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The unit of electrical resistance is the ohm (Ω), named after German physicist Georg Ohm. The ohm is defined as the electrical resistance between two points of a conductor when a constant potential difference of one volt (V), applied to these points, produces in the conductor a current of one ampere (A). The ohm is part of the International System of Units (SI) and is used to measure how much an object opposes the passage of an electric current.
| Characteristics | Values |
|---|---|
| Unit of electrical resistance | Ohm (Ω) |
| Symbol | Ω (uppercase Greek letter omega) |
| Named after | German physicist Georg Ohm |
| Year named | 1867 |
| Derived from | Existing units of mass, length and time |
| Definition | Electrical resistance between two points of a conductor when a constant potential difference of one volt (V), applied to these points, produces in the conductor a current of one ampere (A) |
| Reciprocal quantity | Electrical conductance measured in Siemens (S) |
| Factors influencing resistance | Material, size and shape of an object, temperature |
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What You'll Learn
The ohm is the unit of electrical resistance
The ohm (Ω) is the unit of electrical resistance in the International System of Units (SI). It is named after German physicist Georg Ohm (1789–1854). The ohm is defined as the electrical resistance between two points of a conductor when a constant potential difference of one volt (V), applied to these points, produces in the conductor a current of one ampere (A), with the conductor not being the seat of any electromotive force.
The ohm is used to measure how much an object resists or opposes the flow of an electric current. This is similar to how mechanical friction works. The resistance of an object depends largely on the material it is made of. Objects made of electrical insulators like rubber tend to have very high resistance, while objects made of electrical conductors like metals tend to have very low resistance.
The ohm belongs to a coherent system of units, with each quantity having its corresponding SI unit: watt for power (P), ohm for resistance (R), volt for potential difference (V), and ampere for current (I). This system was developed in response to the rapid rise of electrotechnology in the last half of the 19th century, which created a demand for a rational, coherent, consistent, and international system of units for electrical quantities.
The British Association for the Advancement of Science proposed a unit of electrical resistance derived from existing units of mass, length, and time in 1861. The unit was subsequently named after Georg Ohm and formally defined in 1873. The ohm was then redefined in absolute terms in 1948 and again in 2019 following a revision of the SI system.
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Resistance is dependent on the material, shape, and size of an object
The ohm (Ω) is the unit of electrical resistance in the International System of Units (SI). It is named after German physicist Georg Ohm. The resistance of an object depends on its material composition, shape, and size.
Firstly, resistance depends on the material of which the object is composed. Different materials offer different resistance to the flow of charge. Resistivity (ρ) is an intrinsic property of a material, independent of its shape or size, and the resistance (R) of an object is directly proportional to ρ. Materials can be broadly categorized into conductors, semiconductors, and insulators, with conductors having the smallest resistivities and insulators having the largest. Conductors have varying but large free charge densities, while most charges in insulators are bound to atoms and are not free to move.
Secondly, resistance depends on the shape of the object. For example, the resistance of a cylinder is directly proportional to its length and inversely proportional to its cross-sectional area. The longer the cylinder, the more collisions charges will make with its atoms, increasing resistance. On the other hand, the greater the diameter of the cylinder, the more current it can carry, decreasing resistance.
Thirdly, resistance depends on the size of the object. For instance, the resistance of a given metallic wire depends on its length and thickness. Additionally, the resistance of an object can also depend on its temperature, as the resistivity of some materials, such as semiconductors, changes with temperature.
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Resistance is measured in ohms, conductance in siemens
The unit of electrical resistance is the ohm (Ω), which is the International System of Units (SI). The ohm is named after German physicist Georg Ohm (1789–1854). The ohm is defined as the electrical resistance between two points of a conductor when a constant potential difference of one volt (V), applied to these points, produces a current of one ampere (A) in the conductor, provided that the conductor is not the seat of any electromotive force. The ohm is also defined by the quantum Hall effect.
The ohm is a coherent part of a system of units for electrical quantities, alongside the watt (W), volt (V), and ampere (A). The rapid rise of electrotechnology in the latter half of the 19th century created a demand for a rational, coherent, consistent, and international system of units for electrical quantities. The British Association for the Advancement of Science proposed a unit derived from existing units of mass, length, and time, in 1861. The International Committee on Electrical Standards, with contributions by Fleeming Jenkin, James Clerk Maxwell, William Thomson, Werner von Siemens, and colleagues, advised the British Association in providing a widely recognised standard for electrical resistance.
The resistance of an object is defined as the ratio of voltage across it to the current through it. The resistance of an object depends on the material it is made of, its shape, and other factors like temperature. 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 zero resistance.
The reciprocal quantity of electrical resistance is electrical conductance, which measures how easily a current passes through an object. Electrical conductance is measured in siemens (S), previously called the "mho" (represented by ℧). One siemens is equal to the reciprocal of one ohm. 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.
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The British Association proposed a unit of resistance in 1861
The unit of electrical resistance in the International System of Units (SI) is the ohm (Ω), represented by the uppercase Greek letter omega. It is named after German physicist Georg Ohm (1789–1854). The ohm is defined as the electrical resistance between two points of a conductor when a constant potential difference of one volt (V), applied to these points, produces in the conductor a current of one ampere (A), provided that the conductor is not the seat of any electromotive force.
The ohm belongs to a coherent system of units, with each quantity having its corresponding SI unit: watt for power (P), ohm for resistance (R), volt for potential difference (V), and ampere for current (I). This formula remains valid when these units are used and thought of as being cancelled or omitted. The rapid rise of electrotechnology in the last half of the 19th century created a demand for a rational, coherent, consistent, and international system of units for electrical quantities.
Telegraphers and other early users of electricity in the 19th century needed a practical standard unit of measurement for resistance. Resistance was often expressed as a multiple of the resistance of a standard length of telegraph wires, but different agencies used different bases for standards, making units not readily interchangeable. Electrical units so defined were not coherent with the units for energy, mass, length, and time, requiring conversion factors to be used in calculations relating energy or power to resistance.
In 1861, the British Association for the Advancement of Science (BAAS) proposed a unit of resistance derived from existing units of mass, length, and time, and of a convenient scale for practical work. This proposal was made by Latimer Clark (1822–1898) and Charles Bright (1832–1888) in a paper presented at the BAAS meeting, which suggested that standards for electrical units be established and that these units be named after eminent philosophers: 'Ohma', 'Farad', and 'Volt'. The BAAS responded by appointing the British Association Committee on Electrical Standards, with Fleeming Jenkin as secretary, and led by William Thomson (later Lord Kelvin) and James Clerk Maxwell. This committee proposed the electrical units of resistance based on the metric system, which are still in use today.
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The ohm is defined as the resistance of a conductor
The ohm, represented by the uppercase Greek letter omega (Ω), is the unit of electrical resistance in the International System of Units (SI). It is named after German physicist Georg Ohm (1789–1854).
The ohm is defined as the electrical resistance between two points of a conductor when a constant potential difference of one volt (V), applied to these points, produces in the conductor a current of one ampere (A), provided that the conductor is not the seat of any electromotive force. This definition is based on the work of the International Committee on Electrical Standards, which included contributions by Fleeming Jenkin, James Clerk Maxwell, William Thomson, Werner von Siemens, and others. Their work led to the standardisation of the ohm as a unit of electrical resistance, which was subsequently named after Georg Ohm.
The ohm is a crucial concept in understanding electrical resistance and conductance. Electrical resistance is a measure of how much an object opposes the flow of electric current. All objects resist electrical current, except for superconductors, which have zero resistance. The resistance of an object depends on various factors, including the material it is made of, its size, and its shape. For example, a long, thin copper wire has higher resistance than a short, thick copper wire due to its increased length and reduced cross-sectional area.
The nature of the material is a significant factor in determining resistance. Objects made of electrical insulators, such as rubber, tend to have high resistance, while conductors like metals have low resistance. This relationship is quantified by resistivity or conductivity, which takes into account the microscopic structure and electron configuration of the material. Additionally, factors like temperature can influence resistance; as temperature increases, atoms vibrate more strongly, causing more collisions and higher resistance.
Ohm's law states that the current through a material is proportional to the voltage applied across it. This relationship holds for many materials, and objects or components made from these materials are said to have constant resistance or conductance. Examples of ohmic components include wires and resistors. However, there are also non-ohmic materials where the current is not proportional to the voltage, resulting in variable resistance.
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Frequently asked questions
The unit of electrical resistance in the International System of Units (SI) is the ohm (Ω).
Electrical resistance is a measure of how much an object resists or opposes the flow of an electric current.
Electrical resistance is calculated as the ratio of voltage across an object to the current through it.
The electrical resistance of an object depends on its material, size, and shape. For example, a long, thin copper wire has higher resistance than a short, thick wire of the same material.
