is, the net charge within an isolated system will always remain constant regardless of any changes taking place within that system.[8] Within the system, charge may be transferred between bodies, either by direct contact, or by passing along a conducting material, such as a wire.[9] The informal term static electricity refers to the net presence (or 'imbalance') of charge on a body, usually caused when dissimilar materials are rubbed together, transferring charge from one to the other.

Charge on a gold-leaf electroscope causes the leaves to visibly repel each other
Charge on a gold-leaf electroscope causes the leaves to visibly repel each other

The presence of charge gives rise to the electromagnetic force: charges exert a force on each other, an effect that was known, though not understood, in antiquity.[10] A lightweight ball suspended from a string can be charged by touching it with a glass rod that has itself been charged by rubbing with a cloth. If a similar ball is charged by the same glass rod, it is found to repel the first: the charge acts to force the two balls apart. Two balls that are charged with an rubbed amber rod also repel each other. However, if one ball is charged by the glass rod, and the other by an amber rod, the two balls are found to attract each other. Clearly, charge manifests itself in two opposing forms, leading to the well-known axiom: like-charged objects repel and opposite-charged objects attract.[10]

The force acts on the charged particles themselves, hence charge has a tendency to spread itself as evenly as possible over a conducting surface. The magnitude of the electromagnetic force, whether attractive or repulsive, is given by Coulomb's Law, which relates the force to the product of the charges and has an inverse square relation to the distance between them.[11][12] The electromagnetic force is very strong, second only in strength to the strong interaction, but unlike that force it operates over all distances. In comparison with the much weaker gravitational force, the electromagnetic force pushing two electrons apart is 1042 times that of the gravitational attraction pulling them together.[13]

The charge on electrons and protons is opposite in sign, hence an amount of charge may be expressed as being either negative or positive. By convention, the charge carried by electrons is deemed negative, and that by protons positive, a custom that originated with the work of Benjamin Franklin.[14] The amount of charge is usually given the symbol Q and expressed in coulombs;[15] each electron carries the same charge of approximately −1.6022×10−19 coulomb. The proton has a charge that is equal and opposite, and thus +1.6022×10−19  coulomb. Charge is possessed not just by matter, but also by antimatter, each antiparticle bearing an equal and opposite charge to its corresponding particle.[16]

Charge can be measured by a number of means, an early instrument being the gold-leaf electroscope, which although still in use for classroom demonstrations, has been superseded by the electronic electrometer.[9]

Electric current

Main article: Current (electricity)
A basic electric circuit. The voltage source V on the left drives a current I around the circuit, delivering electrical energy into the resistance R.
A basic electric circuit. The voltage source V on the left drives a current I around the circuit, delivering electrical energy into the resistance R.

The movement of electric charge is known as an electric current, the intensity of which is usually measured in amperes. Current can consist of any moving charged particles; most commonly these are electrons, but any charge in motion constitutes a current.

By historical convention, a positive current is defined as having the same direction of flow as any positive charge it contains, or to flow from the most positive part of a circuit to the most negative part. Current defined in this manner is called conventional current. The motion of negatively-charged electrons around an electric circuit, one of the most familiar forms of current, is thus deemed positive in the opposite direction to that of the electrons.[17] However, depending on the conditions, an electric current can consist of a flow of charged particles in either direction, or even in both directions at once. The positive-to-negative convention is widely used to simplify this situation. If another definition is used—for example, "electron current"—it needs to be explicitly stated.

The process by which electric current passes through a material is termed electric conduction, and its nature varies with that of the charged particles and the material through which they are travelling. Examples of electric currents include metallic conduction, where electrons flow through a conductor such as metal, and electrolysis, where ions (charged atoms) flow through liquids. While the particles themselves can move quite slowly, sometimes with a drift velocity only fractions of a millimetre per second,[9] the electric field that drives them itself propagates at close to the speed of light, enabling electrical signals to pass rapidly along wires.[18]

Ohm's law is an important relationship describing the behaviour of electric currents, relating them to voltage.

Electric field

Main article: Electric field
See also: Electrostatics

The concept of the electric field was introduced by Michael Faraday. An electric field is created by a charged body in the space that surrounds it, and results in a force exerted on any other charges placed within the field. The electric field acts between two charges in a similar manner to the way that the gravitational field acts between two masses, and like it, is infinite in extent and shows an inverse square relationship with distance. However, there is an important difference. Gravity can only act in attraction, drawing two masses together, while the electric field can result in either attraction or repulsion. Since large bodies such as planets generally carry no net charge, the electric field at a distance is usually zero. Thus gravity is the dominant force at distance in the universe, despite being much the weaker.[13]

Field lines emanating from a positive charge above a plane conductor
Field lines emanating from a positive charge above a plane conductor

An electric field generally varies in space,[19] and its strength at any one point is defined as the force (per unit charge) that would be felt by a stationary, negligible charge if placed at that point.[20] The conceptual charge, termed a test charge, must be vanishingly small to prevent its own electric field disturbing the main field and must also be stationary to prevent the effect of magnetic fields. As the electric field is defined in terms of force, and force is a vector, so it follows that an electric field is also a vector, having both magnitude and direction. Specifically, it is a vector field.[20]

The study of electric fields created by stationary charges is called electrostatics. The field may be visualised by a set of lines whose direction at any point is the same as that of the field. This concept was introduced by Faraday, whose term 'lines of force' still sometimes sees use. The field lines are the paths that a point positive charge would seek to make as it was forced to move within the field. Field lines emanating from stationary charges have several key properties: first, that they originate at positive charges and terminate at negative charges; second, that they must enter any good conductor at right angles, and third, that they may never cross nor close in on themselves.[21]

The principals of electrostatics are important when designing items of high-voltage equipment. There is a finite limit to the electric field strength that may withstood by any medium. Beyond this point, electrical breakdown occurs and an electrical arc causes flashover between the charged parts.[22] Air, for example, tends to arc at electric field strengths which exceed 30 kV per centimetre across small gaps. Over larger gaps, its breakdown strength is weaker, perhaps 1 kV per centimetre.[22] The most visible natural occurrence of this is lightning, caused when charge becomes separated in the clouds by rising columns of air, and raises the electric field in the air to greater than it can withstand. The voltage of a large lightning cloud may be as high as 100 MV and have discharge energies as great as 250 kWh.[23]

The field strength is greatly affected by nearby conducting objects, and it is particularly intense when it is forced to curve around sharply pointed objects. This principal is exploited in the lightning conductor, the sharp spike of which acts to encourage the lightning stroke to develop there, rather than to the building it serves to protect.[24]

Electric potential

Main article: Electric potential
See also: Voltage

The concept of electric potential is closely linked to that of the electric field. A small charge placed within an electric field experiences a force, and to have brought that charge to that point against the force requires work. The electric potential at any point is defined as the energy required to bring a unit charge from an infinite distance slowly to that point. It is usually measured in volts, and one volt is the potential for which one joule of work must be expended to bring a charge of one coulomb from infinity.[25] This definition of potential, while formal, has little practical application, and a more useful concept is that of electric potential difference, and is the energy required to move a unit charge between two specified points. An electric field has the special property that it is conservative, which means that the path taken by the test charge is irrelevant: all paths between two specified points expend the same energy, and thus a unique value for potential difference may be stated.[25] The volt is so strongly identified as the unit of choice for measurement and description of electric potential difference that the term voltage sees greater common usage.

A 0–300 volts voltmeter
A 0–300 volts voltmeter

Electric potential is a scalar quantity, that is, it has only magnitude and not direction. It may be viewed as analogous to temperature: as there is a certain temperature at every point in space, and the temperature gradient indicates the direction and magnitude of the driving force behind heat flow, similarly, there is an electric potential at every point in space, and its gradient, or field strength, indicates the direction and magnitude of the driving force behind charge movement.

For practical purposes, it is useful to define a common reference point to which potentials may be expressed and compared. While this could be at infinity, a much more useful reference is the Earth itself, which is assumed to be at the same potential everywhere. This reference point naturally takes the name earth or ground. Earth is assumed to be an infinite source of equal amounts of positive and negative charge, and is therefore electrically uncharged and unchargeable.

Electric energy

Main article: Electric energy

Electric power

Main article: Electric power

A direct current (DC) is a unidirectional flow, while an alternating current (AC) reverses direction repeatedly. The time average of an alternating current is zero, but its energy capability (RMS value) is not zero.

Production and uses of electricity

Main article: Electricity generation

Thales' experiments with amber rods were the first studies into the production of electrical energy. While this method, now known as the triboelectric effect, is capable of lifting light objects and even generating sparks, it is extremely inefficient. It was not until the invention of the voltaic pile in the eighteenth century that a viable source of electricity became available. The voltaic pile, and its modern descendant, the electrical battery, store energy chemically and make it available on demand in the form of electrical energy. The battery is a versatile and very common power source which is well-suited to many consumer applications, but it is incapable of supplying large quantities of energy. For this purpose electrical energy must be generated and transmitted in bulk.

Electricity and the natural world

Effects of electricity on living organisms

Electrical phenomena in nature

References

  1. ^ a b Stewart, Joseph (2001), Intermediate Electromagnetic Theory, World Scientific, p. 50, ISBN 9-8102-4471-1
  2. ^ Simpson, Brian (2003), Electrical Stimulation and the Relief of Pain, Elsevier Health Sciences, pp. 6–7, ISBN 0-4445-1258-6
  3. ^ Corder, Gregory, "Using an Unconventional History of the Battery to engage students and explore the importance of evidence", Virginia Journal of Science Education 1
  4. ^ Baigrie, Brian (2006), Electricity and Magnetism: A Historical Perspective, Greenwood Press, pp. 7–8, ISBN 0-3133-3358-0
  5. ^ Chalmers, Gordon (1937), "The Lodestone and the Understanding of Matter in Seventeenth Century England", Philosophy of Science 4 (1): 75–95
  6. ^ Socket to me! How electricity came to be. (2007). IEEE Virtual History Museum.
  7. ^ Uman, Martin (1987). All About Lightning. Dover Publications. ISBN 048625237X. 
  8. ^ Trefil, James (2003), The Nature of Science: An A-Z Guide to the Laws and Principles Governing Our Universe, Houghton Mifflin Books, p. 74, ISBN 0-6183-1938-7
  9. ^ a b c Duffin, W.J. (1980), Electricity and Magnetism, 3rd edition, McGraw-Hill, pp. 2–5, ISBN 07084111X
  10. ^ a b Sears, et al., Francis (1982), University Physics, Sixth Edition, Addison Wesley, p. 457, ISBN 0-2010-7199-1
  11. ^ "The repulsive force between two small spheres charged with the same type of electricity is inversely proportional to the square of the distance between the centres of the two spheres." Charles Augustin Coulomb, Histoire de l'Academie Royal des Sciences, Paris 1785.
  12. ^ Duffin, W.J. (1980), Electricity and Magnetism, 3rd edition, McGraw-Hill, p. 35, ISBN 07084111X
  13. ^ a b Hawking, Stephen (1988), A Brief History of Time, Bantam Press, p. 77, ISBN 0-553-17521-1
  14. ^ Shectman, Jonathan (2003), Groundbreaking Scientific Experiments, Inventions, and Discoveries of the 18th Century, Greenwood Press, pp. 87–91, ISBN 0-3133-2015-2
  15. ^ Sewell, Tyson (1902), The Elements of Electrical Engineering, Lockwood, p. 18. The Q originally stood for 'quantity of electricity', the term 'electricity' now more commonly expressed as 'charge'.
  16. ^ Close, Frank (2007), The New Cosmic Onion: Quarks and the Nature of the Universe, CRC Press, p. 51, ISBN 1-5848-8798-2
  17. ^ Ward, Robert (1960), Introduction to Electrical Engineering, Prentice-Hall, p. 18
  18. ^ Solymar, L. (1984), Lectures on electromagnetic theory, Oxford University Press, p. 140, ISBN 0-19-856169-5
  19. ^ Almost all electric fields vary in space. An exception is the electric field surrounding a planar conductor of infinite extent, the field of which is uniform.
  20. ^ a b Sears, et al., Francis (1982), University Physics, Sixth Edition, Addison Wesley, pp. 469–470, ISBN 0-2010-7199-1
  21. ^ Sears, et al., Francis (1982), University Physics, Sixth Edition, Addison Wesley, p. 479, ISBN 0-2010-7199-1
  22. ^ a b Naidu, M.S. & Kamataru, V. (1982), High Voltage Engineering, Tata McGraw-Hill, p. 2, ISBN 0-07-451786-4
  23. ^ Naidu, M.S. & Kamataru, V. (1982), High Voltage Engineering, Tata McGraw-Hill, pp. 201–202, ISBN 0-07-451786-4
  24. ^ Rickards, Teresa (1985), Thesaurus of Physics, HarperCollins, p. 167, ISBN 0-0601-5214-1
  25. ^ a b Sears, et al., Francis (1982), University Physics, Sixth Edition, Addison Wesley, pp. 494–498, ISBN 0-2010-7199-1
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