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My name is Armand and I am studying Biological Sciences and Industrial and Labor Relations at Harads / Sweden.<br><br>My web blog; [http://www.allmusic.com/artist/arthur-falcone-mn0002077183/discography Arthur Falcone]
{{electromagnetism|cTopic=[[Electrical network|Electrical Network]]}}
 
The '''electrical resistance''' of an [[electrical conductor]] is the opposition to the passage of an [[electric current]] through that conductor; the inverse quantity is '''electrical conductance''', the ease at which an electric current passes.  Electrical resistance shares some conceptual parallels with the mechanical notion of [[friction]]. The [[International System of Units|SI]] unit of electrical resistance is the [[ohm]] ([[Omega|Ω]]), while electrical conductance is measured in [[siemens (unit)|siemens]] (S).
 
An object of uniform cross section has a resistance proportional to its [[resistivity]] and length and inversely proportional to its cross-sectional area. All materials show some resistance, except for [[superconductor]]s, which have a resistance of zero.
 
The resistance (R) of an object is defined as the ratio of [[voltage]] across it (''V'') to [[Electric current|current]] through it (''I''), while the conductance (G) is the inverse:
:<math>R = {V\over I}, \qquad G = {I\over V}, \qquad G = \frac{1}{R}</math>
 
For a wide variety of materials and conditions, ''V'' and ''I'' are directly proportional to each other, and therefore ''R'' and ''G'' are [[Constant (mathematics)|constant]] (although they can depend on other factors like temperature or strain). This proportionality is called [[Ohm's law]], and materials that satisfy it are called "Ohmic" materials.
 
In other cases, such as a [[diode]] or [[battery (electricity)|battery]], ''V'' and ''I'' are ''not'' directly proportional, or in other words the [[I–V curve|''I–V'' curve]] is not a straight line through the origin, and Ohm's law does not hold. In this case, resistance and conductance are less useful concepts, and more difficult to define. The ratio V/I is sometimes still useful, and is referred to as a "chordal resistance" or "static resistance",<ref name=brown>{{cite book | title = Engineering System Dynamics | author = Forbes T. Brown | publisher = CRC Press | year = 2006 | isbn = 978-0-8493-9648-9 | page = 43 | url = http://books.google.com/books?id=UzqX4j9VZWcC&pg=PA43&dq=%22chordal+resistance%22&as_brr=3&ei=Z0x0Se2yNZHGlQSpjMyvDg }}</ref><ref name=kaiser>{{cite book | title = Electromagnetic Compatibility Handbook | author = Kenneth L. Kaiser | publisher = CRC Press | year = 2004 | isbn = 978-0-8493-2087-3 | pages = 13–52 | url = http://books.google.com/books?id=nZzOAsroBIEC&pg=PT1031&dq=%22static+resistance%22+%22dynamic+resistance%22+nonlinear&lr=&as_brr=3&ei=Kk50Ser1MJeOkAS9wNTwDg#PPT1031,M1 }}</ref> as it corresponds to the inverse slope of a chord between the origin and an [[I–V curve|''I–V'' curve]]. In other situations, the [[derivative]] <math> \frac{dV}{dI} \,\!</math> may be most useful; this is called the "differential resistance".
 
==Introduction==
 
[[File:ResistanceHydraulicAnalogy.svg|thumb|The [[hydraulic analogy]] compares electric current flowing through circuits to water flowing through pipes. When a pipe (left) is filled with hair (right), it takes a larger pressure to achieve the same flow of water. Pushing electric current through a large resistance is like pushing water through a pipe clogged with hair: It requires a larger push ([[electromotive force]]) to drive the same flow ([[electric current]]).]]
In the [[hydraulic analogy]], current flowing through a wire (or [[resistor]]) is like water flowing through a pipe, and the [[voltage drop]] across the wire is like the [[pressure drop]] which pushes water through the pipe. Conductance is proportional to how much flow occurs for a given pressure, and resistance is proportional to how much pressure is required to achieve a given flow. (Conductance and resistance are [[Multiplicative inverse|reciprocals]].)
 
The [[voltage drop|voltage ''drop'']] (i.e., difference in voltage between one side of the resistor and the other), not the [[voltage]] itself, provides the driving force pushing current through a resistor. In hydraulics, it is similar: The pressure ''difference'' between two sides of a pipe, not the pressure itself, determines the flow through it. For example, there may be a large water pressure above the pipe, which tries to push water down through the pipe. But there may be an equally large water pressure below the pipe, which tries to push water back up through the pipe. If these pressures are equal, no water will flow. (In the image at right, the water pressure below the pipe is zero.)
 
The resistance and conductance of a wire, resistor, or other element is primarily determined by two factors: geometry (shape) and materials.
 
Geometry is important because it is more difficult to push water through a long, narrow pipe than a wide, short pipe. In the same way, a long, thin copper wire has higher resistance (lower conductance) than a short, thick copper wire.
 
Materials are important as well. A pipe filled with hair restricts the flow of water more than a clean pipe of the same shape and size. In a similar way, [[electron]]s can flow freely and easily through a [[copper]] wire, but cannot as easily flow through a [[steel]] wire of the same shape and size, and they essentially cannot flow at all through an [[insulator (electrical)|insulator]] like [[rubber]], regardless of its shape. The difference between, copper, steel, and rubber is related to their microscopic structure and [[electron configuration]], and is quantified by a property called [[resistivity]].
 
==Conductors and resistors==
[[File:65-ohm resistor.jpg|thumb|250px|A 65 Ω [[resistor]], as identified by its [[electronic color code]] (blue–green–black-gold). <!-- This resistor uses the 4 band variant of the colour code so the bands represent 6 - 5 - 0 - x0.1 making 65Ω ---> An [[ohmmeter]] could be used to verify this value.]]
 
Those substances through which electricity can flow are called [[electrical conductor|conductors]].  A piece of conducting material of a particular resistance meant for use in a circuit is called a [[resistor]].  Conductors are made of high-[[Electrical resistivity and conductivity|conductivity]] materials such as metals, in particular copper and aluminium. Resistors, on the other hand, are made of a wide variety of materials depending on factors such as the desired resistance, amount of energy that it needs to dissipate, precision, and costs.
 
==Ohm's law==
[[File:FourIVcurves.svg|thumb|500px|The [[current-voltage characteristic]]s of four devices: Two [[resistor]]s, a [[diode]], and a [[Battery (electricity)|battery]]. The horizontal axis is [[voltage drop]], the vertical axis is [[electric current|current]]. Ohm's law is satisfied when the graph is a straight line through the origin. Therefore, the two resistors are "ohmic", but the diode and battery are not.]]
{{main|Ohm's law}}
Ohm's law is an empirical law relating the voltage ''V'' across an element to the current ''I'' through it:
:<math>V \propto I</math>
(''V'' is directly proportional to ''I''). This law is not always true: For example, it is false for [[diode]]s, [[battery (electrical)|batteries]], etc. However, it ''is'' true to a very good approximation for wires and [[resistor]]s (assuming that other conditions, including temperature, are held fixed). Materials or objects where Ohm's law is true are called "ohmic", whereas objects which do not obey Ohm's law are '"non-ohmic".
 
== Relation to resistivity and conductivity ==
[[File:Resistivity geometry.png|thumb|A piece of resistive material with electrical contacts on both ends.]]
{{main|Electrical resistivity and conductivity}}
The resistance of a given object depends primarily on two factors: What material it is made of, and its shape. For a given material, the resistance is inversely proportional to the cross-sectional area; for example, a thick copper wire has lower resistance than an otherwise-identical thin copper wire. Also, for a given material, the resistance is proportional to the length; for example, a long copper wire has higher resistance than an otherwise-identical short copper wire. The resistance {{math|R}} and conductance {{math|G}} of a conductor of uniform cross section, therefore, can be computed as
 
:<math>R = \rho \frac{\ell}{A},</math>
:<math>G= \sigma \frac{A}{\ell}.</math>
 
where <math>\ell</math> is the length of the conductor, measured in [[metre]]s [m], ''A'' is the cross-section area of the conductor measured in [[square metre]]s [m²], σ ([[Sigma (letter)|sigma]]) is the [[electrical conductivity]] measured in [[Siemens (unit)|siemens]] per meter (S·m<sup>−1</sup>), and ρ ([[Rho (letter)|rho]]) is the [[electrical resistivity]] (also called ''specific electrical resistance'') of the material, measured in ohm-metres (Ω·m). The resistivity and conductivity are proportionality constants, and therefore depend only on the material the wire is made of, not the geometry of the wire. Resistivity and conductivity are [[multiplicative inverse|reciprocals]]: <math>\rho=1/\sigma</math>. Resistivity is a measure of the material's ability to oppose electric current.
 
This formula is not exact: It assumes the [[current density]] is totally uniform in the conductor, which is not always true in practical situations. However, this formula still provides a good approximation for long thin conductors such as wires.
 
Another situation for which this formula is not exact is with [[alternating current]] (AC), because the [[skin effect]] inhibits current flow near the center of the conductor. Then, the ''geometrical'' cross-section is different from the ''effective'' cross-section in which current is actually flowing, so the resistance is higher than expected. Similarly, if two conductors are near each other carrying AC current, their resistances will increase due to the [[proximity effect (electromagnetism)|proximity effect]]. At [[utility frequency|commercial power frequency]], these  effects are significant for large conductors carrying large currents, such as [[busbar]]s in an [[electrical substation]],<ref>Fink and Beaty, ''Standard Handbook for Electrical Engineers 11th Edition'', page 17-19</ref> or large power cables carrying more than a few hundred amperes.
 
===What determines resistivity?===
{{main|Electrical resistivity and conductivity}}
The resistivity of different materials varies by an enormous amount: For example, the conductivity of [[PTFE|teflon]] is about 10<sup>30</sup> times lower than the conductivity of copper. Why is there such a difference? Loosely speaking, a metal has large numbers of "delocalized" electrons that are not stuck in any one place, but free to move across large distances, whereas in an insulator (like teflon), each electron is tightly bound to a single molecule, and a great force is required to pull it away. [[Semiconductor]]s lie between these two extremes. More details can be found in the article: [[Electrical resistivity and conductivity]]. For the case of [[electrolyte]] solutions, see the article: [[Conductivity (electrolytic)]].
 
Resistivity varies with temperature. In semiconductors, resistivity also changes when light is shining on it. These are discussed below.
 
==Measuring resistance==
{{main|ohmmeter}}
 
An instrument for measuring resistance is called an [[ohmmeter]].  Simple ohmmeters cannot measure low resistances accurately because the resistance of their measuring leads causes a voltage drop that interferes with the measurement, so more accurate devices use [[four-terminal sensing]].
 
==Typical resistances==
{{see also|Electrical resistivities of the elements (data page)|Electrical resistivity and conductivity}}
 
{| class="wikitable"
|-
|'''Component'''
|'''Resistance''' (Ω)
|-
|1 meter of copper wire<br>with 1mm diameter
|0.02<ref>The resistivity of copper is about 1.7×10<sup>-8</sup>Ωm. See [http://hypertextbook.com/facts/2004/BridgetRitter.shtml].</ref>
|-
|1&nbsp;km [[overhead power line]]<br>''(typical)''
|0.03<ref>''Electric power substations engineering'' by John Douglas McDonald, p 18-37, [http://books.google.com/books?id=e__hltcUQIQC&pg=PT363 google books link]</ref>
|-
|[[AA battery]] ''(typical<br>[[internal resistance]])''
|0.1<ref>[http://data.energizer.com/PDFs/BatteryIR.pdf] For a fresh Energizer E91 AA alkaline battery, the internal resistance varies from 0.9Ω at -40°C, to 0.1Ω at +40°C.</ref>
|-
|[[Incandescent light bulb]]<br>filament ''(typical)''
|200-1000<ref>A 60W light bulb in the USA (120V [[mains electricity]]) draws RMS current 60W/120V=500mA, so its resistance is 120V/500mA=240 ohms. The resistance of a 60W light bulb in Europe (230V mains) would be 900 ohms. The resistance of a filament is temperature-dependent; these values are for when the filament is already heated up and the light is already glowing.</ref>
|-
|Human body
|1000 to 100,000<ref>100,000 ohms for dry skin contact, 1000 ohms for wet or broken skin contact. Other factors and conditions are relevant as well. See [[electric shock]] article for more details. Also see: {{cite web|url=http://www.cdc.gov/niosh/docs/98-131/overview.html|accessdate=2008-08-16|title=Publication No. 98-131: Worker Deaths by Electrocution|publisher=[[National Institute for Occupational Safety and Health]]}}</ref>
|}
 
==Static and differential resistance==
{{multiple image
| align    = right
| direction = horizontal
| width    = 200
| image1    = DifferentialChordalResistance.svg
| width1    = 110
| alt1      = Differential versus chordal resistance
| caption1  = The [[Current–voltage characteristic|IV curve]] of a non-ohmic device (purple).  The '''static resistance''' at point ''A'' is the [[Multiplicative inverse|inverse]] [[slope]] of line ''B'' through the origin. The '''differential resistance''' at ''A'' is the inverse slope of [[tangent line]] ''C''.
| image2    = Negative_differential_resistance.svg
| width2    = 90
| alt2      = Negative differential resistance
| caption2  = The [[Current–voltage characteristic|IV curve]] of a component with [[negative resistance|negative differential resistance]], an unusual phenomenon where the IV curve is non-[[Monotonic function|monotonic]].}}
{{see also|Small-signal model}}
 
Many electrical elements, such as [[diode]]s and [[battery (electricity)|batteries]] do ''not'' satisfy [[Ohm's law]]. These are called ''non-ohmic'' or ''nonlinear'', and are characterized by an [[I–V curve|''I–V'' curve]] which is ''not'' a straight line through the origin.
 
Resistance and conductance can still be defined for non-ohmic elements.  However, unlike ohmic resistance, nonlinear resistance is not constant but varies with the voltage or current through the device; its [[Biasing|operating point]].  There are two types:<ref name=brown/><ref name=kaiser/>
 
*'''Static resistance''' (also called ''chordal'' or ''DC resistance'') - This corresponds to the usual definition of resistance; the voltage divided by the current
:<math>R_\mathrm{static} = \frac {V}{I} \,</math>.
:It is the slope of the line ([[chord (geometry)|chord]]} from the origin through the point on the curve.  Static resistance determines the power dissipation in an electrical component.  Points on the ''IV'' curve located in the 2nd or 4th quadrants, for which the slope of the chordal line is negative, have ''negative static resistance''.  [[Passivity (engineering)|Passive]] devices, which have no source of energy, cannot have negative static resistance. However active devices such as transistors or [[op-amp]]s can synthesize negative static resistance with feedback, and it is used in some circuits such as [[gyrator]]s.
 
*'''Differential resistance''' (also called ''dynamic'', ''incremental'' or ''small signal resistance'') - [[Electrical resistance#Static and differential resistance|Differential resistance]] is the derivative of the voltage with respect to the current; the [[slope]] of the ''IV'' curve at a point
:<math>R_\mathrm{diff} = \frac {dV}{dI}  \,</math>.
:If the ''IV'' curve is non[[monotonic]] (with peaks and troughs), the curve will have a negative slope in some regions; so in these regions the device has ''negative differential resistance''.  Devices with negative differential resistance can amplify a signal applied to them, and are used to make amplifiers and oscillators.  These include [[tunnel diode]]s, [[Gunn diode]]s, [[IMPATT diode]]s, [[magnetron]] tubes, and [[unijunction transistor]]s.
 
==AC circuits==
===Impedance and admittance===
[[File:VI phase.png|thumb|right|300px|The voltage (red) and current (blue) versus time (horizontal axis) for a [[capacitor]] (top) and [[inductor]] (bottom). Since the [[amplitude]] of the current and voltage [[Sine wave|sinusoid]]s are the same, the [[absolute value]] of [[electrical impedance|impedance]] is 1 for both the capacitor and the inductor (in whatever units the graph is using). On the other hand, the [[phase (waves)|phase difference]] between current and voltage is -90° for the capacitor; therefore, the [[argument (complex analysis)|complex phase]] of the [[electrical impedance|impedance]] of the capacitor is -90°. Similarly, the [[phase (waves)|phase difference]] between current and voltage is +90° for the inductor; therefore, the complex phase of the impedance of the inductor is +90°.]]
{{main|Electrical impedance|Admittance}}
When an alternating current flows through a circuit, the relation between current and voltage across a circuit element is characterized not only by the ratio of their magnitudes, but also the difference in their [[Phase (waves)|phases]]. For example, in an ideal resistor, the moment when the voltage reaches its maximum, the current also reaches its maximum (current and voltage are oscillating in phase). But for a [[capacitor]] or [[inductor]], the maximum current flow occurs as the voltage passes through zero and vice-versa (current and voltage are oscillating 90° out of phase, see image at right). [[Complex number]]s are used to keep track of both the phase and magnitude of current and voltage:
:<math>V(t)=\text{Re}(V_0 e^{j\omega t}), \quad I(t)=\text{Re}(I_0 e^{j\omega t}), \quad Z=\frac{V_0}{I_0}, \quad Y=\frac{I_0}{V_0}</math>
where:
*''t'' is time,
*''V''(''t'') and ''I''(''t'') are, respectively, voltage and current as a function of time,
*''V<sub>0</sub>'', ''I<sub>0</sub>'', ''Z'', and ''Y'' are complex numbers,
*''Z'' is called [[electrical impedance|impedance]],
*''Y'' is called [[admittance]],
*Re indicates [[real part]],
*<math>\omega</math> is the [[angular frequency]] of the AC current,
*<math>j=\sqrt{-1}</math> is the [[imaginary unit]].
The impedance and admittance may be expressed as complex numbers which can be broken into real and imaginary parts:
:<math>Z=R+jX, \quad Y=G+jB</math>
where ''R'' and ''G'' are resistance and conductance respectively, ''X'' is [[electrical reactance|reactance]], and ''B'' is [[susceptance]]. For ideal resistors, ''Z'' and ''Y'' reduce to ''R'' and ''G'' respectively, but for AC networks containing [[capacitor]]s and [[inductor]]s, ''X'' and ''B'' are nonzero.
 
<math>Z=1/Y</math> for AC circuits, just as <math>R=1/G</math> for DC circuits.
 
===Frequency dependence of resistance===
Another complication of AC circuits is that the resistance and conductance can be frequency-dependent. One reason, mentioned above is the [[skin effect]] (and the related [[proximity effect (electromagnetism)|proximity effect]]). Another reason is that the resistivity itself may depend on frequency (see [[Drude model]], [[deep-level trap]]s, [[resonant frequency]], [[Kramers–Kronig relations]], etc.)
 
==Energy dissipation and Joule heating==
[[File:Cartridge-heater-hot.jpg|thumb|Running current through a material with high resistance creates heat, in a phenomenon called [[Joule heating]]. In this picture, a [[cartridge heater]], warmed by Joule heating, is [[Incandescence|glowing red hot]].]]
{{main|Joule heating}}
Resistors (and other elements with resistance) oppose the flow of electric current; therefore, electrical energy is required to push current through the resistance. This electrical energy is dissipated, heating the resistor in the process. This is called ''[[Joule heating]]'' (after [[James Prescott Joule]]), also called ''ohmic heating'' or ''resistive heating''.
 
The dissipation of electrical energy is often undesired, particularly in the case of [[electric power transmission|transmission losses]] in [[overhead power line|power lines]]. [[Electric power transmission|High voltage transmission]] helps reduce the losses by reducing the current for a given power.
 
On the other hand, Joule heating is sometimes useful, for example in [[electric stove]]s and other [[electric heating|electric heaters]] (also called ''resistive heaters''). As another example, [[incandescent lamp]]s rely on Joule heating: the filament is heated to such a high temperature that it glows "white hot" with [[thermal radiation]] (also called [[incandescence]]).
 
The formula for Joule heating is:
:<math>P=I^2R</math>
where ''P'' is the [[electric power|power]] (energy per unit time) converted from electrical energy to thermal energy, ''R'' is the resistance, and ''I'' is the current through the resistor.
 
== Dependence of resistance on other conditions ==
=== Temperature dependence ===
{{main|Electrical resistivity and conductivity#Temperature dependence}}
Near room temperature, the resistivity of metals typically increases as temperature is increased, while the resistivity of semiconductors typically decreases as temperature is increased. The resistivity of insulators and electrolytes may increase or decrease depending on the system. For the detailed behavior and explanation, see [[Electrical resistivity and conductivity]].
 
As a consequence, the resistance of wires, resistors, and other components often change with temperature. This effect may be undesired, causing an electronic circuit to malfunction at extreme temperatures. In some cases, however, the effect is put to good use. When temperature-dependent resistance of a component is used purposefully, the component is called a [[resistance thermometer]] or [[thermistor]]. (A resistance thermometer is made of metal, usually platinum, while a thermistor is made of ceramic or polymer.)
 
Resistance thermometers and thermistors are generally used in two ways. First, they can be used as [[thermometer]]s: By measuring the resistance, the temperature of the environment can be inferred. Second, they can be used in conjunction with [[Joule heating]] (also called self-heating): If a large current is running through the resistor, the resistor's temperature rises and therefore its resistance changes. Therefore, these components can be used in a circuit-protection role similar to [[fuse (electrical)|fuse]]s, or for [[feedback]] in circuits, or for many other purposes. In general, self-heating can turn a resistor into a [[nonlinear element|nonlinear]] and [[hysteresis|hysteretic]] circuit element. For more details see [[Thermistor#Self-heating effects]].
 
If the temperature ''T'' does not vary too much, a [[linear approximation]] is typically used:
:<math>R(T) = R_0[1+\alpha (T - T_0)]</math>
where <math>\alpha</math> is called the ''temperature coefficient of resistance'', <math>T_0</math> is a fixed reference temperature (usually room temperature), and <math>R_0</math> is the resistance at temperature <math>T_0</math>. The parameter <math>\alpha</math> is an empirical parameter fitted from measurement data. Because the linear approximation is only an approximation, <math>\alpha</math> is different for different reference temperatures. For this reason it is usual to specify the temperature that <math>\alpha</math> was measured at with a suffix, such as <math>\alpha_{15}</math>, and the relationship only holds in a range of temperatures around the reference.<ref>Ward, MR, ''Electrical Engineering Science'', pp36–40, McGraw-Hill, 1971.</ref>
 
The temperature coefficient <math>\alpha</math> is typically +3×10<sup>−3</sup> K<sup>−1</sup> to +6×10<sup>−3</sup> K<sup>−1</sup> for metals near room temperature. It is usually negative for semiconductors and insulators, with highly variable magnitude.<ref>See [[Electrical resistivity and conductivity]] for a table. The temperature coefficient of resistivity is similar but not identical to the temperature coefficient of resistance. The small difference is due to [[thermal expansion]] changing the dimensions of the resistor.</ref>
 
=== Strain dependence ===
{{main|Strain gauge}}
Just as the resistance of a conductor depends upon temperature, the resistance of a conductor depends upon [[strain (materials science)|strain]]. By placing a conductor under [[tension (mechanics)|tension]] (a form of [[stress (physics)|stress]] that leads to strain in the form of stretching of the conductor), the length of the section of conductor under tension increases and its cross-sectional area decreases. Both these effects contribute to increasing the resistance of the strained section of conductor. Under compression (strain in the opposite direction), the resistance of the strained section of conductor decreases. See the discussion on [[strain gauge]]s for details about devices constructed to take advantage of this effect.
 
=== Light illumination dependence ===
{{main|Photoresistor|Photoconductivity}}
Some resistors, particularly those made from [[semiconductor]]s, exhibit ''[[photoconductivity]]'', meaning that their resistance changes when light is shining on them. Therefore they are called ''[[photoresistor]]s'' (or ''light dependent resistors''). These are a common type of [[photodetector|light detector]].
 
==Superconductivity==
{{main|Superconductivity}}
[[Superconductor]]s are materials that have exactly zero resistance and infinite conductance, because they can have V=0 and I≠0. This also means there is no [[joule heating]], or in other words no [[dissipation]] of electrical energy. Therefore, if superconductive wire is made into a closed loop, current will keep flowing around the loop forever. Superconductors require cooling to temperatures near 4 K with [[liquid helium]] for most metallic superconductors like [[Niobium|Nb]][[Tin|Sn]] alloys, or cooling to temperatures near 77K with [[liquid nitrogen]] for the expensive, brittle and delicate ceramic [[high temperature superconductors]].
Nevertheless, there are many [[technological applications of superconductivity]], including [[superconducting magnet]]s.
 
==See also==
 
{{Portal|Electronics}}
 
<div class= style="-moz-column-count:2; column-count:2;">
* [[Electrical measurements]]
* [[Resistor]]
* [[Electrical conduction]] for more information about the physical mechanisms for conduction in materials.
* [[Voltage divider]]
* [[Voltage drop]]
* [[Thermal resistance]]
* [[Sheet resistance]]
* [[SI electromagnetism units]]
* [[Quantum Hall effect]], a standard for high-accuracy resistance measurements.
* [[Series and parallel circuits]]
* [[Johnson–Nyquist noise]]
</div>
 
==References==
{{Reflist}}
 
==External links==
* [http://independent.academia.edu/Csoliverez/Papers/1848699/The_Notion_of_Electrical_Resistance_by_Soliverez The Notion of Electrical Resistance].  Review of the equations that determine the value of electrical resistance.
*[http://www.cvel.clemson.edu/emc/calculators/Resistance_Calculator/index.html''Clemson Vehicular Electronics Laboratory: Resistance Calculator'']
 
{{DEFAULTSORT:Electrical Resistance}}
[[Category:Electricity]]
[[Category:Physical quantities]]
[[Category:Electromagnetism]]

Revision as of 18:36, 23 December 2013

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Unique specialty cookware items served a la carte to compliment any cookware set are constructed of a sturdy Stainless Steel with a brushed exterior finish. Designed with an impression bonded, aluminum disk encapsulated base which distributes warmth quickly and evenly to permit exact temperature management. Handles are riveted for durability and efficiency. The New Specialty Cookware is compatible for all vary sorts together with induction. Along with the multi use function, another unique characteristic is bottom to top interior quantity markings in each quarts and metric measurement; and each bit comes with a tempered glass lid, oven safe to 350°F.
Whether or not you are a cooking enthusiasts, knowledgeable chef or simply cooking for your family you know the importance of getting a totally stocked kitchen. Not solely do you want the fitting elements, but you also need the right instruments to get the job performed. In any sort of fundamental cooking coaching lesson, you will be taught that chrome steel is your new finest pal when it comes to kitchen cookware. What additionally, you will study is that quality cooking tools does not normally come at a reduced price. Because of this, you will need to take good care of your cookware! Listed here are some fundamentals for stainless steel care.
To combat the uneven heating downside, most stainless steel pans are laminations of aluminum or copper on the underside to spread the warmth around, and stainless-steel contained in the pan to provide a cooking floor that's impervious to whatever you might put inside. In my experience, this chrome steel floor continues to be too sticky to fry on, and when you ever burn it you get a permanent hassle spot. But, sometimes a stainless-steel cooking surface turns out to be useful when you can't use aluminum (see below) so I keep some round. Choose one thing with a fairly thick aluminum layer on the underside.
Well, until you’re a metals professional and go examine the factory where the steel is made to see whether or not their manufacturing course of creates a pure austenite without corrosive supplies shaped, you’re not going to know for sure whether or not the craftsmanship of your stainless is of the very best quality. I think your greatest wager is to simply buy high-quality chrome steel from the start, from a brand with a reputation for good quality. But, I believe I have found out one way that you would be able to decide if the stainless cookware you already have is probably reactive.

The electrical resistance of an electrical conductor is the opposition to the passage of an electric current through that conductor; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical notion of friction. The SI unit of electrical resistance is the ohm (Ω), while electrical conductance is measured in siemens (S).

An object of uniform cross section has a resistance proportional to its resistivity and length and inversely proportional to its cross-sectional area. All materials show some resistance, except for superconductors, which have a resistance of zero.

The resistance (R) of an object is defined as the ratio of voltage across it (V) to current through it (I), while the conductance (G) is the inverse:

For a wide variety of materials and conditions, V and I are directly proportional to each other, and therefore R and G are constant (although they can depend on other factors like temperature or strain). This proportionality is called Ohm's law, and materials that satisfy it are called "Ohmic" materials.

In other cases, such as a diode or battery, V and I are not directly proportional, or in other words the I–V curve is not a straight line through the origin, and Ohm's law does not hold. In this case, resistance and conductance are less useful concepts, and more difficult to define. The ratio V/I is sometimes still useful, and is referred to as a "chordal resistance" or "static resistance",[1][2] as it corresponds to the inverse slope of a chord between the origin and an I–V curve. In other situations, the derivative may be most useful; this is called the "differential resistance".

Introduction

The hydraulic analogy compares electric current flowing through circuits to water flowing through pipes. When a pipe (left) is filled with hair (right), it takes a larger pressure to achieve the same flow of water. Pushing electric current through a large resistance is like pushing water through a pipe clogged with hair: It requires a larger push (electromotive force) to drive the same flow (electric current).

In the hydraulic analogy, current flowing through a wire (or resistor) is like water flowing through a pipe, and the voltage drop across the wire is like the pressure drop which pushes water through the pipe. Conductance is proportional to how much flow occurs for a given pressure, and resistance is proportional to how much pressure is required to achieve a given flow. (Conductance and resistance are reciprocals.)

The voltage drop (i.e., difference in voltage between one side of the resistor and the other), not the voltage itself, provides the driving force pushing current through a resistor. In hydraulics, it is similar: The pressure difference between two sides of a pipe, not the pressure itself, determines the flow through it. For example, there may be a large water pressure above the pipe, which tries to push water down through the pipe. But there may be an equally large water pressure below the pipe, which tries to push water back up through the pipe. If these pressures are equal, no water will flow. (In the image at right, the water pressure below the pipe is zero.)

The resistance and conductance of a wire, resistor, or other element is primarily determined by two factors: geometry (shape) and materials.

Geometry is important because it is more difficult to push water through a long, narrow pipe than a wide, short pipe. In the same way, a long, thin copper wire has higher resistance (lower conductance) than a short, thick copper wire.

Materials are important as well. A pipe filled with hair restricts the flow of water more than a clean pipe of the same shape and size. In a similar way, electrons can flow freely and easily through a copper wire, but cannot as easily flow through a steel wire of the same shape and size, and they essentially cannot flow at all through an insulator like rubber, regardless of its shape. The difference between, copper, steel, and rubber is related to their microscopic structure and electron configuration, and is quantified by a property called resistivity.

Conductors and resistors

A 65 Ω resistor, as identified by its electronic color code (blue–green–black-gold). An ohmmeter could be used to verify this value.

Those substances through which electricity can flow are called conductors. A piece of conducting material of a particular resistance meant for use in a circuit is called a resistor. Conductors are made of high-conductivity materials such as metals, in particular copper and aluminium. Resistors, on the other hand, are made of a wide variety of materials depending on factors such as the desired resistance, amount of energy that it needs to dissipate, precision, and costs.

Ohm's law

The current-voltage characteristics of four devices: Two resistors, a diode, and a battery. The horizontal axis is voltage drop, the vertical axis is current. Ohm's law is satisfied when the graph is a straight line through the origin. Therefore, the two resistors are "ohmic", but the diode and battery are not.

Mining Engineer (Excluding Oil ) Truman from Alma, loves to spend time knotting, largest property developers in singapore developers in singapore and stamp collecting. Recently had a family visit to Urnes Stave Church. Ohm's law is an empirical law relating the voltage V across an element to the current I through it:

(V is directly proportional to I). This law is not always true: For example, it is false for diodes, batteries, etc. However, it is true to a very good approximation for wires and resistors (assuming that other conditions, including temperature, are held fixed). Materials or objects where Ohm's law is true are called "ohmic", whereas objects which do not obey Ohm's law are '"non-ohmic".

Relation to resistivity and conductivity

A piece of resistive material with electrical contacts on both ends.

Mining Engineer (Excluding Oil ) Truman from Alma, loves to spend time knotting, largest property developers in singapore developers in singapore and stamp collecting. Recently had a family visit to Urnes Stave Church. The resistance of a given object depends primarily on two factors: What material it is made of, and its shape. For a given material, the resistance is inversely proportional to the cross-sectional area; for example, a thick copper wire has lower resistance than an otherwise-identical thin copper wire. Also, for a given material, the resistance is proportional to the length; for example, a long copper wire has higher resistance than an otherwise-identical short copper wire. The resistance Buying, selling and renting HDB and personal residential properties in Singapore are simple and transparent transactions. Although you are not required to engage a real property salesperson (generally often known as a "public listed property developers In singapore agent") to complete these property transactions, chances are you'll think about partaking one if you are not accustomed to the processes concerned.

Professional agents are readily available once you need to discover an condominium for hire in singapore In some cases, landlords will take into account you more favourably in case your agent comes to them than for those who tried to method them by yourself. You need to be careful, nevertheless, as you resolve in your agent. Ensure that the agent you are contemplating working with is registered with the IEA – Institute of Estate Brokers. Whereas it might sound a hassle to you, will probably be worth it in the end. The IEA works by an ordinary algorithm and regulations, so you'll protect yourself in opposition to probably going with a rogue agent who prices you more than they should for his or her service in finding you an residence for lease in singapore.

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Another method by way of which you could find out whether the agent is sweet is by checking the feedback, of the shoppers, on the website. There are various individuals would publish their comments on the web site of the Singapore property agent. You can take a look at these feedback and the see whether it will be clever to hire that specific Singapore property agent. You may even get in contact with the developer immediately. Many Singapore property brokers know the developers and you may confirm the goodwill of the agent by asking the developer. and conductance Buying, selling and renting HDB and personal residential properties in Singapore are simple and transparent transactions. Although you are not required to engage a real property salesperson (generally often known as a "public listed property developers In singapore agent") to complete these property transactions, chances are you'll think about partaking one if you are not accustomed to the processes concerned.

Professional agents are readily available once you need to discover an condominium for hire in singapore In some cases, landlords will take into account you more favourably in case your agent comes to them than for those who tried to method them by yourself. You need to be careful, nevertheless, as you resolve in your agent. Ensure that the agent you are contemplating working with is registered with the IEA – Institute of Estate Brokers. Whereas it might sound a hassle to you, will probably be worth it in the end. The IEA works by an ordinary algorithm and regulations, so you'll protect yourself in opposition to probably going with a rogue agent who prices you more than they should for his or her service in finding you an residence for lease in singapore.

There isn't any deal too small. Property agents who are keen to find time for any deal even if the commission is small are the ones you want on your aspect. Additionally they present humbleness and might relate with the typical Singaporean higher. Relentlessly pursuing any deal, calling prospects even without being prompted. Even if they get rejected a hundred times, they still come again for more. These are the property brokers who will find consumers what they need eventually, and who would be the most successful in what they do. 4. Honesty and Integrity

This feature is suitable for you who need to get the tax deductions out of your PIC scheme to your property agency firm. It's endorsed that you visit the correct site for filling this tax return software. This utility must be submitted at the very least yearly to report your whole tax and tax return that you're going to receive in the current accounting 12 months. There may be an official website for this tax filling procedure. Filling this tax return software shouldn't be a tough thing to do for all business homeowners in Singapore.

A wholly owned subsidiary of SLP Worldwide, SLP Realty houses 900 associates to service SLP's fast rising portfolio of residential tasks. Real estate is a human-centric trade. Apart from offering comprehensive coaching applications for our associates, SLP Realty puts equal emphasis on creating human capabilities and creating sturdy teamwork throughout all ranges of our organisational hierarchy. Worldwide Presence At SLP International, our staff of execs is pushed to make sure our shoppers meet their enterprise and investment targets. Under is an inventory of some notable shoppers from completely different industries and markets, who've entrusted their real estate must the expertise of SLP Worldwide.

If you're looking for a real estate or Singapore property agent online, you merely need to belief your instinct. It is because you don't know which agent is sweet and which agent will not be. Carry out research on a number of brokers by looking out the internet. As soon as if you find yourself certain that a selected agent is dependable and trustworthy, you'll be able to choose to utilize his partnerise find you a house in Singapore. More often than not, a property agent is considered to be good if she or he places the contact data on his web site. This is able to imply that the agent does not thoughts you calling them and asking them any questions regarding properties in Singapore. After chatting with them you too can see them of their office after taking an appointment.

Another method by way of which you could find out whether the agent is sweet is by checking the feedback, of the shoppers, on the website. There are various individuals would publish their comments on the web site of the Singapore property agent. You can take a look at these feedback and the see whether it will be clever to hire that specific Singapore property agent. You may even get in contact with the developer immediately. Many Singapore property brokers know the developers and you may confirm the goodwill of the agent by asking the developer. of a conductor of uniform cross section, therefore, can be computed as

where is the length of the conductor, measured in metres [m], A is the cross-section area of the conductor measured in square metres [m²], σ (sigma) is the electrical conductivity measured in siemens per meter (S·m−1), and ρ (rho) is the electrical resistivity (also called specific electrical resistance) of the material, measured in ohm-metres (Ω·m). The resistivity and conductivity are proportionality constants, and therefore depend only on the material the wire is made of, not the geometry of the wire. Resistivity and conductivity are reciprocals: . Resistivity is a measure of the material's ability to oppose electric current.

This formula is not exact: It assumes the current density is totally uniform in the conductor, which is not always true in practical situations. However, this formula still provides a good approximation for long thin conductors such as wires.

Another situation for which this formula is not exact is with alternating current (AC), because the skin effect inhibits current flow near the center of the conductor. Then, the geometrical cross-section is different from the effective cross-section in which current is actually flowing, so the resistance is higher than expected. Similarly, if two conductors are near each other carrying AC current, their resistances will increase due to the proximity effect. At commercial power frequency, these effects are significant for large conductors carrying large currents, such as busbars in an electrical substation,[3] or large power cables carrying more than a few hundred amperes.

What determines resistivity?

Mining Engineer (Excluding Oil ) Truman from Alma, loves to spend time knotting, largest property developers in singapore developers in singapore and stamp collecting. Recently had a family visit to Urnes Stave Church. The resistivity of different materials varies by an enormous amount: For example, the conductivity of teflon is about 1030 times lower than the conductivity of copper. Why is there such a difference? Loosely speaking, a metal has large numbers of "delocalized" electrons that are not stuck in any one place, but free to move across large distances, whereas in an insulator (like teflon), each electron is tightly bound to a single molecule, and a great force is required to pull it away. Semiconductors lie between these two extremes. More details can be found in the article: Electrical resistivity and conductivity. For the case of electrolyte solutions, see the article: Conductivity (electrolytic).

Resistivity varies with temperature. In semiconductors, resistivity also changes when light is shining on it. These are discussed below.

Measuring resistance

Mining Engineer (Excluding Oil ) Truman from Alma, loves to spend time knotting, largest property developers in singapore developers in singapore and stamp collecting. Recently had a family visit to Urnes Stave Church.

An instrument for measuring resistance is called an ohmmeter. Simple ohmmeters cannot measure low resistances accurately because the resistance of their measuring leads causes a voltage drop that interferes with the measurement, so more accurate devices use four-terminal sensing.

Typical resistances

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Component Resistance (Ω)
1 meter of copper wire
with 1mm diameter
0.02[4]
1 km overhead power line
(typical)
0.03[5]
AA battery (typical
internal resistance)
0.1[6]
Incandescent light bulb
filament (typical)
200-1000[7]
Human body 1000 to 100,000[8]

Static and differential resistance

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The Hexacube is offering new ec launch singapore business property for sale Singapore investors want to contemplate. Residents of the realm will likely appreciate that they'll customize the business area that they wish to purchase as properly. This venture represents one of the crucial expansive buildings offered in Singapore up to now. Many investors will possible want to try how they will customise the property that they do determine to buy by means of here. This location has offered folks the prospect that they should understand extra about how this course of can work as well.

Singapore has been beckoning to traders ever since the value of properties in Singapore started sky rocketing just a few years again. Many businesses have their places of work in Singapore and prefer to own their own workplace area within the country once they decide to have a everlasting office. Rentals in Singapore in the corporate sector can make sense for some time until a business has discovered a agency footing. Finding Commercial Property Singapore takes a variety of time and effort but might be very rewarding in the long term.

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In 12 months 2013, c ommercial retails, shoebox residences and mass market properties continued to be the celebrities of the property market. Models are snapped up in report time and at document breaking prices. Builders are having fun with overwhelming demand and patrons need more. We feel that these segments of the property market are booming is a repercussion of the property cooling measures no.6 and no. 7. With additional buyer's stamp responsibility imposed on residential properties, buyers change their focus to commercial and industrial properties. I imagine every property purchasers need their property funding to understand in value.

Many electrical elements, such as diodes and batteries do not satisfy Ohm's law. These are called non-ohmic or nonlinear, and are characterized by an I–V curve which is not a straight line through the origin.

Resistance and conductance can still be defined for non-ohmic elements. However, unlike ohmic resistance, nonlinear resistance is not constant but varies with the voltage or current through the device; its operating point. There are two types:[1][2]

  • Static resistance (also called chordal or DC resistance) - This corresponds to the usual definition of resistance; the voltage divided by the current
.
It is the slope of the line (chord} from the origin through the point on the curve. Static resistance determines the power dissipation in an electrical component. Points on the IV curve located in the 2nd or 4th quadrants, for which the slope of the chordal line is negative, have negative static resistance. Passive devices, which have no source of energy, cannot have negative static resistance. However active devices such as transistors or op-amps can synthesize negative static resistance with feedback, and it is used in some circuits such as gyrators.
  • Differential resistance (also called dynamic, incremental or small signal resistance) - Differential resistance is the derivative of the voltage with respect to the current; the slope of the IV curve at a point
.
If the IV curve is nonmonotonic (with peaks and troughs), the curve will have a negative slope in some regions; so in these regions the device has negative differential resistance. Devices with negative differential resistance can amplify a signal applied to them, and are used to make amplifiers and oscillators. These include tunnel diodes, Gunn diodes, IMPATT diodes, magnetron tubes, and unijunction transistors.

AC circuits

Impedance and admittance

The voltage (red) and current (blue) versus time (horizontal axis) for a capacitor (top) and inductor (bottom). Since the amplitude of the current and voltage sinusoids are the same, the absolute value of impedance is 1 for both the capacitor and the inductor (in whatever units the graph is using). On the other hand, the phase difference between current and voltage is -90° for the capacitor; therefore, the complex phase of the impedance of the capacitor is -90°. Similarly, the phase difference between current and voltage is +90° for the inductor; therefore, the complex phase of the impedance of the inductor is +90°.

Mining Engineer (Excluding Oil ) Truman from Alma, loves to spend time knotting, largest property developers in singapore developers in singapore and stamp collecting. Recently had a family visit to Urnes Stave Church. When an alternating current flows through a circuit, the relation between current and voltage across a circuit element is characterized not only by the ratio of their magnitudes, but also the difference in their phases. For example, in an ideal resistor, the moment when the voltage reaches its maximum, the current also reaches its maximum (current and voltage are oscillating in phase). But for a capacitor or inductor, the maximum current flow occurs as the voltage passes through zero and vice-versa (current and voltage are oscillating 90° out of phase, see image at right). Complex numbers are used to keep track of both the phase and magnitude of current and voltage:

where:

The impedance and admittance may be expressed as complex numbers which can be broken into real and imaginary parts:

where R and G are resistance and conductance respectively, X is reactance, and B is susceptance. For ideal resistors, Z and Y reduce to R and G respectively, but for AC networks containing capacitors and inductors, X and B are nonzero.

for AC circuits, just as for DC circuits.

Frequency dependence of resistance

Another complication of AC circuits is that the resistance and conductance can be frequency-dependent. One reason, mentioned above is the skin effect (and the related proximity effect). Another reason is that the resistivity itself may depend on frequency (see Drude model, deep-level traps, resonant frequency, Kramers–Kronig relations, etc.)

Energy dissipation and Joule heating

Running current through a material with high resistance creates heat, in a phenomenon called Joule heating. In this picture, a cartridge heater, warmed by Joule heating, is glowing red hot.

Mining Engineer (Excluding Oil ) Truman from Alma, loves to spend time knotting, largest property developers in singapore developers in singapore and stamp collecting. Recently had a family visit to Urnes Stave Church. Resistors (and other elements with resistance) oppose the flow of electric current; therefore, electrical energy is required to push current through the resistance. This electrical energy is dissipated, heating the resistor in the process. This is called Joule heating (after James Prescott Joule), also called ohmic heating or resistive heating.

The dissipation of electrical energy is often undesired, particularly in the case of transmission losses in power lines. High voltage transmission helps reduce the losses by reducing the current for a given power.

On the other hand, Joule heating is sometimes useful, for example in electric stoves and other electric heaters (also called resistive heaters). As another example, incandescent lamps rely on Joule heating: the filament is heated to such a high temperature that it glows "white hot" with thermal radiation (also called incandescence).

The formula for Joule heating is:

where P is the power (energy per unit time) converted from electrical energy to thermal energy, R is the resistance, and I is the current through the resistor.

Dependence of resistance on other conditions

Temperature dependence

Mining Engineer (Excluding Oil ) Truman from Alma, loves to spend time knotting, largest property developers in singapore developers in singapore and stamp collecting. Recently had a family visit to Urnes Stave Church. Near room temperature, the resistivity of metals typically increases as temperature is increased, while the resistivity of semiconductors typically decreases as temperature is increased. The resistivity of insulators and electrolytes may increase or decrease depending on the system. For the detailed behavior and explanation, see Electrical resistivity and conductivity.

As a consequence, the resistance of wires, resistors, and other components often change with temperature. This effect may be undesired, causing an electronic circuit to malfunction at extreme temperatures. In some cases, however, the effect is put to good use. When temperature-dependent resistance of a component is used purposefully, the component is called a resistance thermometer or thermistor. (A resistance thermometer is made of metal, usually platinum, while a thermistor is made of ceramic or polymer.)

Resistance thermometers and thermistors are generally used in two ways. First, they can be used as thermometers: By measuring the resistance, the temperature of the environment can be inferred. Second, they can be used in conjunction with Joule heating (also called self-heating): If a large current is running through the resistor, the resistor's temperature rises and therefore its resistance changes. Therefore, these components can be used in a circuit-protection role similar to fuses, or for feedback in circuits, or for many other purposes. In general, self-heating can turn a resistor into a nonlinear and hysteretic circuit element. For more details see Thermistor#Self-heating effects.

If the temperature T does not vary too much, a linear approximation is typically used:

where is called the temperature coefficient of resistance, is a fixed reference temperature (usually room temperature), and is the resistance at temperature . The parameter is an empirical parameter fitted from measurement data. Because the linear approximation is only an approximation, is different for different reference temperatures. For this reason it is usual to specify the temperature that was measured at with a suffix, such as , and the relationship only holds in a range of temperatures around the reference.[9]

The temperature coefficient is typically +3×10−3 K−1 to +6×10−3 K−1 for metals near room temperature. It is usually negative for semiconductors and insulators, with highly variable magnitude.[10]

Strain dependence

Mining Engineer (Excluding Oil ) Truman from Alma, loves to spend time knotting, largest property developers in singapore developers in singapore and stamp collecting. Recently had a family visit to Urnes Stave Church. Just as the resistance of a conductor depends upon temperature, the resistance of a conductor depends upon strain. By placing a conductor under tension (a form of stress that leads to strain in the form of stretching of the conductor), the length of the section of conductor under tension increases and its cross-sectional area decreases. Both these effects contribute to increasing the resistance of the strained section of conductor. Under compression (strain in the opposite direction), the resistance of the strained section of conductor decreases. See the discussion on strain gauges for details about devices constructed to take advantage of this effect.

Light illumination dependence

Mining Engineer (Excluding Oil ) Truman from Alma, loves to spend time knotting, largest property developers in singapore developers in singapore and stamp collecting. Recently had a family visit to Urnes Stave Church. Some resistors, particularly those made from semiconductors, exhibit photoconductivity, meaning that their resistance changes when light is shining on them. Therefore they are called photoresistors (or light dependent resistors). These are a common type of light detector.

Superconductivity

Mining Engineer (Excluding Oil ) Truman from Alma, loves to spend time knotting, largest property developers in singapore developers in singapore and stamp collecting. Recently had a family visit to Urnes Stave Church. Superconductors are materials that have exactly zero resistance and infinite conductance, because they can have V=0 and I≠0. This also means there is no joule heating, or in other words no dissipation of electrical energy. Therefore, if superconductive wire is made into a closed loop, current will keep flowing around the loop forever. Superconductors require cooling to temperatures near 4 K with liquid helium for most metallic superconductors like NbSn alloys, or cooling to temperatures near 77K with liquid nitrogen for the expensive, brittle and delicate ceramic high temperature superconductors. Nevertheless, there are many technological applications of superconductivity, including superconducting magnets.

See also

Sportspersons Hyslop from Nicolet, usually spends time with pastimes for example martial arts, property developers condominium in singapore singapore and hot rods. Maintains a trip site and has lots to write about after touring Gulf of Porto: Calanche of Piana.

References

43 year old Petroleum Engineer Harry from Deep River, usually spends time with hobbies and interests like renting movies, property developers in singapore new condominium and vehicle racing. Constantly enjoys going to destinations like Camino Real de Tierra Adentro.

External links

  1. 1.0 1.1 20 year-old Real Estate Agent Rusty from Saint-Paul, has hobbies and interests which includes monopoly, property developers in singapore and poker. Will soon undertake a contiki trip that may include going to the Lower Valley of the Omo.

    My blog: http://www.primaboinca.com/view_profile.php?userid=5889534
  2. 2.0 2.1 20 year-old Real Estate Agent Rusty from Saint-Paul, has hobbies and interests which includes monopoly, property developers in singapore and poker. Will soon undertake a contiki trip that may include going to the Lower Valley of the Omo.

    My blog: http://www.primaboinca.com/view_profile.php?userid=5889534
  3. Fink and Beaty, Standard Handbook for Electrical Engineers 11th Edition, page 17-19
  4. The resistivity of copper is about 1.7×10-8Ωm. See [1].
  5. Electric power substations engineering by John Douglas McDonald, p 18-37, google books link
  6. [2] For a fresh Energizer E91 AA alkaline battery, the internal resistance varies from 0.9Ω at -40°C, to 0.1Ω at +40°C.
  7. A 60W light bulb in the USA (120V mains electricity) draws RMS current 60W/120V=500mA, so its resistance is 120V/500mA=240 ohms. The resistance of a 60W light bulb in Europe (230V mains) would be 900 ohms. The resistance of a filament is temperature-dependent; these values are for when the filament is already heated up and the light is already glowing.
  8. 100,000 ohms for dry skin contact, 1000 ohms for wet or broken skin contact. Other factors and conditions are relevant as well. See electric shock article for more details. Also see: Template:Cite web
  9. Ward, MR, Electrical Engineering Science, pp36–40, McGraw-Hill, 1971.
  10. See Electrical resistivity and conductivity for a table. The temperature coefficient of resistivity is similar but not identical to the temperature coefficient of resistance. The small difference is due to thermal expansion changing the dimensions of the resistor.