Molly Ayala
The SI unit of electrical resistance is the ohm, represented by the Greek letter Ω. The ohm is named after German physicist Georg Ohm. The unit was developed in the 19th century, when the rapid rise of electrotechnology created a demand for a coherent system of units for electrical quantities. 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).
| Characteristics | Values |
|---|---|
| SI unit of electrical resistance | Ohm (Ω) |
| Symbol | Ω, the uppercase Greek letter omega |
| Named after | German physicist Georg Ohm |
| 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), the conductor not being the seat of any electromotive force |
| Formula | 1 Ω = 1 V/A |
| Non-linear resistors | Value may vary depending on the applied voltage (or current) |
| Linear resistors | Resistance is approximately constant within a certain range of voltages, temperatures, and other parameters |
| Conductance | Reciprocal quantity of electrical resistance, measuring the ease with which an electric current passes |
| Siemens (S) | SI derived unit of electric conductance and admittance, historically known as the "mho" (represented by ℧); it is one reciprocal ohm: 1 S = 1 Ω-1 |
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What You'll Learn
The SI unit for electrical resistance is the ohm, Ω
The ohm is represented by the uppercase Greek letter omega (Ω). The symbol Ω is also used to represent angular velocity in physics and geometry. The use of Ω as a symbol for the unit of electrical resistance is distinct from its use as a symbol for angular velocity. The ohm is a coherent unit within the International System of Units (SI), where each quantity has a corresponding unit: watt for power, ohm for resistance, volt for voltage, and ampere for current.
The ohm is derived from early empirical standard units for electrical resistance, which were developed in connection with telegraphy practice in the 19th century. Telegraphers and other early users of electricity needed a practical standard unit of measurement for resistance, as resistance was often expressed as a multiple of the resistance of a standard length of telegraph wire. The British Association for the Advancement of Science proposed a unit derived from existing units of mass, length, and time as early as 1861.
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 ohm is now an integral part of this system, used to measure electrical resistance in a wide range of applications.
The electrical resistance of an object is a measure of its opposition to the flow of electric current. It is influenced by the material's microscopic structure and electron configuration, also known as resistivity. The resistance of an object also depends on its size and shape, as these properties are extensive rather than intrinsic. 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.
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Electrical conductance is measured in siemens (S)
The SI unit of electrical resistance is the ohm, represented by the Greek letter Ω (omega). It 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 belongs to a coherent system of units, with each quantity having its corresponding SI unit: watt for power, ohm for resistance, volt for voltage, and ampere for current. This formula remains valid when these units are used, and they can be thought of as being cancelled or omitted.
The electrical resistance of an object is a measure of its opposition to the flow of electric current. 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 conductors like metals tend to have very low resistance.
The reciprocal quantity of electrical resistance is electrical conductance, which measures the ease with which an electric current passes. Electrical conductance is measured in Siemens (S), formerly called the 'mho' and represented by the symbol ℧. The Siemens is one reciprocal ohm: 1 S = 1 Ω-1.
The resistance and conductance of a wire, resistor, or other element are mostly determined by two properties: geometry and material. For example, a long, thin copper wire has higher resistance (lower conductance) than a short, thick copper wire. Materials are also important; electrons can flow freely through a copper wire but not as easily through a steel wire of the same shape and size.
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Resistance is influenced by material and shape
The SI 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 a current of one ampere (A).
Resistance is influenced by both the material and shape of an object. The nature of the material is a primary factor in determining resistance. Materials with high electrical conductivity, such as metals, tend to have very low resistance and high conductance. On the other hand, electrical insulators like rubber have high resistance and low conductance. The difference in resistance between materials like copper, steel, and rubber is related to their microscopic structure, electron configuration, and resistivity.
The geometry and shape of an object also play a significant role in influencing resistance. For a given material, resistance is inversely proportional to the cross-sectional area. For example, a thick wire will have lower resistance compared to a thin wire of the same length. Additionally, resistance is proportional to the length of the object; a long wire will have higher resistance than a shorter one of the same thickness. This relationship can be observed in Ohm's law, where the resistance of objects is dependent on their size and shape, in addition to the material they are made of.
Other factors that influence resistance include temperature, frequency of the current, and the presence of impurities. Temperature changes can affect resistance, typically increasing it in conductors and decreasing it in semiconductors. Higher frequencies of current can impact resistance by affecting the movement of electrons within the conductor. Finally, impurities in the material can obstruct the flow of electrons, increasing resistance.
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Resistivity is an intrinsic property
The ohm (Ω), represented by the uppercase Greek letter omega, is the SI unit of electrical resistance. 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, which is not the seat of any electromotive force.
The resistivity of a material is influenced by factors such as its chemical composition and temperature. For instance, the difference in resistivity between copper, steel, and rubber is due to their distinct microscopic structures and electron configurations.
Resistivity is also related to the scattering time of electrons within a material. The Drude formula expresses resistivity in terms of the scattering time resulting from collisions of electrons with impurities, photons, and other electrons.
In summary, resistivity is an intrinsic property of a material that quantifies its opposition to the flow of electric current. It is expressed in ohm-metres and is influenced by the material's chemical composition, temperature, and electron behaviour.
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Resistance and temperature are inversely proportional
The SI unit of electrical resistance is the ohm (Ω), which is the Greek letter omega. This unit was 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).
Resistance and temperature are related. As the temperature of a conductor increases, so does the resistance. This is because the speed at which electrons travel is affected by temperature, which changes how electricity flows across an electrical circuit. However, this is not true for all materials. For example, as the temperature of an NTC thermistor rises, the resistance lowers.
Resistance is also impacted by the material and shape of an object. 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. For a given material, the resistance is inversely proportional to the cross-sectional area; a thick copper wire has lower resistance than a thin copper wire. Additionally, for a given material, the resistance is proportional to the length; a long copper wire has higher resistance than a short copper wire.
Resistivity is the intrinsic property of a material that is defined as the measure of a material's resistance to the flow of an electric current. Resistivity is the reciprocal of conductivity, meaning that the higher the conductivity, the lower the resistivity of the material. Resistivity is indirectly proportional to temperature. As the temperature of a material increases, its resistivity decreases.
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Frequently asked questions
The SI unit for electrical resistance is the ohm, represented by the Greek letter Ω (omega).
Electrical resistance measures how much electric charge is restricted within a circuit. All objects resist electric current to some degree, except for superconductors, which have zero resistance.
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.
Resistance depends on the geometry of an object, whereas resistivity is an intrinsic property and does not depend on geometric properties. For example, all pure copper wires have the same resistivity, but a long, thin copper wire has a much larger resistance than a thick, short wire.
