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78-488: Sometime around 1913 several copper samples from 14 important refiners and wire manufacturers were analyzed by the U.S. Bureau of Standards . The average resistance of the samples was determined to be 0.15292 Ω for copper wires with a mass of 1 gram of uniform cross section and 1 meter in length at 20 °C. In the United States this is usually written as "0.15292 ohm (meter, gram) at 20 °C". Germany proposed

156-733: A bridge circuit . The cathode-ray oscilloscope works by amplifying the voltage and using it to deflect an electron beam from a straight path, so that the deflection of the beam is proportional to the voltage. A common voltage for flashlight batteries is 1.5 volts (DC). A common voltage for automobile batteries is 12 volts (DC). Common voltages supplied by power companies to consumers are 110 to 120 volts (AC) and 220 to 240 volts (AC). The voltage in electric power transmission lines used to distribute electricity from power stations can be several hundred times greater than consumer voltages, typically 110 to 1200 kV (AC). The voltage used in overhead lines to power railway locomotives

234-430: A capacitor ), and from an electromotive force (e.g., electromagnetic induction in a generator ). On a macroscopic scale, a potential difference can be caused by electrochemical processes (e.g., cells and batteries), the pressure-induced piezoelectric effect , and the thermoelectric effect . Since it is the difference in electric potential, it is a physical scalar quantity . A voltmeter can be used to measure

312-423: A chordal resistance or static resistance , since it corresponds to the inverse slope of a chord between the origin and an I – V curve . In other situations, the derivative d V d I {\textstyle {\frac {\mathrm {d} V}{\mathrm {d} I}}} may be most useful; this is called the differential resistance . In the hydraulic analogy , current flowing through

390-410: A linear approximation is typically used: R ( T ) = R 0 [ 1 + α ( T − T 0 ) ] {\displaystyle R(T)=R_{0}[1+\alpha (T-T_{0})]} where α {\displaystyle \alpha } is called the temperature coefficient of resistance , T 0 {\displaystyle T_{0}}

468-532: 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 below). Complex numbers are used to keep track of both

546-406: 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. For many materials, the current I through the material is proportional to

624-420: A conductor of uniform cross section, therefore, can be computed as R = ρ ℓ A , G = σ A ℓ . {\displaystyle {\begin{aligned}R&=\rho {\frac {\ell }{A}},\\[5pt]G&=\sigma {\frac {A}{\ell }}\,.\end{aligned}}} where ℓ {\displaystyle \ell }

702-671: A design engineer. A measurement of the electric conductivity of aluminum alloys can be used to verify that a heat treatment process has been done correctly. For example a component made of "7075" alloy which was correctly treated with the process "T73" to gain resistance to stress corrosion cracking will fall in the range of 38.0 to 43.0 % IACS. The acceptance criteria for electrical conductivity of finished or semi-finished parts of wrought aluminum alloys are contained in SAE International specification AMS2658 Hardness and Conductivity Inspection of Wrought Aluminum Alloy Parts. Here

780-493: A measure of its opposition to the flow of electric current . Its reciprocal quantity is electrical conductance , measuring the ease with which an electric current passes. Electrical resistance shares some conceptual parallels with mechanical friction . The SI unit of electrical resistance is the ohm ( Ω ), while electrical conductance is measured in siemens (S) (formerly called the 'mho' and then represented by ℧ ). The resistance of an object depends in large part on

858-654: A slight modification of this value to "0.15328 ohm (meter, gram) at 20 °C", this being equivalent to a conductivity of exactly 58 × 10  S /m at 20 °C. The German modification was adopted by the International Electrotechnical Commission in 1913 and subsequently published by the United States Department of Commerce on October 1, 1914 as the International Annealed Copper Standard (IACS). The standard

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936-432: A straight line through the origin with positive slope . Other components and materials used in electronics do not obey Ohm's law; the current is not proportional to the voltage, so the resistance varies with the voltage and current through them. These are called nonlinear or non-ohmic . Examples include diodes and fluorescent lamps . The resistance of a given object depends primarily on two factors: what material it

1014-457: A voltage drop that interferes with the measurement, so more accurate devices use four-terminal sensing . Many electrical elements, such as diodes and batteries do not satisfy Ohm's law . These are called non-ohmic or non-linear , and their current–voltage curves are not straight lines through the origin. Resistance and conductance can still be defined for non-ohmic elements. However, unlike ohmic resistance, non-linear resistance

1092-423: A well-defined voltage between nodes in the circuit, since the electric force is not a conservative force in those cases. However, at lower frequencies when the electric and magnetic fields are not rapidly changing, this can be neglected (see electrostatic approximation ). The electric potential can be generalized to electrodynamics, so that differences in electric potential between points are well-defined even in

1170-415: A wire (or resistor ) is like water flowing through a pipe, and the voltage drop across the wire is like the pressure drop that 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. The voltage drop (i.e., difference between voltages on one side of the resistor and

1248-465: Is a fixed reference temperature (usually room temperature), and R 0 {\displaystyle R_{0}} is the resistance at temperature T 0 {\displaystyle T_{0}} . The parameter α {\displaystyle \alpha } is an empirical parameter fitted from measurement data. Because the linear approximation is only an approximation, α {\displaystyle \alpha }

1326-463: Is a well-defined voltage across the inductor's terminals. This is the reason that measurements with a voltmeter across an inductor are often reasonably independent of the placement of the test leads. The volt (symbol: V ) is the derived unit for electric potential , voltage, and electromotive force . The volt is named in honour of the Italian physicist Alessandro Volta (1745–1827), who invented

1404-548: Is affected by thermodynamics. The quantity measured by a voltmeter is the negative of the difference of the electrochemical potential of electrons ( Fermi level ) divided by the electron charge and commonly referred to as the voltage difference, while the pure unadjusted electrostatic potential (not measurable with a voltmeter) is sometimes called Galvani potential . The terms "voltage" and "electric potential" are ambiguous in that, in practice, they can refer to either of these in different contexts. The term electromotive force

1482-427: Is between 12 kV and 50 kV (AC) or between 0.75 kV and 3 kV (DC). Inside a conductive material, the energy of an electron is affected not only by the average electric potential but also by the specific thermal and atomic environment that it is in. When a voltmeter is connected between two different types of metal, it measures not the electrostatic potential difference, but instead something else that

1560-417: Is defined so that negatively charged objects are pulled towards higher voltages, while positively charged objects are pulled towards lower voltages. Therefore, the conventional current in a wire or resistor always flows from higher voltage to lower voltage. Historically, voltage has been referred to using terms like "tension" and "pressure". Even today, the term "tension" is still used, for example within

1638-453: Is different for different reference temperatures. For this reason it is usual to specify the temperature that α {\displaystyle \alpha } was measured at with a suffix, such as α 15 {\displaystyle \alpha _{15}} , and the relationship only holds in a range of temperatures around the reference. The temperature coefficient α {\displaystyle \alpha }

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1716-414: Is exactly -90° or +90°, respectively, and X and B are nonzero. Ideal resistors have an angle of 0°, since X is zero (and hence B also), and Z and Y reduce to R and G respectively. In general, AC systems are designed to keep the phase angle close to 0° as much as possible, since it reduces the reactive power , which does no useful work at a load. In a simple case with an inductive load (causing

1794-452: Is higher than expected. Similarly, if two conductors near each other carry AC current, their resistances 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 , or large power cables carrying more than a few hundred amperes. The resistivity of different materials varies by an enormous amount: For example,

1872-603: Is made into a closed loop, current flows around the loop forever. Superconductors require cooling to temperatures near 4 K with liquid helium for most metallic superconductors like niobium–tin alloys, or cooling to temperatures near 77 K with liquid nitrogen for the expensive, brittle and delicate ceramic high temperature superconductors . Nevertheless, there are many technological applications of superconductivity , including superconducting magnets . Voltage Voltage , also known as (electrical) potential difference , electric pressure , or electric tension

1950-411: 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

2028-435: 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 R and conductance G of

2106-551: Is most often used as a comparative property in the specification of the conductivity of other metals. For example, the conductivity of a particular grade of titanium may be specified as 1.2 % IACS, meaning that its electrical conductivity is 1.2 % of the copper specified as the IACS standard. The standard can be found at https://archive.org/stream/copperwiretables31unituoft#page/n0/mode/1up . Heat treatment will alter several properties of an alloy, many of which are important to

2184-459: 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. For this reason, the geometrical cross-section is different from the effective cross-section in which current actually flows, so resistance

2262-506: Is not constant but varies with the voltage or current through the device; i.e., its operating point . There are two types of resistance: Also called chordal or DC resistance This corresponds to the usual definition of resistance; the voltage divided by the current R s t a t i c = V I . {\displaystyle R_{\mathrm {static} }={V \over I}.} Also called dynamic , incremental , or small-signal resistance It

2340-399: 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:

2418-438: Is only true in the special cases of either DC or reactance-free current. The complex angle   θ = arg ⁡ ( Z ) = − arg ⁡ ( Y )   {\displaystyle \ \theta =\arg(Z)=-\arg(Y)\ } is the phase difference between the voltage and current passing through a component with impedance Z . For capacitors and inductors , this angle

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2496-417: Is related to their microscopic structure and electron configuration , and is quantified by a property called resistivity . In addition to geometry and material, there are various other factors that influence resistance and conductance, such as temperature; see below . Substances in which electricity can flow are called conductors . A piece of conducting material of a particular resistance meant for use in

2574-1012: Is resistance, G is conductance, X is reactance , and B is susceptance . These lead to the complex number identities R = G   G 2 + B 2     , X = − B     G 2 + B 2     , G = R   R 2 + X 2     , B = − X     R 2 + X 2     , {\displaystyle {\begin{aligned}R&={\frac {G}{\ G^{2}+B^{2}\ }}\ ,\qquad &X={\frac {-B~}{\ G^{2}+B^{2}\ }}\ ,\\G&={\frac {R}{\ R^{2}+X^{2}\ }}\ ,\qquad &B={\frac {-X~}{\ R^{2}+X^{2}\ }}\ ,\end{aligned}}} which are true in all cases, whereas   R = 1 / G   {\displaystyle \ R=1/G\ }

2652-422: Is shining on them. Therefore, they are called photoresistors (or light dependent resistors ). These are a common type of light detector . 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

2730-548: Is that the resistivity itself may depend on frequency (see Drude model , deep-level traps , resonant frequency , Kramers–Kronig relations , etc.) 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

2808-547: Is the joule per coulomb , where 1 volt = 1 joule (of work) per 1 coulomb of charge. The old SI definition for volt used power and current ; starting in 1990, the quantum Hall and Josephson effect were used, and in 2019 physical constants were given defined values for the definition of all SI units. Voltage is denoted symbolically by Δ V {\displaystyle \Delta V} , simplified V , especially in English -speaking countries. Internationally,

2886-406: Is the derivative of the voltage with respect to the current; the slope of the current–voltage curve at a point R d i f f = d V d I . {\displaystyle R_{\mathrm {diff} }={{\mathrm {d} V} \over {\mathrm {d} I}}.} When an alternating current flows through a circuit, the relation between current and voltage across

2964-475: Is the difference in electric potential between two points. In a static electric field , it corresponds to the work needed per unit of charge to move a positive test charge from the first point to the second point. In the International System of Units (SI), the derived unit for voltage is the volt (V) . The voltage between points can be caused by the build-up of electric charge (e.g.,

3042-530: Is the intensity of the electric field. In this case, the voltage increase from point A to point B is equal to the work done per unit charge, against the electric field, to move the charge from A to B without causing any acceleration. Mathematically, this is expressed as the line integral of the electric field along that path. In electrostatics, this line integral is independent of the path taken. Under this definition, any circuit where there are time-varying magnetic fields, such as AC circuits , will not have

3120-462: Is the length of the conductor, measured in metres (m), A is the cross-sectional area of the conductor measured in square metres (m ), σ ( sigma ) is the electrical conductivity measured in siemens per meter (S·m ), 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

3198-450: Is the sum of the voltage between A and B and the voltage between B and C . The various voltages in a circuit can be computed using Kirchhoff's circuit laws . When talking about alternating current (AC) there is a difference between instantaneous voltage and average voltage. Instantaneous voltages can be added for direct current (DC) and AC, but average voltages can be meaningfully added only when they apply to signals that all have

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3276-409: Is typically +3 × 10  K−1 to +6 × 10  K−1 for metals near room temperature. It is usually negative for semiconductors and insulators, with highly variable magnitude. 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

3354-430: The voltaic pile , possibly the first chemical battery . A simple analogy for an electric circuit is water flowing in a closed circuit of pipework , driven by a mechanical pump . This can be called a "water circuit". The potential difference between two points corresponds to the pressure difference between two points. If the pump creates a pressure difference between two points, then water flowing from one point to

3432-444: 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 exposed to light. See below . An instrument for measuring resistance is called an ohmmeter . Simple ohmmeters cannot measure low resistances accurately because the resistance of their measuring leads causes

3510-413: The circuit are not negligible, then their effects can be modelled by adding mutual inductance elements. In the case of a physical inductor though, the ideal lumped representation is often accurate. This is because the external fields of inductors are generally negligible, especially if the inductor has a closed magnetic path . If external fields are negligible, we find that is path-independent, and there

3588-400: The conductance G is the reciprocal: R = V I , G = I V = 1 R . {\displaystyle R={\frac {V}{I}},\qquad G={\frac {I}{V}}={\frac {1}{R}}.} For a wide variety of materials and conditions, V and I are directly proportional to each other, and therefore R and G are constants (although they will depend on

3666-468: The conductivity of teflon is about 10 times lower than the conductivity of copper. Loosely speaking, this is because metals have large numbers of "delocalized" electrons that are not stuck in any one place, so they are free to move across large distances. In an insulator, such as Teflon, each electron is tightly bound to a single molecule so a great force is required to pull it away. Semiconductors lie between these two extremes. More details can be found in

3744-589: 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. Some resistors, particularly those made from semiconductors , exhibit photoconductivity , meaning that their resistance changes when light

3822-431: The device with respect to a common reference point (or ground ). The voltage drop is the difference between the two readings. Two points in an electric circuit that are connected by an ideal conductor without resistance and not within a changing magnetic field have a voltage of zero. Any two points with the same potential may be connected by a conductor and no current will flow between them. The voltage between A and C

3900-446: The electric field in the region exterior to each component is conservative, and voltages between nodes in the circuit are well-defined, where as long as the path of integration does not pass through the inside of any component. The above is the same formula used in electrostatics. This integral, with the path of integration being along the test leads, is what a voltmeter will actually measure. If uncontained magnetic fields throughout

3978-428: The electric field, rather than to differences in electric potential. In this case, the voltage rise along some path P {\displaystyle {\mathcal {P}}} from r A {\displaystyle \mathbf {r} _{A}} to r B {\displaystyle \mathbf {r} _{B}} is given by: However, in this case the "voltage" between two points depends on

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4056-401: 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: P = I 2 R {\displaystyle P=I^{2}R} 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

4134-422: The material it is made of. Objects made of electrical insulators like rubber tend to have very high resistance and low conductance, while objects made of electrical conductors like metals tend to have very low resistance and high conductance. This relationship is quantified by resistivity or conductivity . The nature of a material is not the only factor in resistance and conductance, however; it also depends on

4212-399: The material the wire is made of, not the geometry of the wire. Resistivity and conductivity are reciprocals : ρ = 1 / σ {\displaystyle \rho =1/\sigma } . Resistivity is a measure of the material's ability to oppose electric current. This formula is not exact, as it assumes the current density is totally uniform in the conductor, which

4290-407: The other will be able to do work, such as driving a turbine . Similarly, work can be done by an electric current driven by the potential difference provided by a battery . For example, the voltage provided by a sufficiently-charged automobile battery can "push" a large current through the windings of an automobile's starter motor . If the pump is not working, it produces no pressure difference, and

4368-409: 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

4446-424: The path taken. In circuit analysis and electrical engineering , lumped element models are used to represent and analyze circuits. These elements are idealized and self-contained circuit elements used to model physical components. When using a lumped element model, it is assumed that the effects of changing magnetic fields produced by the circuit are suitably contained to each element. Under these assumptions,

4524-1245: The phase and magnitude of current and voltage: u ( t ) = R e ⁡ ( U 0 ⋅ e j ω t ) i ( t ) = R e ⁡ ( I 0 ⋅ e j ( ω t + φ ) ) Z = U   I   Y =   1   Z =   I   U {\displaystyle {\begin{array}{cl}u(t)&=\operatorname {\mathcal {R_{e}}} \left(U_{0}\cdot e^{j\omega t}\right)\\i(t)&=\operatorname {\mathcal {R_{e}}} \left(I_{0}\cdot e^{j(\omega t+\varphi )}\right)\\Z&={\frac {U}{\ I\ }}\\Y&={\frac {\ 1\ }{Z}}={\frac {\ I\ }{U}}\end{array}}} where: The impedance and admittance may be expressed as complex numbers that can be broken into real and imaginary parts: Z = R + j X Y = G + j B   . {\displaystyle {\begin{aligned}Z&=R+jX\\Y&=G+jB~.\end{aligned}}} where R

4602-491: The phase to increase), a capacitor may be added for compensation at one frequency, since the capacitor's phase shift is negative, bringing the total impedance phase closer to 0° again. Y is the reciprocal of Z (   Z = 1 / Y   {\displaystyle \ Z=1/Y\ } ) for all circuits, just as R = 1 / G {\displaystyle R=1/G} for DC circuits containing only resistors, or AC circuits for which either

4680-697: The phrase " high tension " (HT) which is commonly used in thermionic valve ( vacuum tube ) based and automotive electronics. In electrostatics , the voltage increase from point r A {\displaystyle \mathbf {r} _{A}} to some point r B {\displaystyle \mathbf {r} _{B}} is given by the change in electrostatic potential V {\textstyle V} from r A {\displaystyle \mathbf {r} _{A}} to r B {\displaystyle \mathbf {r} _{B}} . By definition, this is: where E {\displaystyle \mathbf {E} }

4758-409: The pipe, which tries to push water back up through the pipe. If these pressures are equal, no water flows. (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 mostly determined by two properties: Geometry is important because it is more difficult to push water through a long, narrow pipe than a wide, short pipe. In

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4836-430: The points across which the voltage is measured. When using a voltmeter to measure voltage, one electrical lead of the voltmeter must be connected to the first point, one to the second point. A common use of the term "voltage" is in describing the voltage dropped across an electrical device (such as a resistor). The voltage drop across the device can be understood as the difference between measurements at each terminal of

4914-414: The presence of time-varying fields. However, unlike in electrostatics, the electric field can no longer be expressed only in terms of the electric potential. Furthermore, the potential is no longer uniquely determined up to a constant, and can take significantly different forms depending on the choice of gauge . In this general case, some authors use the word "voltage" to refer to the line integral of

4992-404: The reactance or susceptance happens to be zero ( X or B = 0 , respectively) (if one is zero, then for realistic systems both must be zero). A key feature of AC circuits is that the resistance and conductance can be frequency-dependent, a phenomenon known as the universal dielectric response . One reason, mentioned above is the skin effect (and the related proximity effect ). Another reason

5070-411: 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

5148-435: 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,

5226-409: The resistor. 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,

5304-415: The same frequency and phase. Instruments for measuring voltages include the voltmeter , the potentiometer , and the oscilloscope . Analog voltmeters , such as moving-coil instruments, work by measuring the current through a fixed resistor, which, according to Ohm's law , is proportional to the voltage across the resistor. The potentiometer works by balancing the unknown voltage against a known voltage in

5382-555: 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. Similarly, electrons can flow freely and easily through a copper wire, but cannot flow as easily 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

5460-420: The size and shape of an object because these properties are extensive rather than intensive . For example, a wire's resistance is higher if it is long and thin, and lower if it is short and thick. All objects resist electrical current, except for superconductors , which have a resistance of zero. The resistance R of an object is defined as the ratio of voltage V across it to current I through it, while

5538-409: The size and shape of the object, the material it is made of, and 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 transformer , diode or battery , V and I are not directly proportional. The ratio ⁠ V / I ⁠ is sometimes still useful, and is referred to as

5616-465: The symbol U is standardized. It is used, for instance, in the context of Ohm's or Kirchhoff's circuit laws . The electrochemical potential is the voltage that can be directly measured with a voltmeter. The Galvani potential that exists in structures with junctions of dissimilar materials is also work per charge but cannot be measured with a voltmeter in the external circuit (see § Galvani potential vs. electrochemical potential ). Voltage

5694-401: The turbine will not rotate. Likewise, if the automobile's battery is very weak or "dead" (or "flat"), then it will not turn the starter motor. The hydraulic analogy is a useful way of understanding many electrical concepts. In such a system, the work done to move water is equal to the " pressure drop" (compare p.d.) multiplied by the volume of water moved. Similarly, in an electrical circuit,

5772-459: The values are given in reference to the IACS. A method for measuring electrical conductivity is described in ASTM International specification ASTM E 1004 Electromagnetic (Eddy-Current) Measurements of Electrical Conductivity. Electrical conductivity meters with direct readout in %IACS are commercially available. Electrical resistance The electrical resistance of an object is

5850-481: The voltage V applied across it: I ∝ V {\displaystyle I\propto V} over a wide range of voltages and currents. Therefore, the resistance and conductance of objects or electronic components made of these materials is constant. This relationship is called Ohm's law , and materials which obey it are called ohmic materials. Examples of ohmic components are wires and resistors . The current–voltage graph of an ohmic device consists of

5928-400: The voltage between two points in a system. Often a common reference potential such as the ground of the system is used as one of the points. In this case, voltage is often mentioned at a point without completely mentioning the other measurement point. A voltage can be associated with either a source of energy or the loss, dissipation, or storage of energy. The SI unit of work per unit charge

6006-465: The work done to move electrons or other charge carriers is equal to "electrical pressure difference" multiplied by the quantity of electrical charges moved. In relation to "flow", the larger the "pressure difference" between two points (potential difference or water pressure difference), the greater the flow between them (electric current or water flow). (See " electric power ".) Specifying a voltage measurement requires explicit or implicit specification of

6084-612: Was first used by Volta in a letter to Giovanni Aldini in 1798, and first appeared in a published paper in 1801 in Annales de chimie et de physique . Volta meant by this a force that was not an electrostatic force, specifically, an electrochemical force. The term was taken up by Michael Faraday in connection with electromagnetic induction in the 1820s. However, a clear definition of voltage and method of measuring it had not been developed at this time. Volta distinguished electromotive force (emf) from tension (potential difference):

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