text:
Dermatome (anatomy)
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For other uses, see Dermatome (disambiguation).
Dermatome
Dermatomes of the Upper and Lower Limbs (Modified, after Keegan, J. J., and Garrett, F. D.)
Dermatomes of the Upper Parts of the Body, displaying significant overlapping (Modified, from Fender, after Foerster)
Identifiers
Anatomical terminology
[edit on Wikidata]
A dermatome is an area of skin that is mainly supplied by a single spinal nerve.[1] There are 8 cervical nerves (C1 being an exception with no dermatome), 12 thoracic nerves, 5 lumbar nerves and 5 sacral nerves. Each of these nerves relays sensation (including pain) from a particular region of skin to the brain.
A dermatome also refers to the part of an embryonic somite.
Along the thorax and abdomen the dermatomes are like a stack of discs forming a human, each supplied by a different spinal nerve. Along the arms and the legs, the pattern is different: the dermatomes run longitudinally along the limbs. Although the general pattern is similar in all people, the precise areas of innervation are as unique to an individual as fingerprints.
A similar area innervated by peripheral nerves is called a peripheral nerve field.
Contents
[hide]
1 Clinical significance
2 Important dermatomes and anatomical landmarks
3 Additional images
4 See also
5 References
6 External links
Clinical significance[edit]
Referred pain: Conscious perception of visceral sensations map to specific regions of the body, as shown in this chart. Some sensations are felt locally, whereas others are perceived as affecting areas that are quite distant from the involved organ.
A dermatome is an area of skin supplied by sensory neurons that arise from a spinal nerve ganglion. Symptoms that follow a dermatome (e.g. like pain or a rash) may indicate a pathology that involves the related nerve root. Examples include somatic dysfunction of the spine or viral infection. Referred pain usually involves a specific, "referred" location so is not associated with a dermatome. Certain skin problems tend to orient the lesions in the dermatomal direction.
Viruses that lie dormant in nerve ganglia (e.g. varicella zoster virus, which causes both chickenpox and herpes zoster commonly shingles), often cause either pain, rash or both in a pattern defined by a dermatome (a zosteriform pattern). However, the symptoms may not appear across the entire dermatome.
Important dermatomes and anatomical landmarks[edit]
Following is a list of spinal nerves and points that are characteristically belonging to the dermatome of each nerve:[2]
Dermatomes of the lower limb (modified, from Fender, after Foerster)
C2 - At least one cm lateral to the occipital protuberance at the base of the skull. Alternately, a point at least 3 cm behind the ear.
C3 - In the supraclavicular fossa, at the midclavicular line.
C4 - Over the acromioclavicular joint.
C5 - On the lateral (radial) side of the antecubital fossa, just proximally to the elbow.
C6 - On the dorsal surface of the proximal phalanx of the thumb.
C7 - On the dorsal surface of the proximal phalanx of the middle finger.
C8 - On the dorsal surface of the proximal phalanx of the little finger.
T1 - On the medial (ulnar) side of the antecubital fossa, just proximally to the medial epicondyle of the humerus.
T2 - At the apex of the axilla.
T3 - Intersection of the midclavicular line and the third intercostal space
T4 - Intersection of the midclavicular line and the fourth intercostal space, located at the level of the nipples.
T5 - Intersection of the midclavicular line and the fifth intercostal space, horizontally located midway between the level of the nipples and the level of the xiphoid process.
T6 - Intersection of the midclavicular line and the horizontal level of the xiphoid process.
T7 - Intersection of the midclavicular line and the horizontal level at one quarter the distance between the level of the xiphoid process and the level of the umbilicus.
T8 - Intersection of the midclavicular line and the horizontal level at one half the distance between the level of the xiphoid process and the level of the umbilicus.
T9 - Intersection of the midclavicular line and the horizontal level at three quarters of the distance between the level of the xiphoid process and the level of the umbilicus.
T10 - Intersection of the midclavicular line, at the horizontal level of the umbilicus.
T11 - Intersection of the midclavicular line, at the horizontal level midway between the level of the umbilicus and the inguinal ligament.
T12 - Intersection of the midclavicular line and the midpoint of the inguinal ligament.
L1 - Midway between the key sensory points for T12 and L2.
L2 - On the anterior medial thigh, at the midpoint of a line connecting the midpoint of the inguinal ligament and the medial epicondyle of the femur.
L3 - At the medial epicondyle of the femur.
L4 - Over the medial malleolus.
L5 - On the dorsum of the foot at the third metatarsophalangeal joint.
S1 - On the lateral aspect of the calcaneus.
S2 - At the midpoint of the popliteal fossa.
S3 - Over the tuberosity of the ischium or infragluteal fold
S4 and S5 - In the perianal area, less than one cm lateral to the mucocutaneous zone
Following is a list of sensory cranial nerves:
V1 (1st division of the Trigeminal nerve) - associated with Herpes zoster ophthalmicus
V2 (2nd division of the Trigeminal nerve)
V3 (3rd division of the Trigeminal nerve)
Additional images[edit]
Diagram of segmental distribution of the cutaneous nerves of the right upper extremity.
Lower limb.
Foot.
Major dermatomes and cutaneous nerves (anterior view).
Major dermatomes and cutaneous nerves. (posterior view).
See also[edit]
Cutaneous innervation
Dorsal root
Peripheral nerve field
References[edit]
Jump up ^ "Dermatomes Anatomy". eMedicine. Retrieved 2013-10-09.
Jump up ^ "Key Sensory Points" (PDF). American Spinal Injury Association (ASIA). June 2008. Archived from the original (PDF) on 2016-03-04.
External links[edit]
Wikimedia Commons has media related to Dermatome (anatomy).
3D Dermatomes Web App, Instamedic
Hand kinesiology at the University of Kansas Medical Center
Diagram "Adult Dermatome", The New York Times
Retrieved from "https://en.wikipedia.org/w/index.php?title=Dermatome_(anatomy)&oldid=837550350"
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Alternating current
From Wikipedia, the free encyclopedia
Jump to: navigation, search
Alternating current (green curve). The horizontal axis measures time; the vertical, current or voltage.
Part of a series of articles about
Electromagnetism
Electricity
Magnetism
Electrostatics[show]
Electric charge
Static electricity
Electric field
Conductor
Insulator
Triboelectricity
Electrostatic discharge
Induction
Coulomb's law
Gauss's law
Electric flux / potential energy
Electric dipole moment
Polarization density
Magnetostatics[show]
Ampère's law
Magnetic field
Magnetization
Magnetic flux
Biot–Savart law
Magnetic dipole moment
Gauss's law for magnetism
Electrodynamics[show]
Lorentz force law
Electromagnetic induction
Faraday's law
Lenz's law
Displacement current
Magnetic potential
Maxwell's equations
Electromagnetic field
Electromagnetic pulse
Electromagnetic radiation
Maxwell tensor
Poynting vector
Liénard–Wiechert potential
Jefimenko's equations
Eddy current
London equations
Mathematical descriptions of the electromagnetic field
Electrical network[hide]
Electric current
Electric potential
Voltage
Resistance
Ohm's law
Series circuit
Parallel circuit
Direct current
Alternating current
Electromotive force
Capacitance
Inductance
Impedance
Resonant cavities
Waveguides
Covariant formulation[show]
Electromagnetic tensor
(stress–energy tensor)
Four-current
Electromagnetic four-potential
Scientists[show]
Ampère
Coulomb
Faraday
Gauss
Heaviside
Henry
Hertz
Lorentz
Maxwell
Tesla
Volta
Weber
Ørsted
v
t
e
Alternating current (AC) is an electric current which periodically reverses direction, in contrast to direct current (DC) which flows only in one direction. Alternating current is the form in which electric power is delivered to businesses and residences, and it is the form of electrical energy that consumers typically use when they plug kitchen appliances, televisions, fans and electric lamps into a wall socket. A common source of DC power is a battery cell in a flashlight. The abbreviations AC and DC are often used to mean simply alternating and direct, as when they modify current or voltage.[1][2]
The usual waveform of alternating current in most electric power circuits is a sine wave. In certain applications, different waveforms are used, such as triangular or square waves. Audio and radio signals carried on electrical wires are also examples of alternating current. These types of alternating current carry information such as sound (audio) or images (video) sometimes carried by modulation of an AC carrier signal. These currents typically alternate at higher frequencies than those used in power transmission.
Contents
[hide]
1 Transmission, distribution, and domestic power supply
2 AC power supply frequencies
3 Effects at high frequencies
3.1 Techniques for reducing AC resistance
3.2 Techniques for reducing radiation loss
3.2.1 Twisted pairs
3.2.2 Coaxial cables
3.2.3 Waveguides
3.2.4 Fiber optics
4 Mathematics of AC voltages
4.1 Power
4.2 Root mean square voltage
4.3 Example
5 Information transmission
6 History
6.1 Transformers
6.2 Pioneers
7 See also
8 References
9 Further reading
10 External links
Transmission, distribution, and domestic power supply[edit]
Main articles: Electric power transmission and Electric power distribution
A schematic representation of long distance electric power transmission. C=consumers, D=step down transformer, G=generator, I=current in the wires, Pe=power reaching the end of the transmission line, Pt=power entering the transmission line, Pw=power lost in the transmission line, R=total resistance in the wires, V=voltage at the beginning of the transmission line, U=step up transformer.
Electrical energy is distributed as alternating current because AC voltage may be increased or decreased with a transformer. This allows the power to be transmitted through power lines efficiently at high voltage, which reduces the energy lost as heat due to resistance of the wire, and transformed to a lower, safer, voltage for use. Use of a higher voltage leads to significantly more efficient transmission of power. The power losses ( P w {\displaystyle P_{\rm {w}}} ) in the wire are a product of the square of the current (I) and the resistance (R) of the wire, described by the formula
P w = I 2 R . {\displaystyle P_{\rm {w}}=I^{2}R\,.}
This means that when transmitting a fixed power on a given wire, if the current is halved (i.e. the voltage is doubled), the power loss will be four times less.
The power transmitted is equal to the product of the current and the voltage (assuming no phase difference); that is,
P t = I V . {\displaystyle P_{\rm {t}}=IV\,.}
Consequently, power transmitted at a higher voltage requires less loss-producing current than for the same power at a lower voltage. Power is often transmitted at hundreds of kilovolts, and transformed to 100 V – 240 V for domestic use.
High voltage transmission lines deliver power from electric generation plants over long distances using alternating current. These lines are located in eastern Utah.
High voltages have disadvantages, such as the increased insulation required, and generally increased difficulty in their safe handling. In a power plant, energy is generated at a convenient voltage for the design of a generator, and then stepped up to a high voltage for transmission. Near the loads, the transmission voltage is stepped down to the voltages used by equipment. Consumer voltages vary somewhat depending on the country and size of load, but generally motors and lighting are built to use up to a few hundred volts between phases. The voltage delivered to equipment such as lighting and motor loads is standardized, with an allowable range of voltage over which equipment is expected to operate. Standard power utilization voltages and percentage tolerance vary in the different mains power systems found in the world. High-voltage direct-current (HVDC) electric power transmission systems have become more viable as technology has provided efficient means of changing the voltage of DC power. Transmission with high voltage direct current was not feasible in the early days of electric power transmission, as there was then no economically viable way to step down the voltage of DC for end user applications such as lighting incandescent bulbs.
Three-phase electrical generation is very common. The simplest way is to use three separate coils in the generator stator, physically offset by an angle of 120° (one-third of a complete 360° phase) to each other. Three current waveforms are produced that are equal in magnitude and 120° out of phase to each other. If coils are added opposite to these (60° spacing), they generate the same phases with reverse polarity and so can be simply wired together. In practice, higher "pole orders" are commonly used. For example, a 12-pole machine would have 36 coils (10° spacing). The advantage is that lower rotational speeds can be used to generate the same frequency. For example, a 2-pole machine running at 3600 rpm and a 12-pole machine running at 600 rpm produce the same frequency; the lower speed is preferable for larger machines. If the load on a three-phase system is balanced equally among the phases, no current flows through the neutral point. Even in the worst-case unbalanced (linear) load, the neutral current will not exceed the highest of the phase currents. Non-linear loads (e.g. the switch-mode power supplies widely used) may require an oversized neutral bus and neutral conductor in the upstream distribution panel to handle harmonics. Harmonics can cause neutral conductor current levels to exceed that of one or all phase conductors.
For three-phase at utilization voltages a four-wire system is often used. When stepping down three-phase, a transformer with a Delta (3-wire) primary and a Star (4-wire, center-earthed) secondary is often used so there is no need for a neutral on the supply side. For smaller customers (just how small varies by country and age of the installation) only a single phase and neutral, or two phases and neutral, are taken to the property. For larger installations all three phases and neutral are taken to the main distribution panel. From the three-phase main panel, both single and three-phase circuits may lead off. Three-wire single-phase systems, with a single center-tapped transformer giving two live conductors, is a common distribution scheme for residential and small commercial buildings in North America. This arrangement is sometimes incorrectly referred to as "two phase". A similar method is used for a different reason on construction sites in the UK. Small power tools and lighting are supposed to be supplied by a local center-tapped transformer with a voltage of 55 V between each power conductor and earth. This significantly reduces the risk of electric shock in the event that one of the live conductors becomes exposed through an equipment fault whilst still allowing a reasonable voltage of 110 V between the two conductors for running the tools.
A third wire, called the bond (or earth) wire, is often connected between non-current-carrying metal enclosures and earth ground. This conductor provides protection from electric shock due to accidental contact of circuit conductors with the metal chassis of portable appliances and tools. Bonding all non-current-carrying metal parts into one complete system ensures there is always a low electrical impedance path to ground sufficient to carry any fault current for as long as it takes for the system to clear the fault. This low impedance path allows the maximum amount of fault current, causing the overcurrent protection device (breakers, fuses) to trip or burn out as quickly as possible, bringing the electrical system to a safe state. All bond wires are bonded to ground at the main service panel, as is the neutral/identified conductor if present.
AC power supply frequencies[edit]
Further information: Mains electricity by country
The frequency of the electrical system varies by country and sometimes within a country; most electric power is generated at either 50 or 60 Hertz. Some countries have a mixture of 50 Hz and 60 Hz supplies, notably electricity power transmission in Japan. A low frequency eases the design of electric motors, particularly for hoisting, crushing and rolling applications, and commutator-type traction motors for applications such as railways. However, low frequency also causes noticeable flicker in arc lamps and incandescent light bulbs. The use of lower frequencies also provided the advantage of lower impedance losses, which are proportional to frequency. The original Niagara Falls generators were built to produce 25 Hz power, as a compromise between low frequency for traction and heavy induction motors, while still allowing incandescent lighting to operate (although with noticeable flicker). Most of the 25 Hz residential and commercial customers for Niagara Falls power were converted to 60 Hz by the late 1950s, although some[which?] 25 Hz industrial customers still existed as of the start of the 21st century. 16.7 Hz power (formerly 16 2/3 Hz) is still used in some European rail systems, such as in Austria, Germany, Norway, Sweden and Switzerland. Off-shore, military, textile industry, marine, aircraft, and spacecraft applications sometimes use 400 Hz, for benefits of reduced weight of apparatus or higher motor speeds. Computer mainframe systems were often powered by 400 Hz or 415 Hz for benefits of ripple reduction while using smaller internal AC to DC conversion units.[3] In any case, the input to the M-G set is the local customary voltage and frequency, variously 200 V (Japan), 208 V, 240 V (North America), 380 V, 400 V or 415 V (Europe), and variously 50 Hz or 60 Hz.
Effects at high frequencies[edit]
Main article: Skin effect
Play media
A Tesla coil producing high-frequency current that is harmless to humans, but lights a fluorescent lamp when brought near it
A direct current flows uniformly throughout the cross-section of a uniform wire. An alternating current of any frequency is forced away from the wire's center, toward its outer surface. This is because the acceleration of an electric charge in an alternating current produces waves of electromagnetic radiation that cancel the propagation of electricity toward the center of materials with high conductivity. This phenomenon is called skin effect. At very high frequencies the current no longer flows in the wire, but effectively flows on the surface of the wire, within a thickness of a few skin depths. The skin depth is the thickness at which the current density is reduced by 63%. Even at relatively low frequencies used for power transmission (50 Hz – 60 Hz), non-uniform distribution of current still occurs in sufficiently thick conductors. For example, the skin depth of a copper conductor is approximately 8.57 mm at 60 Hz, so high current conductors are usually hollow to reduce their mass and cost. Since the current tends to flow in the periphery of conductors, the effective cross-section of the conductor is reduced. This increases the effective AC resistance of the conductor, since resistance is inversely proportional to the cross-sectional area. The AC resistance often is many times higher than the DC resistance, causing a much higher energy loss due to ohmic heating (also called I2R loss).
Techniques for reducing AC resistance[edit]
For low to medium frequencies, conductors can be divided into stranded wires, each insulated from one another, and the relative positions of individual strands specially arranged within the conductor bundle. Wire constructed using this technique is called Litz wire. This measure helps to partially mitigate skin effect by forcing more equal current throughout the total cross section of the stranded conductors. Litz wire is used for making high-Q inductors, reducing losses in flexible conductors carrying very high currents at lower frequencies, and in the windings of devices carrying higher radio frequency current (up to hundreds of kilohertz), such as switch-mode power supplies and radio frequency transformers.
Techniques for reducing radiation loss[edit]
As written above, an alternating current is made of electric charge under periodic acceleration, which causes radiation of electromagnetic waves. Energy that is radiated is lost. Depending on the frequency, different techniques are used to minimize the loss due to radiation.
Twisted pairs[edit]
At frequencies up to about 1 GHz, pairs of wires are twisted together in a cable, forming a twisted pair. This reduces losses from electromagnetic radiation and inductive coupling. A twisted pair must be used with a balanced signalling system, so that the two wires carry equal but opposite currents. Each wire in a twisted pair radiates a signal, but it is effectively cancelled by radiation from the other wire, resulting in almost no radiation loss.
Coaxial cables[edit]
Coaxial cables are commonly used at audio frequencies and above for convenience. A coaxial cable has a conductive wire inside a conductive tube, separated by a dielectric layer. The current flowing on the surface of the inner conductor is equal and opposite to the current flowing on the inner surface of the outer tube. The electromagnetic field is thus completely contained within the tube, and (ideally) no energy is lost to radiation or coupling outside the tube. Coaxial cables have acceptably small losses for frequencies up to about 5 GHz. For microwave frequencies greater than 5 GHz, the losses (due mainly to the electrical resistance of the central conductor) become too large, making waveguides a more efficient medium for transmitting energy. Coaxial cables with an air rather than solid dielectric are preferred as they transmit power with lower loss.
Waveguides[edit]
Waveguides are similar to coaxial cables, as both consist of tubes, with the biggest difference being that the waveguide has no inner conductor. Waveguides can have any arbitrary cross section, but rectangular cross sections are the most common. Because waveguides do not have an inner conductor to carry a return current, waveguides cannot deliver energy by means of an electric current, but rather by means of a guided electromagnetic field. Although surface currents do flow on the inner walls of the waveguides, those surface currents do not carry power. Power is carried by the guided electromagnetic fields. The surface currents are set up by the guided electromagnetic fields and have the effect of keeping the fields inside the waveguide and preventing leakage of the fields to the space outside the waveguide. Waveguides have dimensions comparable to the wavelength of the alternating current to be transmitted, so they are only feasible at microwave frequencies. In addition to this mechanical feasibility, electrical resistance of the non-ideal metals forming the walls of the waveguide cause dissipation of power (surface currents flowing on lossy conductors dissipate power). At higher frequencies, the power lost to this dissipation becomes unacceptably large.
Fiber optics[edit]
At frequencies greater than 200 GHz, waveguide dimensions become impractically small, and the ohmic losses in the waveguide walls become large. Instead, fiber optics, which are a form of dielectric waveguides, can be used. For such frequencies, the concepts of voltages and currents are no longer used.
Mathematics of AC voltages[edit]
A sinusoidal alternating voltage.
1 = Peak, also amplitude,
2 = Peak-to-peak,
3 = Effective value,
4 = Period
A sine wave, over one cycle (360°). The dashed line represents the root mean square (RMS) value at about 0.707
Alternating currents are accompanied (or caused) by alternating voltages. An AC voltage v can be described mathematically as a function of time by the following equation:
v ( t ) = V p e a k ⋅ sin  ( ω t ) {\displaystyle v(t)=V_{\mathrm {peak} }\cdot \sin(\omega t)} ,
where
V p e a k {\displaystyle \displaystyle V_{\rm {peak}}} is the peak voltage (unit: volt),
ω {\displaystyle \displaystyle \omega } is the angular frequency (unit: radians per second)
The angular frequency is related to the physical frequency, f {\displaystyle \displaystyle f} (unit = hertz), which represents the number of cycles per second, by the equation ω = 2 π f {\displaystyle \displaystyle \omega =2\pi f} .
t {\displaystyle \displaystyle t} is the time (unit: second).
The peak-to-peak value of an AC voltage is defined as the difference between its positive peak and its negative peak. Since the maximum value of sin  ( x ) {\displaystyle \sin(x)} is +1 and the minimum value is −1, an AC voltage swings between + V p e a k {\displaystyle +V_{\rm {peak}}} and − V p e a k {\displaystyle -V_{\rm {peak}}} . The peak-to-peak voltage, usually written as V p p {\displaystyle V_{\rm {pp}}} or V P − P {\displaystyle V_{\rm {P-P}}} , is therefore V p e a k − ( − V p e a k ) = 2 V p e a k {\displaystyle V_{\rm {peak}}-(-V_{\rm {peak}})=2V_{\rm {peak}}} .
Power[edit]
Main article: AC power
The relationship between voltage and the power delivered is
p ( t ) = v 2 ( t ) R {\displaystyle p(t)={\frac {v^{2}(t)}{R}}} where R {\displaystyle R} represents a load resistance.
Rather than using instantaneous power, p ( t ) {\displaystyle p(t)} , it is more practical to use a time averaged power (where the averaging is performed over any integer number of cycles). Therefore, AC voltage is often expressed as a root mean square (RMS) value, written as V r m s {\displaystyle V_{\rm {rms}}} , because
P t i m e a v e r a g e d = V r m s 2 R . {\displaystyle P_{\rm {time~averaged}}={\frac {{V_{\rm {rms}}}^{2}}{R}}.}
Power oscillation
v ( t ) = V p e a k sin  ( ω t ) {\displaystyle v(t)=V_{\mathrm {peak} }\sin(\omega t)}
i ( t ) = v ( t ) R = V p e a k R sin  ( ω t ) {\displaystyle i(t)={\frac {v(t)}{R}}={\frac {V_{\mathrm {peak} }}{R}}\sin(\omega t)}
P ( t ) = v ( t ) i ( t ) = ( V p e a k ) 2 R sin 2  ( ω t ) {\displaystyle P(t)=v(t)\ i(t)={\frac {(V_{\mathrm {peak} })^{2}}{R}}\sin ^{2}(\omega t)}
Root mean square voltage[edit]
Further information: RMS amplitude
For a broader coverage related to this topic, see Root mean square voltage.
Below it is assumed an AC waveform (with no DC component).
For an arbitrary periodic waveform v ( t ) {\displaystyle v(t)} of period T {\displaystyle T} :
V r m s = 1 T ∫ 0 T [ v ( t ) ] 2 d t . {\displaystyle V_{\mathrm {rms} }={\sqrt {{\frac {1}{T}}\int _{0}^{T}{[v(t)]^{2}dt}}}.}
For a sinusoidal voltage:
V r m s = 1 T ∫ 0 T [ V p k sin  ( ω t + ϕ ) ] 2 d t = V p k 1 2 T ∫ 0 T [ 1 − cos  ( 2 ω t + 2 ϕ ) ] d t = V p k 1 2 T ∫ 0 T d t = V p k 2 {\displaystyle {\begin{aligned}V_{\mathrm {rms} }&={\sqrt {{\frac {1}{T}}\int _{0}^{T}[{V_{pk}\sin(\omega t+\phi )]^{2}dt}}}\\&=V_{pk}{\sqrt {{\frac {1}{2T}}\int _{0}^{T}[{1-\cos(2\omega t+2\phi )]dt}}}\\&=V_{pk}{\sqrt {{\frac {1}{2T}}\int _{0}^{T}{dt}}}\\&={\frac {V_{pk}}{\sqrt {2}}}\end{aligned}}}
where the trigonometric identity sin 2  x = 1 − cos  2 x 2 {\displaystyle \sin ^{2}x={\frac {1-\cos 2x}{2}}} has been used and the factor 2 {\displaystyle {\sqrt {2}}} is called the crest factor, which varies for different waveforms.
For a triangle waveform centered about zero
V r m s = V p e a k 3 . {\displaystyle V_{\mathrm {rms} }={\frac {V_{\mathrm {peak} }}{\sqrt {3}}}.}
For a square waveform centered about zero
V r m s = V p e a k . {\displaystyle \displaystyle V_{\mathrm {rms} }=V_{\mathrm {peak} }.}
Example[edit]
To illustrate these concepts, consider a 230 V AC mains supply used in many countries around the world. It is so called because its root mean square value is 230 V. This means that the time-averaged power delivered is equivalent to the power delivered by a DC voltage of 230 V. To determine the peak voltage (amplitude), we can rearrange the above equation to:
V p e a k = 2 V r m s . {\displaystyle V_{\mathrm {peak} }={\sqrt {2}}\ V_{\mathrm {rms} }.}
For 230 V AC, the peak voltage V p e a k {\displaystyle V_{\mathrm {peak} }} is therefore 230 V × 2 {\displaystyle 230V\times {\sqrt {2}}} , which is about 325 V. During the course of one cycle the voltage rises from zero to 325 V, falls through zero to -325 V, and returns to zero.
Information transmission[edit]
Alternating current is used to transmit information, as in the cases of telephone and cable television. Information signals are carried over a wide range of AC frequencies. POTS telephone signals have a frequency of about 3 kHz, close to the baseband audio frequency. Cable television and other cable-transmitted information currents may alternate at frequencies of tens to thousands of megahertz. These frequencies are similar to the electromagnetic wave frequencies often used to transmit the same types of information over the air.
History[edit]
The first alternator to produce alternating current was a dynamo electric generator based on Michael Faraday's principles constructed by the French instrument maker Hippolyte Pixii in 1832.[4] Pixii later added a commutator to his device to produce the (then) more commonly used direct current. The earliest recorded practical application of alternating current is by Guillaume Duchenne, inventor and developer of electrotherapy. In 1855, he announced that AC was superior to direct current for electrotherapeutic triggering of muscle contractions.[5] Alternating current technology had first developed in Europe due to the work of Guillaume Duchenne (1850s), the Hungarian Ganz Works company (1870s), and in the 1880s: Sebastian Ziani de Ferranti, Lucien Gaulard, and Galileo Ferraris.
In 1876, Russian engineer Pavel Yablochkov invented a lighting system where sets of induction coils were installed along a high voltage AC line. Instead of changing voltage, the primary windings transferred power to the secondary windings which were connected to one or several 'electric candles' (arc lamps) of his own design,[6][7] used to keep the failure of one lamp from disabling the entire circuit.[6] In 1878, the Ganz factory, Budapest, Hungary, began manufacturing equipment for electric lighting and, by 1883, had installed over fifty systems in Austria-Hungary. Their AC systems used arc and incandescent lamps, generators, and other equipment.[8]
Transformers[edit]
Alternating current systems can use transformers to change voltage from low to high level and back, allowing generation and consumption at low voltages but transmission, possibly over great distances, at high voltage, with savings in the cost of conductors and energy losses. A bipolar open-core power transformer developed by Lucien Gaulard and John Dixon Gibbs was demonstrated in London in 1881, and attracted the interest of Westinghouse. They also exhibited the invention in Turin in 1884. However these early induction coils with open magnetic circuits are inefficient at transferring power to loads. Until about 1880, the paradigm for AC power transmission from a high voltage supply to a low voltage load was a series circuit. Open-core transformers with a ratio near 1:1 were connected with their primaries in series to allow use of a high voltage for transmission while presenting a low voltage to the lamps. The inherent flaw in this method was that turning off a single lamp (or other electric device) affected the voltage supplied to all others on the same circuit. Many adjustable transformer designs were introduced to compensate for this problematic characteristic of the series circuit, including those employing methods of adjusting the core or bypassing the magnetic flux around part of a coil.[9] The direct current systems did not have these drawbacks, giving it significant advantages over early AC systems.
Pioneers[edit]
The Hungarian "ZBD" Team (Károly Zipernowsky, Ottó Bláthy, Miksa Déri), inventors of the first high efficiency, closed-core shunt connection transformer
The prototype of the ZBD transformer on display at the Széchenyi István Memorial Exhibition, Nagycenk in Hungary
In the autumn of 1884, Károly Zipernowsky, Ottó Bláthy and Miksa Déri (ZBD), three engineers associated with the Ganz factory, determined that open-core devices were impractical, as they were incapable of reliably regulating voltage.[10] In their joint 1885 patent applications for novel transformers (later called ZBD transformers), they described two designs with closed magnetic circuits where copper windings were either a wound around iron wire ring core or b) surrounded by iron wire core.[9] In both designs, the magnetic flux linking the primary and secondary windings traveled almost entirely within the confines of the iron core, with no intentional path through air (see toroidal cores). The new transformers were 3.4 times more efficient than the open-core bipolar devices of Gaulard and Gibbs.[11] The Ganz factory in 1884 shipped the world's first five high-efficiency AC transformers.[12] This first unit had been manufactured to the following specifications: 1,400 W, 40 Hz, 120:72 V, 11.6:19.4 A, ratio 1.67:1, one-phase, shell form.[12]
The ZBD patents included two other major interrelated innovations: one concerning the use of parallel connected, instead of series connected, utilization loads, the other concerning the ability to have high turns ratio transformers such that the supply network voltage could be much higher (initially 1400 V to 2000 V) than the voltage of utilization loads (100 V initially preferred).[13][14] When employed in parallel connected electric distribution systems, closed-core transformers finally made it technically and economically feasible to provide electric power for lighting in homes, businesses and public spaces.[15][16] The other essential milestone was the introduction of 'voltage source, voltage intensive' (VSVI) systems'[17] by the invention of constant voltage generators in 1885.[18] Ottó Bláthy also invented the first AC electricity meter.[19][20][21][22]
The AC power systems was developed and adopted rapidly after 1886 due to its ability to distribute electricity efficiently over long distances, overcoming the limitations of the direct current system. In 1886, the ZBD engineers designed the world's first power station that used AC generators to power a parallel-connected common electrical network, the steam-powered Rome-Cerchi power plant.[23] The reliability of the AC technology received impetus after the Ganz Works electrified a large European metropolis: Rome in 1886.[23]
Westinghouse Early AC System 1887
(US patent 373035)
In the UK, Sebastian de Ferranti, who had been developing AC generators and transformers in London since 1882, redesigned the AC system at the Grosvenor Gallery power station in 1886 for the London Electric Supply Corporation (LESCo) including alternators of his own design and transformer designs similar to Gaulard and Gibbs.[24] In 1890 he designed their power station at Deptford[25] and converted the Grosvenor Gallery station across the Thames into an electrical substation, showing the way to integrate older plants into a universal AC supply system.[26]
In the US William Stanley, Jr. designed one of the first practical devices to transfer AC power efficiently between isolated circuits. Using pairs of coils wound on a common iron core, his design, called an induction coil, was an early (1885) transformer. Stanley also worked on engineering and adapting European designs such as the Gaulard and Gibbs transformer for US entrepreneur George Westinghouse who started building AC systems in 1886. The spread of Westinghouse and other AC systems triggered a push back in late 1887 by Edison (a proponent of direct current) who attempted to discredit alternating current as too dangerous in a public campaign called the "War of Currents". In 1888 alternating current systems gained further viability with introduction of a functional AC motor, something these systems had lacked up till then. The design, an induction motor, was independently invented by Galileo Ferraris and Nikola Tesla (with Tesla's design being licensed by Westinghouse in the US). This design was further developed into the modern practical three-phase form by Mikhail Dolivo-Dobrovolsky and Charles Eugene Lancelot Brown.[27]
The Ames Hydroelectric Generating Plant (spring of 1891) and the original Niagara Falls Adams Power Plant (August 25, 1895) were among the first hydroelectric alternating current power plants. The first long distance transmission of single-phase electricity was from a hydroelectric generating plant in Oregon at Willamette Falls which in 1890 sent power fourteen miles downriver to downtown Portland for street lighting.[28] In 1891, a second transmission system was installed in Telluride Colorado.[29] The San Antonio Canyon Generator was the third commercial single-phase hydroelectric AC power plant in the United States to provide long-distance electricity. It was completed on December 31, 1892 by Almarian William Decker to provide power to the city of Pomona, California which was 14 miles away. In 1893 he next designed the first commercial three-phase power plant in the United States using alternating current was the hydroelectric Mill Creek No. 1 Hydroelectric Plant near Redlands, California. Decker's design incorporated 10 kV three-phase transmission and established the standards for the complete system of generation, transmission and motors used today. The Jaruga Hydroelectric Power Plant in Croatia was set in operation on 28 August 1895. The two generators (42 Hz, 550 kW each) and the transformers were produced and installed by the Hungarian company Ganz. The transmission line from the power plant to the City of Šibenik was 11.5 kilometers (7.1 mi) long on wooden towers, and the municipal distribution grid 3000 V/110 V included six transforming stations. Alternating current circuit theory developed rapidly in the latter part of the 19th and early 20th century. Notable contributors to the theoretical basis of alternating current calculations include Charles Steinmetz, Oliver Heaviside, and many others.[30][31] Calculations in unbalanced three-phase systems were simplified by the symmetrical components methods discussed by Charles Legeyt Fortescue in 1918.
See also[edit]
Electronics portal
Energy portal
AC power
Direct current
Electric current
Electrical wiring
Heavy-duty power plugs
Hertz
Mains power systems
AC power plugs and sockets
Utility frequency
War of Currents
AC/DC receiver design
References[edit]
Jump up ^ N. N. Bhargava & D. C. Kulshreshtha (1983). Basic Electronics & Linear Circuits. Tata McGraw-Hill Education. p. 90. ISBN 978-0-07-451965-3.
Jump up ^ National Electric Light Association (1915). Electrical meterman's handbook. Trow Press. p. 81.
Jump up ^ The Basics of 400-Hz Power Systems
Jump up ^ Pixii Machine invented by Hippolyte Pixii, National High Magnetic Field Laboratory
Jump up ^ Licht, Sidney Herman., "History of Electrotherapy", in Therapeutic Electricity and Ultraviolet Radiation, 2nd ed., ed. Sidney Licht, New Haven: E. Licht, 1967, Pp. 1-70.
^ Jump up to: a b "Stanley Transformer". Los Alamos National Laboratory; University of Florida. Retrieved Jan 9, 2009.
Jump up ^ De Fonveille, W. (Jan 22, 1880). "Gas and Electricity in Paris". Nature. 21 (534): 283. Bibcode:1880Natur..21..282D. doi:10.1038/021282b0. Retrieved Jan 9, 2009.
Jump up ^ Hughes, Thomas P. (1993). Networks of Power: Electrification in Western Society, 1880-1930. Baltimore: The Johns Hopkins University Press. p. 96. ISBN 0-8018-2873-2. Retrieved Sep 9, 2009.
^ Jump up to: a b Uppenborn, F. J. (1889). History of the Transformer. London: E. & F. N. Spon. pp. 35–41.
Jump up ^ Hughes, p. 95
Jump up ^ Jeszenszky, Sándor. "Electrostatics and Electrodynamics at Pest University in the Mid-19th Century" (PDF). University of Pavia. Retrieved Mar 3, 2012.
^ Jump up to: a b Halacsy, A. A.; Von Fuchs, G. H. (April 1961). "Transformer Invented 75 Years Ago". IEEE Transactions of the American Institute of Electrical Engineers. 80 (3): 121–125. doi:10.1109/AIEEPAS.1961.4500994. Retrieved Feb 29, 2012.
Jump up ^ "Hungarian Inventors and Their Inventions". Institute for Developing Alternative Energy in Latin America. Archived from the original on 2012-03-22. Retrieved Mar 3, 2012.
Jump up ^ "Bláthy, Ottó Titusz". Budapest University of Technology and Economics, National Technical Information Centre and Library. Retrieved Feb 29, 2012.
Jump up ^ "Bláthy, Ottó Titusz (1860 - 1939)". Hungarian Patent Office. Retrieved Jan 29, 2004.
Jump up ^ Zipernowsky, K.; Déri, M.; Bláthy, O.T. "Induction Coil" (PDF). U.S. Patent 352 105, issued Nov. 2, 1886. Retrieved July 8, 2009.
Jump up ^ American Society for Engineering Education. Conference - 1995: Annual Conference Proceedings, Volume 2, (PAGE: 1848)
Jump up ^ Thomas Parke Hughes: Networks of Power: Electrification in Western Society, 1880-1930 (PAGE: 96)
Jump up ^ Eugenii Katz. "Blathy". People.clarkson.edu. Archived from the original on June 25, 2008. Retrieved 2009-08-04.
Jump up ^ Ricks, G.W.D. (March 1896). "Electricity Supply Meters". Journal of the Institution of Electrical Engineers. 25 (120): 57–77. doi:10.1049/jiee-1.1896.0005. Student paper read on January 24, 1896 at the Students' Meeting.
Jump up ^ The Electrician, Volume 50. 1923
Jump up ^ Official gazette of the United States Patent Office: Volume 50. (1890)
^ Jump up to: a b "Ottó Bláthy, Miksa Déri, Károly Zipernowsky". IEC Techline. Archived from the original on September 30, 2007. Retrieved Apr 16, 2010.
Jump up ^ Hughes, Thomas P. (1993). Networks of Power: Electrification in Western Society, 1880-1930. Baltimore: The Johns Hopkins University Press. p. 98. ISBN 0-8018-2873-2
Jump up ^ Ferranti Timeline Archived 2015-10-03 at the Wayback Machine. – Museum of Science and Industry (Accessed 22-02-2012)
Jump up ^ Hughes, Thomas P. (1993). Networks of Power: Electrification in Western Society, 1880-1930. Baltimore: The Johns Hopkins University Press. p. 208. ISBN 0-8018-2873-2
Jump up ^ Arnold Heertje, Mark Perlman Evolving Technology and Market Structure: Studies in Schumpeterian Economics, page 138
Jump up ^ "Electric Transmission of Power". General Electric Review. XVIII. 1915.
Jump up ^ "Electric Transmission of Power". General Electric. XVIII. 1915.
Jump up ^ I. Grattan-Guinness, History and Philosophy of the Mathematical Sciences - 2003, Page 1229
Jump up ^ Jeff Suzuki, Mathematics in Historical Context - 2009, page 329
Further reading[edit]
Willam A. Meyers, History and Reflections on the Way Things Were: Mill Creek Power Plant – Making History with AC, IEEE Power Engineering Review, February 1997, pages 22–24
External links[edit]
Wikimedia Commons has media related to Alternating current.
"AC/DC: What's the Difference?". Edison's Miracle of Light, American Experience. (PBS)
"AC/DC: Inside the AC Generator". Edison's Miracle of Light, American Experience. (PBS)
Kuphaldt, Tony R., "Lessons In Electric Circuits : Volume II - AC". March 8, 2003. (Design Science License)
Nave, C. R., "Alternating Current Circuits Concepts". HyperPhysics.
"Alternating Current (AC)". Magnetic Particle Inspection, Nondestructive Testing Encyclopedia.
"Alternating current". Analog Process Control Services.
Hiob, Eric, "An Application of Trigonometry and Vectors to Alternating Current". British Columbia Institute of Technology, 2004.
"Introduction to alternating current and transformers". Integrated Publishing.
Chan. Keelin, "Alternating current Tools". JC Physics, 2002.
Williams, Trip "Kingpin", "Understanding Alternating Current, Some more power concepts".
"Table of Voltage, Frequency, TV Broadcasting system, Radio Broadcasting, by Country".
Professor Mark Csele's tour of the 25 Hz Rankine generating station
50/60 hertz information
AC circuits Animations and explanations of vector (phasor) representation of RLC circuits
Blalock, Thomas J., "The Frequency Changer Era: Interconnecting Systems of Varying Cycles". The history of various frequencies and interconversion schemes in the US at the beginning of the 20th century
(in Italian) Generating an AC voltage. Interactive.
AC Power History and Timeline
Authority control
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