Umair Gaines
The unit of electrical resistance is the ohm (Ω), named after German physicist Georg Simon Ohm (1784-1854), who studied the relationship between voltage, current, and resistance. The ohm is part of the SI system of units, with 1 ohm being equal to 1 volt per ampere. The rapid development of electrotechnology in the 19th century led to a demand for a coherent system of units for electrical quantities, and the ohm was standardised internationally in 1893.
| Characteristics | Values |
|---|---|
| SI unit of electrical resistance | Ohm (Ω) |
| SI unit of electrical conductance | Siemens (S) |
| Reciprocal of electrical conductance | Electrical resistance |
| Reciprocal of electrical resistance | Electrical conductance |
| Reciprocal of resistivity | Conductivity |
| Unit of resistivity | Ohm-metre |
| Resistance of superconductors | Almost equal to zero |
| Resistance of a standard length of telegraph wires | Multiple of resistance |
| Resistance of a resistor | Fixed-resistance value |
| Resistance of an object | Depends on the material it is made of, size, and shape |
| Resistance of a wire | Higher if it is long and thin, and lower if it is short and thick |
| Resistance of a copper wire | Higher than steel wire, lower than insulator like rubber |
| Resistance of a material | Depends on its microscopic structure and electron configuration |
| Resistance of a component or a circuit | Measured to indicate the condition |
| High or infinite resistance | Indicates an open circuit |
| Very low or zero resistance | Indicates a short circuit |
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What You'll Learn
- The unit of electrical resistance is the ohm, symbolised by Ω
- The ohm is named after Georg Simon Ohm, a German physicist
- The SI unit of electrical conductance is the Siemens (S)
- The formula for resistance is V = IR, also known as Ohm's law
- Resistance is influenced by a material's properties, length, cross-sectional area and temperature
The unit of electrical resistance is the ohm, symbolised by Ω
The ohm was formally defined by an international conference of electricians in Paris in 1884 as the resistance of a mercury column of a specified weight and 106 cm long. This standard was not adopted by any national legislation, but it did serve as the basis for the legal definition of the ohm in several countries. The international ohm was later recommended by the International Electrical Congress in 1893, which defined the ohm as equal to 109 units of resistance of the C.G.S. system of electromagnetic units.
The ohm is used to measure the opposition to the flow of current in an electrical circuit, which is influenced by the material's properties, length, cross-sectional area, and temperature. For example, a wire's resistance is higher if it is long and thin, and lower if it is short and thick. Materials with high resistance include rubber, paper, glass, wood, and plastic, while materials with low resistance include metals such as copper and aluminium.
The ohm is also related to the unit of electric conductance, the siemens (S), which was historically known as the "mho" (ohm spelled backwards). The siemens is the reciprocal of the ohm, so one siemens is equal to one ohm to the power of negative one.
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The ohm is named after Georg Simon Ohm, a German physicist
The unit of electrical resistance is the ohm (Ω), named after Georg Simon Ohm (1784-1854), a German physicist who studied the relationship between voltage, current, and resistance. He is credited for formulating Ohm's Law, which states that the current–voltage graph of an ohmic device consists of a straight line through the origin with a positive slope. In other words, for a wide variety of materials and conditions, voltage and current are directly proportional, and therefore resistance and conductance are constants.
The ohm is part of the SI system of units, with 1 ohm being equal to 1 volt per 1 ampere. 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.
A "legal" ohm was defined by the international conference of electricians in Paris in 1884 as the resistance of a mercury column of specified weight and 106 cm long. However, this standard was never adopted by any national legislation. The "international" ohm was later recommended by unanimous resolution at the International Electrical Congress in 1893 in Chicago, based on the C.G.S. system of electromagnetic units.
The introduction of a conventional reference value for the von Klitzing constant had considerable practical advantages in terms of the maintenance and dissemination of the unit ohm. On May 20, 2019, a revision of the SI came into force, allowing for an SI value for the von Klitzing constant to be derived using exactly defined values for the elementary charge and the Planck constant. This made it possible to realize the ohm via the use of quantum Hall resistors within the SI.
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The SI unit of electrical conductance is the Siemens (S)
The ohm is named after Georg Simon Ohm (1784-1854), a German physicist who studied the relationship between voltage, current, and resistance. In 1861, Latimer Clark and Sir Charles Bright presented a paper at the British Association for the Advancement of Science meeting, suggesting that standards for electrical units be established and proposing names for these units derived from eminent philosophers: 'Ohma', 'Farad', and 'Volt'. The resistance unit was referred to as the "B.A. unit, or Ohmad" in the third report of the committee in 1864, and by 1867, it was simply called the ohm.
The ohm is defined as the resistance of a mercury column of specified weight and length, and it is used to measure the opposition to the flow of current in an electrical circuit. The resistance of an object depends on the material it is made of, its length, cross-sectional area, and temperature. 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.
The Siemens, as the reciprocal of the ohm, measures the ease with which an electric current passes through a material. Conductance is influenced by similar factors as resistance, including the material's properties, length, cross-sectional area, and temperature.
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The formula for resistance is V = IR, also known as Ohm's law
The unit of electrical resistance is the ohm, represented by the Greek letter omega (Ω). The ohm is named after Georg Simon Ohm (1784-1854), a German physicist who studied the relationship between voltage, current, and resistance. He is credited for formulating Ohm's law, which states that the current through a conductor is directly proportional to the voltage applied across it, provided that the temperature of the conductor remains constant.
Ohm's law can be expressed by the formula V = IR, where V is the voltage, I is the current, and R is the resistance. This formula is derived from the definition of resistance, which is the ratio of voltage across a conductor to the current through it. In other words, resistance is a measure of the opposition to the flow of current in an electrical circuit. It is influenced by factors such as the material's properties, length, cross-sectional area, and temperature.
The ohm belongs to a coherent system of units, where each quantity has its corresponding SI unit: watt for power (P), ohm for resistance (R), volt for voltage (V), and ampere for current (I). This formula remains valid when these units are used, demonstrating the coherence and consistency of the system. The SI unit of electric conductance, the reciprocal of resistance, is the Siemens (S), historically known as the "mho" (ohm spelled backwards, symbol ℧).
The introduction of a conventional reference value for the von Klitzing constant facilitated the maintenance and dissemination of the ohm unit. However, this reference value was not initially compliant with the International System of Units (SI). A revision of the SI in 2019 allowed for the derivation of an SI value for the von Klitzing constant, enabling the realisation of the ohm through quantum Hall resistors within the SI framework.
The standard for the ohm was established in the 19th century due to the increasing demand for a rational and international system of units for electrical quantities. Early users of electricity, such as telegraphers, required a practical standard unit of measurement for resistance. The "legal" ohm was defined by the international conference of electricians in Paris in 1884, although it was not adopted by any national legislation. The "international" ohm was later recommended by the International Electrical Congress in 1893, based on the resistance offered by a mercury column of specific dimensions and weight.
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Resistance is influenced by a material's properties, length, cross-sectional area and temperature
Electrical resistance is measured in ohms (Ω). A legal ohm was defined by the international conference of electricians in Paris in 1884 as the resistance of a mercury column of specified weight and 106 cm long. This standard, however, was not adopted by any national legislation. The unit was further refined, and in 1893 the "international ohm" was recommended by unanimous resolution at the International Electrical Congress in Chicago.
Resistance is influenced by a material's properties, length, cross-sectional area, and temperature. Firstly, the intrinsic property of a material known as resistivity (ρ) determines its resistance. Resistivity is an inherent characteristic that dictates how much a material resists the flow of electricity. For example, copper has low resistivity, making it a good conductor, while rubber has high resistivity and is used as an insulator.
Secondly, the length of a wire influences resistance. As the length of a wire increases, so does the resistance. This is because longer wires provide more material for electrons to travel through, encountering more resistance. Conversely, shorter wires have lower resistance.
Thirdly, the cross-sectional area of a wire affects its resistance. A larger cross-sectional area results in lower resistance because it allows more current to flow through the wire at any given time, reducing the likelihood of collisions between electrons.
Lastly, temperature also influences resistance. At high temperatures, the resistance of a conductor increases as the atoms vibrate more violently, causing more collisions between electrons. Conversely, the resistance of an insulator can decrease at high temperatures as the atoms vibrate enough to release captive electrons, creating more free electrons to carry a current.
These factors are crucial to consider when designing and working with electrical wires and circuits.
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Frequently asked questions
The unit of electrical resistance is the ohm, symbolized by the Greek letter omega (Ω).
The ohm is named after Georg Simon Ohm (1784-1854), a German physicist who studied the relationship between voltage, current and resistance. In 1861, Latimer Clark and Sir Charles Bright presented a paper suggesting that standards for electrical units be established and proposed names of eminent philosophers for these units, including 'Ohma'. By 1867, the unit was referred to simply as the ohm.
The ohm is defined as the resistance offered to an unvarying electric current in a mercury column of a constant cross-sectional area of 106.3 cm long, weighing 14.4521 grams at 0°C.
Electrical resistance is measured using a digital multimeter. Resistance measurements can indicate the condition of a component or circuit. For example, high resistance can indicate an open circuit, while low resistance can indicate a short circuit.
