The ohm (symbol:?) Is an electrical resistance unit derived from SI, named after German physicist Georg Simon Ohm. Although some empirically derived standard units for expressing electrical barriers were developed in relation to early telegraphic practices, the British Association for the Advancement of Science proposed units derived from existing mass units, length and time and measures convenient for practical work as early as possible. as 1861. The definition of ohm was revised several times. Today, the definition of ohm is expressed from the effects of quantum Hall.
Video Ohm
Definisi
where the following units appear: volts (V), amperes (A), siemens (S), watts (W), seconds (s), farads (F), joules (J), kilograms (kg), meters ( m)), and coulomb (C).
In many cases the resistance of the conductor in the ohms is approximately constant within a certain range of voltages, temperatures, and other parameters. This is called a linear resistor. In other cases, the resistance varies (eg, thermistor).
The vowels of the kiloohm and megaohm units that are initiated are usually removed, resulting in kilohms and megohms.
In alternating current circuits, electrical impedance is also measured in ohms.
Maps Ohm
Conversions
Siemens (symbol: S) is a unit derived from electrical conductance and admittance, also known as mho (ohm spelled backwards, symbol is?); it is the reciprocal of resistance in ohm (?).
Power as a function of resistance
Daya yang dihamburkan oleh resistor dapat dihitung dari hambatannya, dan tegangan atau arus yang terlibat. Rumusnya adalah kombinasi dari Hukum Ohm dan hukum Joule:
Where:
- P is power
- R is resistance
- V is the voltage across the resistor
- I is the current through the resistor
A linear resistor has a constant resistance value of all applied voltages or currents; many practical resistors are linear over a useful range of currents. Non-linear resistors have values ​​that can vary depending on the applied voltage (or current). Where an alternating current is applied to the circuit (or where the resistance value is a function of time), the above relation is true at all times but the calculation of the average power over a period of time will require the integration of "instantaneous" forces during the interval..
Since the ohms belong to the unit coherent system, when each of these quantities has the corresponding SI unit (watts for P , ohms for R , volts for V and amperes for I , as related to ç§ Definitions, this formula remains numerically applicable when these units are used (and are considered canceled or removed).
History
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 electric users in the 19th century required a practical standard unit of measurement for durability. Resistance is often expressed as a multiple of the standard length of standard telegraph cables; Different agencies use different bases for standards, so units are not easily exchanged. The defined electrical unit is not a coherent system with units for energy, mass, length, and time, which require a conversion factor to be used in calculations connecting energy or power to endurance.
Two different methods for building electrical unit systems can be selected. Various artifacts, such as wire length or standard electrochemical cells, can be determined as producing the specified amount for durability, voltage, and so on. Alternatively, electric units may be connected to mechanical units by defining, for example, a current unit providing a certain force between two cables, or a unit of charge that gives a force unit between two unit charges. This latter method ensures coherence with the energy unit. Defining units for coherent resistance with energy units and time applicable also requires the determination of potential and current units. It is desirable that one unit of electric potential will force one unit of electric current through one unit of electrical resistance, perform one unit of work in a unit of time, otherwise all electrical calculations will require a conversion factor.
Since the so-called "absolute" unit of charge and current are expressed as combinations of units of mass, length, and time, the dimensional analysis of the relationships between potential, current, and resistance shows that resistance is expressed in terms of length per time - velocity. Some initial definitions of the unit of resistance, for example, define the unit of resistance as one Earth quadrant per second.
Units of magnetic and electrostatic quantities for the base units of mass metrics, time, and length. These units have the great advantage of simplifying the equations used in the solution of electromagnetic problems, and eliminating the conversion factor in the calculation of electrical quantities. However, the centimeter-gram-second unit, CGS, proved to have practical measures for practical measurements.
Various standard artifacts are proposed as definitions of units of resistance. In 1860 Werner Siemens (1816-1892) published a suggestion for reproducible resistance standards at Poggendorffs Annalen der Physik und Chemie. He proposed a pure mercury column, from a square millimeter cross-section, one meter long: Siemens mercury unit. However, this unit is not coherent with other units. One proposal is to design a unit based on a mercury column to be coherent - applicable, adjusting the length to make one ohm resistance. Not all unit users have the resources to conduct metrology experiments with the precision required, so a work based on notional standards based on physical definition is required.
In 1861, Latimer Clark (1822-1898) and Sir Charles Bright (1832-1888) presented papers at the British Association for an Advancement of Science meeting stating that standards for electrical units were made and suggested names for these units originating from leading philosophers. , 'Ohma', 'Farad' and 'Volt'. BAAS in 1861 appointed a committee including Maxwell and Thomson to report on the Standards of Electrical Resilience. Their goal is to design a comfortable unit, part of a complete system for electrical measurements, coherent with units for energy, stable, reproducible and based on the French metric system. In the committee's third report, 1864, the resistance unit is referred to as "unit B.A, or Ohmad". In 1867 this unit was referred to as just Ohm.
The BA. ohm is meant to be a 10 9 CGS unit but due to errors in calculation, this definition is 1.3% too small. The error is significant for the preparation of work standards.
On September 21, 1881 the CongrÃÆ'¨s internationale des ÃÆ' Â © lectriciens (international conference of electrical experts) defines the practical unit of Ohm for resistance, based on the CGS unit, using the mercury column 1 mm. in a cross section, about 104.9 cm in length at 0 ° C, similar to the equipment recommended by Siemens.
A legal ohm, a reproducible standard, defined by an international electrical conference in Paris in 1884 as a resistance to a certain heavy mercury column and a length of 106 cm; this is the compromise value between unit B. A. (equivalent to 104.7 cm), Siemens unit (100 cm by definition), and CGS unit. Although called "legal", this standard is not adopted by national law. The "international" ohm was recommended by a unanimous resolution at the 1893 International Electrical Congress in Chicago. This unit is based on the same unit with 10 9 resistance of C.G.S. system of electromagnetic units. The International Ohm is represented by the resistance offered to an unchanged electric current in the mercury column of a constant cross-sectional area of ​​106.3 cm in mass length 14.4521 grams and 0 Â ° C This definition becomes the basis for the definition of ohm law in some countries. In 1908, this definition was adopted by scientific representatives of several countries at the International Conference on Electrical and Standard Units in London. Standardized mercury columns were retained until the 1948 General Conference on Weights and Measures, in which ohms were redefined in absolute terms and not as standard artifacts.
At the end of the 19th century, units were well understood and consistent. The definition will change with little effect on the commercial use of the unit. Progress in metrology allows definitions to be formulated with a high degree of precision and repetition.
The historical unit of resistance
Actual default
The mercury column method for realizing the ohm physical standard is difficult to reproduce, due to the effect of the non-constant cross section of the glass tube. Various resistant coils are built by the British Association and others, to serve as standard physical artifacts for the resistance unit. The long-term stability and reproducibility of these artifacts is an ongoing field of research, as the effects of temperature, air pressure, humidity, and time on standards are detected and analyzed.
Standard artifacts are still used, but metrology experiments related to inductors and capacitors with exact dimensions provide a more fundamental foundation for ohm definitions. Since 1990 the quantum Hall effect has been used to define ohms with high precision and repeatability. Hall quantum experiments are used to check the stability of work standards that have convenient value for comparison.
Symbol
Symbols? suggested, ohmlike and omega-like sounds, by William Henry Preece in 1867. In documents pre-World War II, the unit symbols often consisted of lower-case omega (?) letters, resulting in 56? written as 56 ? .
When setting up electronic documents, some document editing software apps have used the Symbols type to create characters? Where fonts are not supported, W is displayed instead ("10 W" instead of "10?", For example). Since W represents a watt, SI unit, this can cause confusion.
Where the character set is limited to ASCII, the IEEE 260.1 standard recommends replacing the Ohm symbol for ?.
In the general electronics industry to use the character R, is not it? symbols, thus, 10? resistors can be represented as 10R. This is the UK standard BS 1852 code. This is used in many instances where the value has a decimal place. For example, 5.6? listed as 5R6. This method avoids facing decimal periods, which may not be reliable on components or when duplicating documents.
Unicode encodes symbols as U 2126 ? ohm sign , is different from the Greek omega among letter symbols, but only included for backwards compatibility and capital letters omega Greek U 03A9 ? CAPITAL LETTER TO OMEGA (HTMLÃ, & amp; # 937; Ã, Ã, Â · & amp; Omega; ) is preferred. In DOS and Windows alt code ALT 234 can generate? symbol. On Mac OS, : border: 1px solid #aaa; -moz-border-radius: 0.2em; -webkit-border-radius: 0.2em; border-radius: 0.2em ; -moz-box-shadow: 0.1em 0.1em 0.2em rgba (0,0,0,0.1); -webkit-box-shadow: 0.1em 0.1em 0.2em rgba (0,0.0.0.1); box -shadow : 0.1em 0.1em 0.2em rgba (0,0,0,0.1); background-color: # f9f9f9; background-image: -moz-linear-gradient (top, #eee, # f9f9f9, #eee); background-image: -o-linear-gradient (top, #eee, # f9f9f9, #eee); background-image: -webkit-linear-gradient (top, #eee, # f9f9f9, #eee); background-image: linear-gradient (down, #eee, # f9f9f9, #eee); padding: 0.1em 0.3em; font-family: inherit; font-size: 0.85em; ">? Select Z do the same thing.
See also
- The big command (resistance)
- Measurement history
- International Committee for Weight and Size
- Prisoner
- Electronic color code
Notes and references
External links
- Books scanned from Georg Simon Ohm at Nuernberg University Science University library
- Official leaflet
- NIST 811 Special Publication
- ohm history at sizes.com
- History of electrical units.
Source of the article : Wikipedia
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