Everything about Electrical totally explained
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 recognizable 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 modern industrial society is, and for the foreseeable future can be expected to remain, the use of electrical power.
History
That certain objects such as rods of
amber could be rubbed with cat's fur and attract light objects like feathers was known to ancient cultures around the Mediterranean.
Thales of Miletos conducted a series of experiments into
static electricity around 600 BC, from which he believed that friction rendered amber
magnetic, in contrast to minerals such as
magnetite, which needed no rubbing. 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
Parthians had some knowledge of
electroplating, based on the 1936 discovery of the
Baghdad Battery, which resembles a
galvanic cell, though this claim lacks evidence supporting the exact nature of the artifact, and whether it was electrical in nature.
Several ancient writers, such as
Pliny the Elder and
Scribonius Largus, attested to the numbing effect of
electric shocks delivered by
catfish and
torpedo rays, and knew that such shocks could travel along conducting objects. Patients suffering from ailments such as
gout or
headache were directed to touch electric fish in the hope that the powerful jolt might cure them.
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 electricity and magnetism, distinguishing the
lodestone effect from static electricity produced by rubbing amber. This association gave rise to the English words "electric" and "electricity", which made their first appearance in print in
Thomas Browne's
Pseudodoxia Epidemica of 1646.
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, selling his possessions to fund his work. In June 1752 he's reputed to have attached a metal key to the bottom of a dampened kite string and flown the kite in a storm-threatened sky. He observed a succession of sparks jumping from the key to the back of his hand, showing that
lightning was indeed electrical in nature.
In 1791
Luigi Galvani published his discovery of
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 more reliable source of electrical energy than the
electrostatic machines previously used.
Concepts
Electric charge
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. Within the system, charge may be transferred between bodies, either by direct contact, or by passing along a conducting material, such as a wire. 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.
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. 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's 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. These phenomena were investigated by
Charles-Augustin de Coulomb in the late eighteenth century, who deduced that charge manifests itself in two opposing forms, leading to the well-known axiom:
like-charged objects repel and opposite-charged objects attract. 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 10
42 times that of the
gravitational attraction pulling them together.
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. The amount of charge is usually given the symbol
Q and expressed in
coulombs; each electron carries the same charge of approximately −1.6022×10
−19 coulomb. The proton has a charge that's 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.
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. 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
electrical conduction, and its nature varies with that of the charged particles and the material through which they're 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 average
drift velocity only fractions of a millimetre per second, the
electric field that drives them itself propagates at close to the
speed of light, enabling electrical signals to pass rapidly along wires.
Current causes several observable effects, which historically were the means of recognising its presence. That water could be decomposed by the current from a voltaic pile was discovered by
Nicholson and
Carlisle in 1800, a process now known as
electrolysis. Their work was greatly expanded upon by
Michael Faraday in 1833. Current through a
resistance causes localised heating, an effect
James Prescott Joule studied mathematically in 1840. He had discovered
electromagnetism, a fundamental interaction between electricity and magnetics.
In engineering or household applications, current is often described as being either
direct current (DC) or
alternating current (AC). These terms refer to how the current varies in time. Direct current, as produced by example from a
battery and required by most
electronic devices, is a unidirectional flow from the positive part of a circuit to the negative. If, as is most common, this flow is carried by electrons, that'll be travelling in the opposite direction. Alternating current is any current that reverses direction repeatedly; almost always this takes the form of a
sinusoidal wave. Alternating current thus pulses back and forth within a conductor without the charge moving any net distance over time. The time-averaged value of an alternating current is zero, but it delivers energy in first one direction, and then the reverse. Alternating current is affected by electrical properties that are not observed under
steady state direct current, such as
inductance and
capacitance. These properties however can become important when circuitry is subjected to
transients, such as when first energised.
Electric field
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, extends towards infinity and shows an inverse square relationship with distance. 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. 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's a
vector field. 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; they're however an imaginary concept with no physical existence, and the field permeates all the intervening space between the lines.
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
electric arc causes flashover between the charged parts. 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. 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.
The field strength is greatly affected by nearby conducting objects, and it's particularly intense when it's 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.
Electric potential
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 test 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. 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's
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.
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's 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's 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. Equally, electric potential may be seen as analogous to
height: just as a released object will fall through a difference in heights caused by a gravitational field, so a charge will 'fall' across the voltage caused by an electric field.
The electric field was formally defined as the force exerted per unit charge, but the concept of potential allows for a more useful and equivalent definition: the electric field is the local gradient of the electric potential. Usually expressed in volts per metre, the vector direction of the field is the line of greatest gradient of potential.
Electromagnetism
Ørsted's discovery in 1821 that a
magnetic field existed around all sides of a wire carrying an electric current indicated that there was a direct relationship between electricity and magnetism. Moreover, the interaction seemed different from gravitational and electrostatic forces, the two forces of nature then known. The force on the compass needle didn't direct it to or away from the current-carrying wire, but acted at right angles to it.
Ørsted didn't fully understand his discovery, but he observed the effect was reciprocal: a current exerts a force on a magnet, and a magnetic field exerts a force on a current. The phenomenon was further investigated by
Ampère, who discovered that two parallel current carrying wires exerted a force upon each other: two wires conducting currents in the same direction are attracted to each other, while wires containing current flowing in opposite directions are forced apart. The interaction is mediated by the magnetic field each current produces and forms the basis for the international
definition of the ampere.
Experimentation by Faraday in 1831 revealed that a wire moving perpendicular to a magnetic field developed a potential difference between its ends. Further analysis of this process, known as
electromagnetic induction, enabled him to state the principal, now known as
Faraday's law of induction, that the potential difference induced in a closed circuit is proportional to the rate of change of
magnetic flux through the loop. Exploitation of this discovery enabled him to invent the first
electrical generator in 1831, in which he converted the mechanical energy of a rotating copper disc to electrical energy. Such a phenomenon has the properties of a
wave, and is naturally referred to as an
electromagnetic wave. Electromagnetic waves were analysed theoretically by
James Clerk Maxwell in 1864. Maxwell discovered a set of equations that could unambiguously describe the interrelationship between electric field, magnetic field, electric charge, and electric current. He could moreover prove that such a wave would necessarily travel at the
speed of light, and thus light itself was a form of electromagnetic radiation.
Maxwell's Laws, which unify light, fields, and charge are one of the great milestones of theoretical physics.
The
resistor is perhaps the simplest of passive circuit elements: as its name suggests, it
resists the flow of current through it, dissipating its energy as heat.
Ohm's law is a basic law of
circuit theory, stating that the current passing through a resistance is directly proportional to the potential difference across it. The
ohm, the unit of resistance, was named in honour of Georg Ohm, and is symbolised by the Greek letter Ω. 1 Ω is the resistance that will produce a potential difference of one volt in response to a current of one amp. It wasn't 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 invention in the late nineteenth century of the
transformer meant that electricity could be generated at centralised
power stations, benefiting from
economies of scale, and be
transmitted across countries with increasing efficiency. Since electrical energy can't easily be stored in quantities large enough to meet demands on a national scale, at all times exactly as much must be produced as is required. a rate of growth that's now being experienced by emerging economies such as those of India or China. Historically, the growth rate for electricity demand has outstripped that for other forms of energy, such as
coal.
Environmental concerns with electricity generation have led to an increased focus on generation from
renewable sources, in particular from
wind- and
hydropower. While debate can be expected to continue over the environmental impact of different means of electricity production, its final form is relatively clean.
Uses
Electricity is an extremely flexible form of energy, and it may be adapted to a huge, and growing, number of uses. The invention of a practical
incandescent light bulb in the 1870s led to
lighting becoming one of the first publicly available applications of electrical power. Although electrification brought with it its own dangers, replacing the naked flames of gas lighting greatly reduced fire hazards within homes and factories. Public utilities were set up in many cities targeting the burgeoning market for electrical lighting.
The
Joule heating effect employed in the light bulb also sees more direct use in
electric heating. While this is versatile and controllable, it can be seen as wasteful, since most electrical generation has already required the production of heat at a power station. A number of countries, such as Denmark, have issued legislation restricting or banning the use of electric heating in new buildings. Electricity is however a highly practical energy source for
refrigeration, with
air conditioning representing a growing sector for electricity demand, the effects of which electricity utilities are increasingly obliged to accommodate.
Electricity is used within
telecommunications, and indeed the
electrical telegraph, demonstrated commercially in 1837 by
Cooke and
Wheatstone, was one of its earliest applications. With the construction of first
intercontinental, and then
transatlantic, telegraph systems in the 1860s, electricity had enabled communications in minutes across the globe.
Optical fibre and
satellite communication technology have taken a share of the market for communications systems, but electricity can be expected to remain an essential part of the process.
The effects of electromagnetism are most visibly employed in the
electric motor, which provides a clean and efficient means of motive power. A stationary motor such as a
winch is easily provided with a supply of power, but a motor that moves with its application, such as an
electric vehicle, is obliged to either carry along a power source such as a battery, or by collecting current from a sliding contact such as a
pantograph, placing restrictions on its range or performance.
Electronic devices make use of the
transistor, perhaps one of the most important inventions of the twentieth century, and a fundamental building block of all modern circuitry. A modern
integrated circuit may contain several billion miniaturised transistors in a region only a few centimetres square.
Electricity and the natural world
Physiological effects
A voltage applied to a human body causes an electric current to flow through the tissues, and although the relationship is non-linear, the greater the voltage, the greater the current. The threshold for perception varies with the supply frequency and with the path of the current, but is about 1 mA for mains-frequency electricity. If the current is sufficiently high, it'll cause muscle contraction,
fibrillation of the heart, and
tissue burns.
Electrical phenomena in nature
Electricity is by no means a purely human invention, and may be observed in several forms in nature, a prominent manifestation of which is
lightning. The
Earth's magnetic field is thought to arise from a
natural dynamo of circulating currents in the planet's core. Certain crystals, such as
quartz, or even
sugarcane, generate a potential difference across their faces when subjected to external pressure. This phenomenon is known as
piezoelectricity, from the
Greek piezein, meaning to press, and was discovered in 1880 by
Pierre and
Jacques Curie. The effect is reciprocal, and when a piezoelectric material is subjected to an electric field, a small change in physical dimensions take place. while others, termed
electrogenic, are able to generate voltages themselves to serve as a predatory or defensive weapon. (Because of this principle, an electric shock can induce temporary or permanent
paralysis by "overloading" the nervous system.) They are also responsible for coordinating activities in certain plants.
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