Amp

Recording depicting a galvanometer (1890)

The ampere, ampere or ampère (symbol A) is the unit of electric current intensity. It is part of the base units in the international system of units and was named after the French mathematician and physicist André-Marie Ampère (1775-1836).

Defined by setting the numeric value of the elementary charge, e, to 1.602 176 634 × 10^-19, when expressed in unit C, equal to A s, where the second is defined in terms of Δν_Cs. The effect of this definition is that the ampere is the electric current corresponding to the flow of 1/(1.602 176 634 × 10⁻¹⁹) = 6.241 509 074 × 10¹⁸ elementary charges per second.

This definition was adopted on November 16, 2018 at the 26th General Conference on Weights and Measures, agreeing to its entry into force on May 20, 2019.

The amperio and the assmbio are related by: 1A=1Cs{displaystyle mathrm {1,a=1{frac {,c}{s}{s} },} While the amperio is a basic unit and the assmbio is not, in practice, the current definition of amperio depends on the assmbio.

Historical definitions

Since the middle of the 19th century, with the development of electromagnetism and electrical engineering, the ampere began to be used as a unit of electrical current. The definition and quantification was not uniform, but each country developed its own standards. The first international standard defining the ampere, as well as other electrical units, was established at the Chicago International Electrical Congress in 1893, and confirmed at the 1908 London International Conference. it was defined in terms of the electrical current that causes the electrolytic deposition of silver from a silver nitrate solution at an average rate of 0.001118 g/s. Its value, expressed in terms of the absolute ampere, was equivalent to 0.99985 A.

The unit of electric charge, the coulomb, is derived from the ampere: one coulomb is the amount of electric charge displaced by a current of one ampere flowing for one second. Therefore, the electric current (I) can be expressed as the average charge (Q) that flows per unit of time (t):

I=Qt{displaystyle {rm {I={frac {Q}{t}}}}}}

Although conceptually it would seem more logical to take the charge as the basic unit, the current was chosen because, for operational reasons, it was easier to measure experimentally. This is how the previous definition of ampere was arrived at, the one established in the 9th General Conference on Weights and Measures in 1948, as follows: An ampere is the constant current that, maintained in two parallel straight conductors of infinite length, of circular section negligible, and placed one meter apart in a vacuum, would produce a force between these conductors equal to 2 x 10^–7 newtons per meter of length.

The redefinition process that led to the definition of the ampere adopted in 2018 was formally initiated in 2010 when the BIPM Committee proposed replacing the then current definitions of various units of the international system, by others based on constants of nature, such as Planck's constant, Boltzmann's constant, elementary charge and Avogadro's number.

Vacuum permeability value

Illustration of the definition of Amperio. The strength exercised in the drivers is the strength of Lorentz

From the 1948 definition of an ampere an exact value for the magnetic permeability of a vacuum followed. The force exerted on two rectilinear parallel conductors through which an intensity of current circulates is given by the Biot-Savart law:

Fl=μ μ 0⋅ ⋅ I22π π ⋅ ⋅ r{displaystyle {frac {F}{l}}}={frac {mu _{0}cdot I^{2}}{2pi cdot r}}}}}}

where:

F{displaystyle F} is the force, which is of attraction when the sense of the current is the same,

l{displaystyle l} is the length of drivers considered,

μ μ 0{displaystyle mu _{0}} is the magnetic permeability of the vacuum,

I{displaystyle I} is the electrical current intensity that circulates through the conductors,

r{displaystyle r} is the distance between the drivers,

Clearing the previous equation we have that:

μ μ 0=2π π ⋅ ⋅ f⋅ ⋅ rI2⋅ ⋅ l=2π π ⋅ ⋅ 2⋅ ⋅ 10− − 7N⋅ ⋅ 1m(1A)2⋅ ⋅ 1m=4π π × × 10− − 7N/A2=1,2566370614...... × × 10− − 6H/m{displaystyle mathbf {mu _{0}} ={frac {2pi cdot fcdot r}{I^{2}cdot l}}={frac {2cdot} 2cdot 10^{-7}Ncdot 1m}{(1st)}{2cdot}{2}}{2cd}}}{x1st}}{x1st}}}{x1st}}}{cd}{cd}}{cd}{cd}{cd}}}{cd}{cd}}{x1st}{x1st}{cd}{cd}{cd}}}}{cd}}{cd}{cd}{cd}{cd}{cd}{cd}{cd}{cd}{cd}{cd}{cd}}}

was the exact value for the permeability of the void. In addition, since the permittivity and the characteristic impedance of the vacuum are related to the permeability and the speed of light in a vacuum, other exact defined values were obtained:

• The allowivity of the vacuum ε ε 0=1c2μ μ 0=8,8541878176...... × × 10− − 12F/m,{displaystyle mathbf {varepsilon _{0}} ={frac {1{c^{2}mu _{0}}}}=8,8541878176ldots times 10^{-12}mathrm {F/m}}
• the characteristic impedance of the void Z0=μ μ 0⋅ ⋅ c=119,9169832π π Ω Ω {displaystyle Z_{0}=mu _{0}cdot c=119,9169832;pi omega }

where c{displaystyle c} is the speed of light in the void.

With the definition of the ampere of the year 2018, these three magnitudes: magnetic permeability, permittivity and characteristic impedance no longer have exact values defined in the International System of Units and must be subject to experimental determination.

Multiples of the amp

The following is a table of the multiples and submultiples of the ampere according to the nomenclature of the International System of Units:

This unit of the International System is named after André-Marie Ampère. In the units of the SI whose name comes from a person's own name, the first letter of the symbol is written with capital (A), while his name always starts with a tiny letter (amperio), except in case you start a sentence or a title.

Based on The International System of UnitsSection 5.2.