amp

The title of this article is ambiguous. Further meanings are listed under Ampere (disambiguation) .

Physical unit
Unit name	amp
Unit symbol	${\displaystyle \mathrm {A} }$
Physical quantity (s)	Amperage , magnetic flow
Formula symbol	${\displaystyle I}$
dimension	${\displaystyle {\mathsf {I}}}$
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

The blue circles represent electrons flowing through the cross-sectional area of ​​the conductor. One ampere corresponds to one coulomb that flows through the conductor cross-section in one second.

See also : International system of units # Definition of SI units

One ampere corresponds to a flow of 1  coulomb (C) per second through the conductor cross-section:

${\displaystyle 1\,\mathrm {A} =1{\frac {\mathrm {C} }{\mathrm {s} }}=1\,\mathrm {C} \cdot \mathrm {s} ^{-1}}$

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. ${\textstyle e=1{,}602\,176\,634\cdot 10^{-19}{\text{As}}}$ ${\textstyle 1/1{,}602\,176\,634\cdot 10^{-19}}$

A flow of 1 A over a voltage of 1  volt (V) means an output of 1  watt (W).

${\displaystyle 1\,\mathrm {A} =1\,{\frac {\mathrm {W} }{\mathrm {V} }}=1\,\mathrm {W} \cdot \mathrm {V} ^{-1}}$

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

See also : Electromagnetic Units of Measure

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

Illustration of the ampere definition from 1948. The forces drawn in blue each have the amount 2 · 10 ⁻⁷ N.

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

Wiktionary: Ampere  - explanations of meanings, word origins, synonyms, translations

Wikisource: Law relating to electrical units. From June 1, 1898 (German Empire)

• 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

1. ↑ ^{a b} New definitions in the International System of Units (SI). (PDF; 1.3 MB) PTB , accessed on October 30, 2019 .  
2. ^ JG Crowther: British Scientists of the Nineteenth Century , Routledge & Kegan Paul, London, 2009, p. 246.
3. ^ Wilhelm Jaeger: The emergence of the international dimensions of electrical engineering , Julius Springer Verlag, Berlin 1932, pp. 8–9.
4. ↑ Law on Wikisource .
5. ↑ 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 .
6. ↑ 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 ).
7. ↑ ^{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). 

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