Electrical resistance and conductance

Introduction to Electrical Resistance and Conductance
– Electrical resistance is a measure of opposition to electric current.
– Electrical conductance measures the ease of electric current flow.
– Resistance and conductance are analogous to mechanical friction.
– The SI unit of resistance is the ohm (Ω), while conductance is measured in siemens (S).
– Resistance and conductance depend on the material, size, and shape of an object.
– Conductors are materials that allow electricity to flow easily.
– Resistors are materials with a specific resistance used in circuits.
– Copper and aluminum are common conductors.
– Resistors are made of various materials based on resistance, energy dissipation, precision, and cost.
– Conductors and resistors play different roles in electrical circuits.

Ohm’s Law and Relationship to Resistivity and Conductivity
– Ohm’s law states that the current through a conductor is proportional to the voltage across it.
– Ohmic materials follow Ohm’s law and have a linear current-voltage relationship.
– Examples of ohmic components are wires and resistors.
– Non-ohmic components, like diodes and fluorescent lamps, do not follow Ohm’s law.
– Non-ohmic devices have a curved current-voltage characteristic.
– Resistance depends on the material and shape of an object.
– Resistance is inversely proportional to cross-sectional area.
– Resistance is directly proportional to length.
– Conductivity is the reciprocal of resistance.
– Resistivity quantifies the material’s resistance to electric current.

Factors Influencing Resistance and Conductance
– Geometry, such as the shape and size of an object, affects resistance.
– Materials with impurities or obstacles restrict electric current flow.
– Temperature can affect resistance, with some materials becoming more conductive at higher temperatures.
– Other factors, such as strain or pressure, can also influence resistance.
– Superconductors have zero resistance and are exceptions to the general resistance-conductance relationship.

Measurement and Typical Values of Resistance
– Electrical resistance is measured using an ohmmeter.
– Simple ohmmeters may not be accurate for low resistances due to voltage drop in the measuring leads.
– More accurate devices use four-terminal sensing to minimize interference.
– The resistance of a 1-meter copper wire with 1mm diameter is approximately 0.02 Ω.
– The resistance of a typical 1km overhead power line is around 0.03 Ω.
– The internal resistance of an AA battery is typically 0.1 Ω.
– The resistance of a typical incandescent light bulb filament ranges from 200 to 1000 Ω.
– The resistance of the human body can vary from 1000 to 100,000 Ω.

Applications and Phenomena Related to Resistance and Conductance
– Impedance and admittance are used to describe the behavior of electrical elements in AC circuits.
– Impedance is a complex number that includes resistance and reactance.
– Reactance can be either inductive (positive) or capacitive (negative).
– Admittance is the reciprocal of impedance and includes conductance and susceptance.
– The phase difference between voltage and current is an important factor in AC circuits.
– Resistors oppose the flow of electric current, requiring electrical energy to push current through the resistance.
– Joule heating, also known as ohmic heating or resistive heating, is the phenomenon of heat generation when current passes through a material with resistance.
– Transmission losses in power lines are a result of the dissipation of electrical energy, which is often undesired.
– Superconductors have zero resistance and infinite conductance, resulting in no dissipation of electrical energy.
– The resistivity of metals typically increases with temperature, while the resistivity of semiconductors typically decreases with temperature.
– The resistance of wires, resistors, and other components often changes with temperature, which can cause circuit malfunction at extreme temperatures.
– Resistance thermometers and thermistors utilize temperature-dependent resistance for measuring and feedback purposes.
– The resistance of a conductor also depends on strain, with tension increasing resistance and compression decreasing resistance.
– Some resistors made from semiconductors exhibit photoconductivity, causing their resistance to change when exposed to light. Source:  https://en.wikipedia.org/wiki/Electrical_resistance

Electrical resistance and conductance (Wikipedia)

This article is about specific applications of conductivity and resistivity in electrical elements. For other types of conductivity, see Conductivity. For electrical conductivity in general, see Electrical resistivity and conductivity.

"Resistive" redirects here. For the term used when referring to touchscreens, see Resistive touchscreen.

The electrical resistance of an object is 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 ℧).

Electric resistance
Common symbols	R
SI unit	ohm (Ω)
In SI base units	kg⋅m2⋅s−3⋅A−2
Dimension	${\displaystyle {\mathsf {M}}{\mathsf {L}}^{2}{\mathsf {T}}^{-3}{\mathsf {I}}^{-2}}$

Electric conductance
Common symbols	G
SI unit	siemens (S)
In SI base units	kg−1⋅m−2⋅s3⋅A2
Dimension	${\displaystyle {\mathsf {M}}^{-1}{\mathsf {L}}^{-2}{\mathsf {T}}^{3}{\mathsf {I}}^{2}}$

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 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 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 the conductance G is the reciprocal:

$${\displaystyle R={\frac {V}{I}},\qquad G={\frac {I}{V}}={\frac {1}{R}}.}$$

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 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 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 ${\textstyle {\frac {\mathrm {d} V}{\mathrm {d} I}}}$ may be most useful; this is called the differential resistance.