# ampere

Current can be measured by a galvanometer, via the deflection of a magnetic needle in the magnetic field created by the current.

The ampere, in practice often shortened to amp, (symbol: A) is a unit of electric current, or amount of electric charge per second. The ampere is an SI base unit, and is named after André-Marie Ampère, one of the main discoverers of electromagnetism.

## Definition

The ampere is a constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross section, and placed 1 metre apart in a vacuum, would produce between these conductors a force equal to 2×10–7 newton per meter of length.[1]

The ampere is a base unit, along with the metre, the second, and the kilogram: it is defined without reference to the quantity of electric charge. The unit of charge, the coulomb, is defined, as a derived unit, to be the amount of charge displaced by a one ampere current in the time of one second.

As a result, electric current is also the time rate of change or displacement of electric charge. One ampere represents the rate of 1 coulomb of charge per second.

## Explanation

Because it is a base unit, the definition of the ampere is not tied to any other electrical unit. The definition for the ampere is equivalent to fixing a value of the permeability of vacuum to μ0 = 4π×10−7 H/m. Prior to 1948, the so-called "international ampere" was used, defined in terms of the electrolytic deposition rate of silver.[2] The older unit is equal to 0.999 85 A.

The ampere is most accurately realized using an watt balance, but is in practice maintained via Ohm's Law from the units of voltage and resistance, the volt and the ohm, since the latter two can be tied to physical phenomena that are relatively easy to reproduce, the Josephson junction and the quantum Hall effect, respectively.

The unit of electric charge, the coulomb, is defined in terms of the ampere: one coulomb is the amount of electric charge (formerly quantity of electricity) carried in a current of one ampere flowing for one second.[3] Current, then, is the rate at which charge flows through a wire or surface. One ampere of current (I) is equal to a flow of one coulomb of charge (Q) per second of time (t):

## Proposed future definition

Since a coulomb is approximately equal to 6.24150948×1018 elementary charges, one ampere is approximately equivalent to 6.24150948×1018 elementary charges, such as electrons, moving past a boundary in one second.

As with other SI base units, there have been proposals to redefine the kilogram in such a way as to define some presently measured physical constants to fixed values. One proposed definition of the kilogram is:

 The kilogram is the mass which would be accelerated at precisely 2×10-7 m/s2 if subjected to the per metre force between two straight parallel conductors of infinite length, of negligible circular cross section, placed 1 metre apart in vacuum, through which flow a constant current of exactly 6 241 509 479 607 717 888 elementary charges per second.

This redefinition of the kilogram has the effect of fixing the elementary charge to be e = 1.6021765310-19 C and would result in a functionally equivalent definition for the coulomb as being the sum of exactly 6 241 509 479 607 717 888 elementary charges and the ampere as being the electrical current of exactly 6 241 509 479 607 717 888 elementary charges per second. This is consistent with the current 2002 CODATA value for the elementary charge which is 1.60217653×10-19 ± 0.00000014×10-19 C.

### CIPM recommendation

International Committee for Weights and Measures (CIPM) Recommendation 1 (CI-2005): Preparative steps towards new definitions of the kilogram, the ampere, the kelvin and the mole in terms of fundamental constants

The International Committee for Weights and Measures (CIPM),
• approve in principle the preparation of new definitions and mises en pratique of the kilogram, the ampere and the kelvin so that if the results of experimental measurements over the next few years are indeed acceptable, all having been agreed with the various Consultative Committees and other relevant bodies, the CIPM can prepare proposals to be put to Member States of the Metre Convention in time for possible adoption by the 24th CGPM in 2011;
• give consideration to the possibility of redefining, at the same time, the mole in terms of a fixed value of the Avogadro constant;
• prepare a Draft Resolution that may be put to the 23rd CGPM in 2007 to alert Member States to these activities;
 This SI unit is named after André-Marie Ampère. As for all SI units whose names are derived from the proper name of a person, the first letter of its symbol is uppercase (A). But when an SI unit is spelled out, it should always be written in lowercase (ampere), unless it begins a sentence or is the name "degree Celsius". — Based on The International System of Units, section 5.2.

## References

1. ^ Paul M. S. Monk, Physical Chemistry: Understanding our Chemical World, John Wiley and Sons, 2004 online.
2. ^ Robert B. Northrop, Introduction to Instrumentation and Measurements, CRC Press, 1997 online
3. ^ Kuzman Ražnjević, Physical Quantities and the Units of the International System (Si), Begell House Publishers, 1995 online

Electric current is the flow (movement) of electric charge. The SI unit of electric current is the ampere (A), which is equal to a flow of one coulomb of charge per second.

## Definition

The amount of electric current (measured in amperes) through some surface, e.g.
Flavour in particle physics

The international system (SI) of units defines seven SI base units: physical units defined by an operational definition.

All other physical units can be derived from these base units: these are known as SI derived units. Derivation is by dimensional analysis.
André-Marie Ampère

André-Marie Ampère (1775-1836)
Born January 22 1775
Poleymieux, Lyon, France
Electromagnetism is the physics of the electromagnetic field: a field which exerts a force on particles that possess the property of electric charge, and is in turn affected by the presence and motion of those particles.
In geometry, a cross section is the intersection of a body in 2-dimensional space with a line, or of a body in 3-dimensional space with a plane, etc. More plainly, when cutting an object into slices one gets many parallel cross sections.
1 metre =
SI units
1000 mm 0 cm
US customary / Imperial units
0 ft 0 in
The metre or meter[1](symbol: m) is the fundamental unit of length in the International System of Units (SI).
The newton (symbol: N) is the SI derived unit of force, named after Sir Isaac Newton in recognition of his work on classical mechanics.

## Definition

A newton
The international system (SI) of units defines seven SI base units: physical units defined by an operational definition.

All other physical units can be derived from these base units: these are known as SI derived units. Derivation is by dimensional analysis.
1 metre =
SI units
1000 mm 0 cm
US customary / Imperial units
0 ft 0 in
The metre or meter[1](symbol: m) is the fundamental unit of length in the International System of Units (SI).
second (SI symbol: s), sometimes abbreviated sec., is the name of a unit of time, and is the International System of Units (SI) base unit of time.

SI prefixes are frequently combined with the word second to denote subdivisions of the second, e.g.
kilogram or kilogramme (symbol: kg) is the SI base unit of mass. The kilogram is defined as being equal to the mass of the International Prototype Kilogram (IPK), which is almost exactly equal to the mass of one liter of water.
Flavour in particle physics

The coulomb (symbol: C) is the SI unit of electric charge. It is named after Charles-Augustin de Coulomb.

## Definition

1 coulomb is the amount of electric charge transported by a current of 1 ampere in 1 second.
second (SI symbol: s), sometimes abbreviated sec., is the name of a unit of time, and is the International System of Units (SI) base unit of time.

SI prefixes are frequently combined with the word second to denote subdivisions of the second, e.g.
In electromagnetism, permeability is the degree of magnetization of a material that responds linearly to an applied magnetic field. Magnetic permeability is represented by the Greek letter μ. The term was coined in September, 1885 by Oliver Heaviside.
electrolysis is a method of separating chemically bonded elements and compounds by passing an electric current through them.

## Overview

Electrolysis involves the passage of an electric current through a typically ionic substance which is either molten or dissolved in an
Silver (IPA: /ˈsɪlvə(ɹ)/) is a chemical element with the symbol Ag (Latin: argentum) and atomic number 47.
watt balance is an experimental electromechanical apparatus that may one day serve as the delineation of an electronic-based definition of the kilogram.

The watt balance is a more accurate version of the ampere balance, in which the force between two current-carrying coils
Ohm's law states that, in an electrical circuit, the current passing through a conductor between two points is proportional to the potential difference (i.e. voltage drop or voltage) across the two points, and inversely proportional to the resistance between them.
Voltage (sometimes also called electric potential difference or electrical tension) is the potential similarity of electrical potential between two points of an electrical or electronic circuit, expressed in volts.
Electrical resistance is a measure of the degree to which an object opposes an electric current through it. The SI unit of electrical resistance is the ohm. Its reciprocal quantity is electrical conductance measured in siemens.
volt (symbol: V) is the SI derived unit of electric potential difference or electromotive force.[1][2] It is named in honor of the Italian physicist Alessandro Volta (1745–1827), who invented the voltaic pile, the first modern chemical battery.
ohm (symbol: Ω) is the SI unit of electrical impedance or, in the direct current case, electrical resistance, named after Georg Ohm.

## Definition

An ohm is the electrical resistance offered by a current-carrying element that produces a voltage drop of one volt when a
The Josephson effect is the phenomenon of current flow across two weakly coupled superconductors, separated by a very thin insulating barrier. This arrangement—two superconductors linked by a non-conducting barrier—is known as a Josephson junction
The quantum Hall effect is a quantum-mechanical version of the Hall effect, observed in two-dimensional electron systems subjected to low temperatures and strong magnetic fields, in which the Hall conductance takes on the quantized values

Flavour in particle physics

The coulomb (symbol: C) is the SI unit of electric charge. It is named after Charles-Augustin de Coulomb.

## Definition

1 coulomb is the amount of electric charge transported by a current of 1 ampere in 1 second.