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Encyclopedia > Electrical resistance
Electromagnetism
Electricity · Magnetism
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A 750-kΩ resistor, as identified by its electronic color code. An ohmmeter could be used to verify this value.
A 750-kΩ resistor, as identified by its electronic color code. An ohmmeter could be used to verify this value.

Electrical resistance is a measure of the degree to which an object opposes an electric current through it, measured in ohms. Its reciprocal quantity is electrical conductance measured in siemens. Assuming a uniform current density, an object's electrical resistance is a function of both its physical geometry and the resistivity of the material it is made from: Electromagnetism is the physics of the electromagnetic field: a field which exerts a force on particles that possess the property of electric charge, and is in turn affected by the presence and motion of those particles. ... Image File history File links Solenoid. ... Electricity (from New Latin Ä“lectricus, amberlike) is a general term for a variety of phenomena resulting from the presence and flow of electric charge. ... For other senses of this word, see magnetism (disambiguation). ... Electrostatics (also known as static electricity) is the branch of physics that deals with the phenomena arising from what seem to be stationary electric charges. ... This box:      Electric charge is a fundamental conserved property of some subatomic particles, which determines their electromagnetic interaction. ... This box:      Coulombs torsion balance Coulombs law, developed in the 1780s by French physicist Charles Augustin de Coulomb, may be stated in scalar form as follows: The magnitude of the electrostatic force between two point electric charges is directly proportional to the product of the magnitudes of each... In physics, the space surrounding an electric charge or in the presence of a time-varying magnetic field has a property called an electric field. ... In physics, Gausss law gives the relation between the electric flux flowing out a closed surface and the charge enclosed in the surface. ... This article does not cite any references or sources. ... This article is about the electromagnetic phenomenon. ... Magnetostatics is the study of static magnetic fields. ... In physics, Ampères Circuital law, discovered by André-Marie Ampère, relates the circulating magnetic field in a closed loop to the electric current passing through the loop. ... This box:      Electric current is the flow (movement) of electric charge. ... For the indie-pop band, see The Magnetic Fields. ... Magnetic flux, represented by the Greek letter Φ (phi), is a measure of quantity of magnetism, taking account of the strength and the extent of a magnetic field. ... The Biot-Savart law is a physical law with applications in both electromagnetics and fluid dynamics. ... A bar magnet. ... Classical electrodynamics (or classical electromagnetism) is a theory of electromagnetism that was developed over the course of the 19th century, most prominently by James Clerk Maxwell. ... In physics, free space is a concept of electromagnetic theory, corresponding roughly to the vacuum, the baseline state of the electromagnetic field, or the replacement for the electromagnetic aether. ... Lorentz force. ... Electromotive force (emf) is the amount of energy gained per unit charge that passes through a device in the opposite direction to the electric field existing across that device. ... For magnetic induction, see Magnetic field. ... Faradays law of induction (more generally, the law of electromagnetic induction) states that the induced emf (electromotive force) in a closed loop equals the negative of the time rate of change of magnetic flux through the loop. ... Displacement current is a quantity related to changing electric field. ... For thermodynamic relations, see Maxwell relations. ... The electromagnetic field is a physical field that is produced by electrically charged objects and which affects the behaviour of charged objects in the vicinity of the field. ... This box:      Electromagnetic (EM) radiation is a self-propagating wave in space with electric and magnetic components. ... The Liénard-Wiechert potential describes the electromagnetic effect of a moving charge. ... In physics, the Maxwell stress tensor is the stress tensor of an electromagnetic field. ... As the circular plate moves down through a small region of constant magnetic field directed into the page, eddy currents are induced in the plate. ... A simple electric circuit made up of a voltage source and a resistor. ... Conduction is the movement of electrically charged particles through a transmission medium (electrical conductor). ... Capacitance is a measure of the amount of electric charge stored (or separated) for a given electric potential. ... An electric current i flowing around a circuit produces a magnetic field and hence a magnetic flux Φ through the circuit. ... Electrical impedance, or simply impedance, is a measure of opposition to a sinusoidal alternating electric current. ... A resonator is a device or part that vibrates (or oscillates) with waves. ... This box:      This page is about waveguides for electromagnetic wave propagation at microwave and radio wave frequencies. ... In special relativity, in order to more clearly express the fact that Maxwells equations (in vacuum) take the same form in any inertial coordinate system, the vacuum Maxwells equations are written in terms of four-vectors and tensors in the manifestly covariant form (cgs units): , and where is... To meet Wikipedias quality standards, this article or section may require cleanup. ... In physics, the electromagnetic stress-energy tensor is the portion of the stress-energy tensor due to the electromagnetic field. ... In special and general relativity, the four-current is the Lorentz covariant four-vector that replaces the electromagnetic current density where c is the speed of light, ρ the charge density, and j the conventional current density. ... The electromagnetic four-potential is a four-vector defined in SI units (and gaussian units in parentheses) as in which φ is the electrical potential, and is the magnetic potential, a vector potential. ... André-Marie Ampère (January 20, 1775 – June 10, 1836), was a French physicist who is generally credited as one of the main discoverers of electromagnetism. ... Charles Augustin de Coulomb (born June 14, 1736, Angoulême, France - died August 23, 1806, Paris, France) was a French physicist. ... Michael Faraday, FRS (September 22, 1791 – August 25, 1867) was an English chemist and physicist (or natural philosopher, in the terminology of that time) who contributed to the fields of electromagnetism and electrochemistry. ... Heinrich Rudolf Hertz (February 22, 1857 - January 1, 1894) was the German physicist and mechanician for whom the hertz, an SI unit, is named. ... Hendrik Antoon Lorentz (July 18, 1853, Arnhem – February 4, 1928, Haarlem) was a Dutch physicist who shared the 1902 Nobel Prize in Physics with Pieter Zeeman for the discovery and elucidation of the Zeeman effect. ... James Clerk Maxwell (13 June 1831 – 5 November 1879) was a Scottish mathematician and theoretical physicist. ... Image File history File linksMetadata Download high resolution version (2274x1248, 116 KB) Description: Register. ... Image File history File linksMetadata Download high resolution version (2274x1248, 116 KB) Description: Register. ... Resistor symbols (American) Resistor symbols (Europe, IEC) Axial-lead resistors on tape. ... The electronic color code discussed here is used to indicate the values or ratings of electronic components, very commonly for resistors, but also for capacitors, inductors, and others. ... An Ohmmeter is an electrical measuring instrument that measures electrical resistance, the opposition to the flow of an electric current. ... This box:      Electric current is the flow (movement) of electric charge. ... A multimeter can be used to measure resistance in ohms. ... The reciprocal function: y = 1/x. ... Electrical conductance is the reciprocal of electrical resistance. ... The siemens (symbol: S) is the SI derived unit of electric conductance. ...

R = {l cdot rho over A} ,

where

"l" is the length
"A" is the cross sectional area, and
"ρ" is the resistivity of the material

Electrical resistance shares some conceptual parallels with the mechanical notion of friction. The SI unit of electrical resistance is the ohm, symbol Ω. The resistance of an object determines the amount of current through the object for a given potential difference across the object. // Headline text POOP!! Danny Hornsby (also known as Gnome) is a measure indicating how strongly a Gnome can opposes the flow of electric current. ... For other uses, see Friction (disambiguation). ... Look up si, Si, SI in Wiktionary, the free dictionary. ... The ohm (symbol: Ω) is the SI unit of electric resistance. ... Omega (Ω ω) is the 24th and last letter of the Greek alphabet. ... Potential difference is a quantity in physics related to the amount of energy that would be required to move an object from one place to another against various types of force. ...

R = frac{V}{I}

where

R is the resistance of the object, measured in ohms, equivalent to J·s/C2
V is the potential difference across the object, measured in volts
I is the current through the object, measured in amperes

For a wide variety of materials and conditions, the electrical resistance does not depend on the amount of current through or the amount of voltage across the object, meaning that the resistance R is constant. The ohm (symbol: Ω) is the SI unit of electric resistance. ... Josephson junction array chip developed by NIST as a standard volt. ... For other uses, see Ampere (disambiguation). ... In mathematics and the mathematical sciences, a constant is a fixed, but possibly unspecified, value. ...

Contents

Resistance of a conductor

DC resistance

As long as the current density is totally uniform in the insulator, the DC resistance R of a conductor of regular cross section can be computed as In electricity, current is the rate of flow of charges, usually through a metal wire or some other electrical conductor. ... Direct current (DC or continuous current) is the continuous flow of electricity through a conductor such as a wire from high to low potential. ...

R = {l cdot rho over A} ,

where

l is the length of the conductor, measured in meters
A is the cross-sectional area, measured in square meters
ρ (Greek: rho) is the electrical resistivity (also called specific electrical resistance) of the material, measured in ohm · meter. Resistivity is a measure of the material's ability to oppose the flow of electric current.

For practical reasons, almost any connections to a real conductor will almost certainly mean the current density is not totally uniform. However, this formula still provides a good approximation for long thin conductors such as wires. The metre, or meter (symbol: m) is the SI base unit of length. ... A square metre (US spelling: square meter) is by definition the area enclosed by a square with sides each 1 metre long. ... // Headline text POOP!! Danny Hornsby (also known as Gnome) is a measure indicating how strongly a Gnome can opposes the flow of electric current. ...


AC resistance

If a wire conducts high-frequency alternating current then the effective cross sectional area of the wire is reduced because of the skin effect. This causes the wire resistance to increase at a rate of 10dB/decade for wire radius much greater than skin depth. The skin effect is the tendency of an alternating electric current (AC) to distribute itself within a conductor so that the current density near the surface of the conductor is greater than that at its core. ... When an electromagnetic wave interacts with a conductive material, mobile charges within the material are made to oscillate back and forth with the same frequency as the impinging fields. ...


In a conductor close to others, the actual resistance is higher than that predicted by the skin effect because of the proximity effect. A changing magnetic field will influence the distribution of an electric current flowing within an electrical conductor. ...


Causes of resistance

In metals

A metal consists of a lattice of atoms, each with a shell of electrons. This can also be known as a positive ionic lattice. The outer electrons are free to dissociate from their parent atoms and travel through the lattice, creating a 'sea' of electrons, making the metal a conductor. When an electrical potential difference (a voltage) is applied across the metal, the electrons drift from one end of the conductor to the other under the influence of the electric field. This article is about metallic materials. ... For other uses, see Atom (disambiguation). ... International safety symbol Caution, risk of electric shock (ISO 3864), colloquially known as high voltage symbol. ... In physics, the space surrounding an electric charge or in the presence of a time-varying magnetic field has a property called an electric field. ...


Near room temperatures, the thermal motion of ions is the primary source of scattering of electrons (due to destructive interference of free electron wave on non-correlating potentials of ions) - thus the prime cause of metal resistance. Imperfections of lattice also contribute into resistance, although their contribution in pure metals is negligible.


The larger the cross-sectional area of the conductor, the more electrons are available to carry the current, so the lower the resistance. The longer the conductor, the more scattering events occur in each electron's path through the material, so the higher the resistance. Different materials also affect the resistance.[1]


In semiconductors and insulators

In metals, the fermi level lies in the conduction band (see Band Theory, below) giving rise to free conduction electrons. However, in semiconductors the position of the fermi level is within the band gap, exactly half way between the conduction band minimum and valence band maximum for intrinsic (undoped) semiconductors. This means that at 0 Kelvin, there are no free conduction electrons and the resistance is infinite. However, the resistance will continue to decrease as the charge carrier density in the conduction band increases. In extrinsic (doped) semiconductors, dopant atoms increase the majority charge carrier by donating electrons to the conduction band or accepting holes in the valence band. For both types of donor or acceptor atoms, increasing the dopant density leads to a reduction in the resistance. Highly doped semiconductors hence behave metallic. At very high temperatures, the contribution of thermally generated carriers will dominate over the contribution from dopant atoms and the resistance will decrease exponentially with temperature.


In ionic liquids/electrolytes

In electrolytes, electrical conduction happens not by band electrons or holes, but by full atomic species (ions) traveling, each carrying an electrical charge. The resistivity of ionic liquids varies tremendously by the salt concentration - while distilled water is almost an insulator, salt water is a very efficient electrical conductor. In biological membranes, currents are carried by ionic salts. Small holes in the membranes, called ion channels, are selective to specific ions and determine the membrane resistance. An electrolyte is any substance containing free ions that behaves as an electrically conductive medium. ... Conduction is the movement of electrically charged particles through a transmission medium (electrical conductor). ... This article is about the electrically charged particle. ... Look up cell membrane in Wiktionary, the free dictionary. ... Ion channels are pore-forming proteins that help to establish and control the small voltage gradient that exists across the plasma membrane of all living cells (see cell potential) by allowing the flow of ions down their electrochemical gradient. ...


Resistance of various materials

Material Resistivity, ρ
ohm-meter
Metals 10 - 8
Semiconductors variable
Electrolytes variable
Insulators 1016
Superconductors 0

This article is about metallic materials. ... A semiconductor is a solid material that has electrical conductivity in between that of a conductor and that of an insulator; it can vary over that wide range either permanently or dynamically. ... An electrolyte is any substance containing free ions that behaves as an electrically conductive medium. ... This article or section is in need of attention from an expert on the subject. ... Superconductivity is a phenomenon occurring in certain materials at low temperatures, characterised by the complete absence of electrical resistance and the damping of the interior magnetic field (the Meissner effect. ...

Band theory simplified

Electron energy levels in an insulator.
Electron energy levels in an insulator.

Quantum mechanics states that the energy of an electron in an atom cannot be any arbitrary value. Rather, there are fixed energy levels which the electrons can occupy, and values in between these levels are impossible. The energy levels are grouped into two bands: the valence band and the conduction band (the latter is generally above the former). Electrons in the conduction band may move freely throughout the substance in the presence of an electrical field. Image File history File links A rough diagram illustrating the forbidden electron energy levels in an insulator. ...


In insulators and semiconductors, the atoms in the substance influence each other so that between the valence band and the conduction band there exists a forbidden band of energy levels, which the electrons cannot occupy. In order for a current to flow, a relatively large amount of energy must be furnished to an electron for it to leap across this forbidden gap and into the conduction band. Thus, even large voltages can yield relatively small currents.


Differential resistance

When resistance may depend on voltage and current, differential resistance, incremental resistance or slope resistance is defined as the slope of the U-I graph at a particular point, thus:

R = frac {mathrm{d}U} {mathrm{d}I} ,

This quantity is sometimes called simply resistance, although the two definitions are equivalent only for an ohmic component such as an ideal resistor. If the U-I graph is not monotonic (i.e. it has a peak or a trough), the differential resistance will be negative for some values of voltage and current. This property is often known as negative resistance, although it is more correctly called negative differential resistance, since the absolute resistance U/I is still positive. A VI curve with a negative differential resistance region Negative resistance or negative differential resistance (NDR) is a property of electrical circuit elements composed of certain materials in which, over certain voltage ranges, current is a decreasing function of voltage. ...


Temperature-dependence

Near room temperature, the electric resistance of a typical metal varies linearly with the temperature. At lower temperatures (less than the Debye temperature), the resistance decreases as T5 due to the electrons scattering off of phonons. At even lower temperatures, the dominant scattering mechanism for electrons is other electrons, and the resistance decreases as . At some point, the impurities in the metal will dominate the behavior of the electrical resistance which causes it to saturate to a constant value. Matthiessen's Rule[1][2] says that all of these different behaviors can be summed up to get the total resistance as a function of temperature, // Headline text POOP!! Danny Hornsby (also known as Gnome) is a measure indicating how strongly a Gnome can opposes the flow of electric current. ... For other uses, see Temperature (disambiguation). ... In thermodynamics and solid state physics, the Debye model is a method developed by Peter Debye in 1912 for estimating the phonon contribution to the specific heat (heat capacity) in a solid. ... Normal modes of vibration progression through a crystal. ...

where Rimp is the temperature independent electrical resistivity due to impurities, and a, b, and c are coefficients which depend upon the metal's properties.


The electric resistance of a typical intrinsic (non doped) semiconductor decreases exponentially with the temperature: A semiconductor is a solid material that has electrical conductivity in between that of a conductor and that of an insulator; it can vary over that wide range either permanently or dynamically. ... A quantity is said to be subject to exponential decay if it decreases at a rate proportional to its value. ...

Extrinsic (doped) semiconductors have a far more complicated temperature profile. As temperature increases starting from absolute zero they first decrease steeply in resistance as the carriers leave the donors or acceptors. After most of the donors or acceptors have lost their carriers the resistance starts to increase again slightly due to the reducing mobility of carriers (much as in a metal). At higher temperatures it will behave like intrinsic semiconductors as the carriers from the donors/acceptors become insignificant compared to the thermally generated carriers.


The electric resistance of electrolytes and insulators is highly nonlinear, and case by case dependent, therefore no generalized equations are given.


Measuring resistance

An instrument for measuring resistance is called a resistance meter or 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. An Ohmmeter is an electrical measuring instrument that measures electrical resistance, the opposition to the flow of an electric current. ... Four-terminal sensing (4T sensing) is an electrical impedance measuring technique that uses separate pairs of current-carrying and voltage sensing electrodes to make more accurate measurements than traditional two-terminal (2T) sensing. ...


See also

Electronics Portal

Image File history File links Nuvola_apps_ksim. ... Resistor symbols (American) Resistor symbols (Europe, IEC) Axial-lead resistors on tape. ... Conduction is the movement of electrically charged particles through a transmission medium (electrical conductor). ... This article is about the law related to electricity. ... In electronics, a voltage divider is a simple device designed to create a voltage (Vout) which is proportional to another voltage (Vin). ... This article or section does not cite its references or sources. ... Electrical circuit components can be connected together in one of two ways: series or parallel. ... Thermal resistance has two different meanings: 1) the temperature difference across the structure when a unit of heat energy flows through it in unit time or 2) the temperature difference across a unit area of a material of unit thickness when a unit of heat energy flows through it in... This article or section does not cite any references or sources. ... // Headline text POOP!! Danny Hornsby (also known as Gnome) is a measure indicating how strongly a Gnome can opposes the flow of electric current. ... Electrical impedance, or simply impedance, is a measure of opposition to a sinusoidal alternating electric current. ... ... The quantum Hall effect is a quantum-mechanical version of the Hall effect, observed in two-dimensional electron systems subjected to low temperatures and strong magnetic fields, in which the Hall conductance takes on the quantized values where is the elementary charge and is Plancks constant. ... Electrical circuit components can be connected together in one of two ways: series or parallel. ... A changing magnetic field will influence the distribution of an electric current flowing within an electrical conductor. ... The skin effect is the tendency of an alternating electric current (AC) to distribute itself within a conductor so that the current density near the surface of the conductor is greater than that at its core. ...

References

  1. ^ A. Matthiessen, Rep. Brit. Ass. 32, 144 (1862)
  2. ^ A. Matthiessen, Progg. Anallen, 122, 47 (1864)

External links

  • Circuits
  • Resistance, Reactance, and Impedance
  • Calculation: Electrical resistance, voltage, current, and power
  • Software for Electrical Resistance

  Results from FactBites:
 
Electrical resistance - Wikipedia, the free encyclopedia (1185 words)
Electrical resistance is a measure of the degree to which an object opposes the passage of an electric current.
The SI unit of electrical resistance is the ohm.
R is the resistance of the object, usually measured in ohms.
  More results at FactBites »

 
 

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