# Electrical resistance and conductance

**resistanceelectrical resistanceconductanceresistiveelectrical conductanceelectric conductanceelectric resistanceresistancesohmic resistanceresistor**

The electrical resistance of an object is a measure of its opposition to the flow of electric current.wikipedia

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### Ohm

**Ωohmsmegohm**

The SI unit of electrical resistance is the ohm, while electrical conductance is measured in siemens (S).

The ohm (symbol: Ω) is the SI derived unit of electrical resistance, named after German physicist Georg Simon Ohm.

### Superconductivity

**superconductingsuperconductorsuperconductors**

All objects show some resistance, except for superconductors, which have a resistance of zero.

Superconductivity is the set of physical properties observed in certain materials, wherein electrical resistance vanishes and from which magnetic flux fields are expelled.

### Siemens (unit)

**siemensmicro-siemensmho**

The SI unit of electrical resistance is the ohm, while electrical conductance is measured in siemens (S).

The siemens (symbol: S) is the derived unit of electric conductance, electric susceptance, and electric admittance in the International System of Units (SI).

### International System of Units

**SISI unitsSI unit**

The SI unit of electrical resistance is the ohm, while electrical conductance is measured in siemens (S).

Derived units apply to derived quantities, which may by definition be expressed in terms of base quantities, and thus are not independent; for example, electrical conductance is the inverse of electrical resistance, with the consequence that the siemens is the inverse of the ohm, and similarly, the ohm and siemens can be replaced with a ratio of an ampere and a volt, because those quantities bear a defined relationship to each other.

### Diode

**diodessemiconductor diodegermanium diode**

In other cases, such as a transformer, diode or battery, V and I are not directly proportional.

A diode is a two-terminal electronic component that conducts current primarily in one direction (asymmetric conductance); it has low (ideally zero) resistance in one direction, and high (ideally infinite) resistance in the other.

### Ohm's law

**ohmicOhmohmic losses**

This proportionality is called Ohm's law, and materials that satisfy it are called ohmic materials.

Introducing the constant of proportionality, the resistance, one arrives at the usual mathematical equation that describes this relationship:

### Resistor

**Resistor controlresistorsResistance**

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 that pushes water through the pipe.

A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element.

### Electric current

**currentelectrical currentcurrents**

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 reciprocal:

Introducing the constant of proportionality, the resistance, one arrives at the usual mathematical equation that describes this relationship:

### Transformer

**power transformerelectrical transformerprimary winding**

In other cases, such as a transformer, diode or battery, V and I are not directly proportional.

### Electrical resistivity and conductivity

**electrical conductivityresistivityconductivity**

This material dependence is quantified by resistivity or conductivity.

### Skin effect

**skin depthconductor skin effectscurrents tend to flow on the surface of conductors**

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.

The skin effect causes the effective resistance of the conductor to increase at higher frequencies where the skin depth is smaller, thus reducing the effective cross-section of the conductor.

### Semiconductor

**semiconductorssemiconductingsemiconductor material**

Semiconductors lie between these two extremes.

Its resistance falls as its temperature rises; metals are the opposite.

### Insulator (electricity)

**insulatorinsulatorsinsulation**

The resistance of an object depends in large part on the material it is made of—objects made of electrical insulators like rubber tend to have very high resistance and low conductivity, while objects made of electrical conductors like metals tend to have very low resistance and high conductivity.

Rapidly the insulator becomes filled with mobile charge carriers, and its resistance drops to a low level.

### Alternating current

**ACalternating-currentalternating**

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.

This allows the power to be transmitted through power lines efficiently at high voltage, which reduces the energy lost as heat due to resistance of the wire, and transformed to a lower, safer, voltage for use.

### Current–voltage characteristic

**current-voltage characteristiccharacteristic curveI-V curve**

The ratio V over I is sometimes still useful, and is referred to as a "chordal resistance" or "static resistance", since it corresponds to the inverse slope of a chord between the origin and an I–V curve.

The reciprocal of the slope is equal to the resistance.

### Current density

**current densitieselectric current densitycurrent**

This formula is not exact, as it assumes the current density is totally uniform in the conductor, which is not always true in practical situations.

Most electrical conductors have a finite, positive resistance, making them dissipate power in the form of heat.

### Ohmmeter

**measuredtwo-terminal (2T) sensing**

An instrument for measuring resistance is called an ohmmeter.

An ohmmeter is an electrical instrument that measures electrical resistance, the opposition to an electric current.

### Voltage drop

**potential dropdrop the voltagedropped**

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 that pushes water through the pipe.

The DC source, the conductors (wires), the resistors, and the light bulb (the load) all have resistance; all use and dissipate supplied energy to some degree.

### Proximity effect (electromagnetism)

**proximity effectproximity**

Similarly, if two conductors near each other carry AC current, their resistances increase due to the proximity effect.

The proximity effect can significantly increase the AC resistance of adjacent conductors when compared to its resistance to a DC current.

### Negative resistance

**negative differential resistancecurrent filamentNegative differential conductivity**

This is in contrast to an ordinary resistor in which an increase of applied voltage causes a proportional increase in current due to Ohm's law, resulting in a positive resistance.

### Conductivity (electrolytic)

**conductivityelectrical conductivityelectrolytic conductivity**

For the case of electrolyte solutions, see the article: Conductivity (electrolytic).

The electrical conductivity of a solution of an electrolyte is measured by determining the resistance of the solution between two flat or cylindrical electrodes separated by a fixed distance.

### Electrical impedance

**impedanceimpedancescomplex impedance**

Impedance extends the concept of resistance to AC circuits, and possesses both magnitude and phase, unlike resistance, which has only magnitude.

### Overhead power line

**power linepower linespowerline**

The dissipation of electrical energy is often undesired, particularly in the case of transmission losses in power lines.

While larger conductors lose less energy because of their lower electrical resistance, they cost more than smaller conductors.

### Electrical reactance

**reactancereactivecapacitive reactance**

The notion of reactance is similar to electric resistance, but it differs in several respects.

### Gyrator

**gyrator circuitgyrator circuitssynthesize inductance**

where is the gyration resistance of the gyrator.