amp
Physical unit | |
---|---|
Unit name | amp |
Unit symbol | |
Physical quantity (s) | Amperage , magnetic flow |
Formula symbol | |
dimension | |
system | International system of units |
In SI units | Base unit |
Named after | André-Marie Ampère |
The ampere [ amˈpɛɐ̯ ] with unit symbol A, named after the French mathematician and physicist André-Marie Ampère , is the SI - base unit of electrical current strength and at the same time SI unit of the derived quantity “ magnetic flux ”.
Although the surname of the namesake is written with accent grave ("Ampère"), the SI unit in the German and English-speaking countries is usually written without an accent, ie "Ampere".
definition
One ampere corresponds to a flow of 1 coulomb (C) per second through the conductor cross-section:
In the International System of Units , the coulomb is defined by the specified elementary charge. One ampere therefore corresponds exactly to a flow of elementary charges per second, with a flow of electrons this is approx. 6.2 · 10 ^{18} (6.2 trillion ) electrons per second. ^{}
A flow of 1 A over a voltage of 1 volt (V) means an output of 1 watt (W).
Historical
Before the ampere was established as the international unit of current, there were a number of different units and definitions. In Germany and some other countries, the " Weber unit" of the current was in use, 1 Weber unit = 0.1 ampere. In Great Britain it was initially suggested that the unit of current strength should be named “Galvat” after the Italian biophysicist Luigi Galvani , which would have roughly corresponded to today's ampere. Later a "Weber unit" was also introduced for the amperage, but it had a value ten times higher than the one commonly used in Germany. It became even more complicated because the name Weber was also used for the unit of electrical charge , so that the current intensity was then equal to "Weber unit / second". At times the Weber unit was also given the name "Farad", which was later used to name the unit of electrical capacitance .
Amperage as the base unit
If one were to measure the current intensity with a derived unit , as is the case with the CGS system of units , the electrical quantities could only be expressed by the base units with non-integer exponents. To avoid this, the unit of amperage was proposed as a further basic unit as early as 1939 .
Definition 1898
1898, 1 ampere in the "Law concerning the electrical units" of the German Empire defined as the strength of that current, which from an aqueous silver nitrate - solution by means of electrolysis in a second 1.118 mg silver deposits. The ampere so defined was later referred to as the international ampere ; the one compatible with the rest of the base units, on the other hand, as an absolute ampere .
Definition 1948
In 1948 the ampere was defined as follows using the Lorentz force of two conductors on each other:
- 1 A is the strength of the temporally constant electric current , which in the vacuum between two parallel , infinitely long, straight conductors with a negligibly small, circular cross-section and the distance of 1 m between these conductors, a force of 2 · 10 ^{−7 } Newtons per meter of conductor length would evoke.
With this definition, the value of the magnetic field constant μ _{0 was also} determined.
Definition since 2019
On November 16, 2018, at the 26th General Conference on Weights and Measures , it was decided to redefine the ampere and other SI base units with effect from May 20, 2019. With this redefinition of the International System of Units , the ampere is based on the elementary charge to which a fixed numerical value has been assigned. Since then, the definition of the ampere depends only on the definition of the second , but no longer on the meter or kilogram . The redefinition was made because it is easier to implement.
Since then, the magnetic field constant μ _{0} has been a measured variable with a measurement uncertainty that has to be determined experimentally.
Web links
- Electricity and Magnetism: Units for electrical quantities. BIPM , accessed on August 2, 2019 . Wording of the definition of ampere.
- Amps: single electron tunneling (SET). Physikalisch-Technische Bundesanstalt, research on the new SI.
Individual evidence
- ↑ ^{a } ^{b } New definitions in the International System of Units (SI). (PDF; 1.3 MB) PTB , accessed on October 30, 2019 .
- ^ JG Crowther: British Scientists of the Nineteenth Century , Routledge & Kegan Paul, London, 2009, p. 246.
- ^ Wilhelm Jaeger: The emergence of the international dimensions of electrical engineering , Julius Springer Verlag, Berlin 1932, pp. 8–9.
- ↑ Law on Wikisource .
- ↑ Ernst Schmidt: State of our knowledge of the basic units and constants of physics and technology . In: Natural Sciences . tape 34 , no. 3 , 1947, pp. 93 , doi : 10.1007 / BF00663124 .
- ↑ The International System of Units (SI) . German translation of the BIPM brochure "Le Système international d'unités / The International System of Units (8e edition, 2006)". In: PTB-Mitteilungen . tape 117 , no. 2 , 2007, p. 156 ( hs-heilbronn.de [PDF; 1.4 MB ]). PDF; 1.4 MB ( memento from March 4, 2016 in the Internet Archive ).
- ↑ ^{a } ^{b } 26th CGPM (2018) - Resolutions adopted / Résolutions adoptées. (PDF; 1.2 MB) Versailles 13–16 November 2018. In: bipm.org. Bureau International des Poids et Mesures, November 19, 2018, pp. 2–5 , accessed on May 6, 2019 (English, French).