The ampere (symbol: A) is the SI unit of electric current (symbol: I) and is one of the seven SI base units. It is named after André-Marie Ampère (1775–1836), French mathematician and physicist, considered the father of electrodynamics. In practice, its name is often shortened to amp.

In practical terms, the ampere is a measure of the amount of electric charge passing a point in an electric circuit per unit time with 6.241 × 10¹⁸ electrons, or one coulomb per second constituting one ampere.

The practical definition may lead to confusion with the definition of a coulomb (i.e., 1 amp-second). The two definitions are closely related. A Coulomb is one amp-second and an Ampere is one Coulomb per second. In practical terms this means that measures of a constant current (e.g., the flow of electric charges per second through a simple circuit) will be defined in amps (e.g., "a 20 mA circuit") and the amount of charge that moves through a circuit will be defined in coulombs ( 10 coulombs of charge moving through a circuit over 5 seconds is due to an average current of 2 amperes"). In this way, amperes can be viewed as a " flow of charges" or "total amount of charge moved divided by total time to move the charges"and coulombs viewed as an "amount of charge."

Definition

Ampère's force law states that there is an attractive force between two parallel wires carrying an electric current. This force is used in the formal definition of the ampere, which states that it is "the constant current that will produce an attractive force of 2 × 10^–7 newton per metre of length between two straight, parallel conductors of infinite length and negligible circular cross section placed one metre apart in a vacuum".

In terms of Ampère's force law, :$2\; \backslash times\; 10^\{-7\}\backslash \; \{\backslash rm\backslash tfrac\; N\; m\}=2\; \backslash times\; k\_A\backslash frac$ so :$1\backslash \; \{\backslash rm\; A\}=\backslash sqrt\{\backslash frac\{2\backslash times\; 10^\{-7\}\backslash rm\backslash \; N\}\{2\backslash times\; k\_A\}\}$

The SI unit of charge, the coulomb, "is the quantity of electricity carried in 1 second by a current of 1 ampere". Conversely, a current of one ampere is one coulomb of charge going past a given point per second: :$\backslash rm\; 1\backslash \; A=1\backslash tfrac\; C\; s$ That is, in general, charge ''Q'' is determined by steady current ''I'' flowing for a time ''t'' as ''Q'' = ''It''.

History

The ampere was originally defined as one tenth of the CGS system electromagnetic unit of current (now known as the abampere), the amount of current that generates a force of two dynes per centimetre of length between two wires one centimetre apart. The size of the unit was chosen so that the units derived from it in the MKSA system would be conveniently sized.

The "international ampere" was an early realization of the ampere, defined as the current that would deposit grams of silver per second from a silver nitrate solution. Later, more accurate measurements revealed that this current is 0.99985 A.

Realization

The standard ampere is most accurately realized using a watt balance, but is in practice maintained via Ohm's Law from the units of electromotive force 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.

At present, techniques to establish the realization of an ampere have a relative uncertainty of approximately a few parts in 10⁷, and involve realizations of the watt, the ohm and the volt.

Proposed future definition

Rather than a definition in terms of the force between two current-carrying wires, it has been proposed to define the ampere in terms of the rate of flow of elementary charges. Since a coulomb is approximately equal to elementary charges, one ampere is approximately equivalent to elementary charges per second, such as electrons, moving past a boundary in one second. The proposed change would define 1 A as being the current in the direction of flow of a particular number of elementary charges per second. In 2005, the International Committee for Weights and Measures (CIPM) agreed to study the proposed change. The new definition is expected to be formally proposed at the 25th General Conference on Weights and Measures (CGPM) in 2015.

Everyday examples

The current drawn by typical constant-voltage energy distribution systems is usually dictated by the power (watts) consumed by the system and the operating voltage. For this reason the examples given below are grouped by voltage level.

Portable gadgets

Hearing aid (typically 1 mW at 1.4 V): 0.7 mA

Motor vehicles – 12 V DC

A typical motor vehicle has a 12 V battery. The various accessories that are powered by the battery might include:

Instrument panel light (typically 2 W): 166 mA.

Headlights (typically 60 W): 5 A each.

Starter Motor (typically 1–2 kW): 80-160 A

US & Canada domestic supply – 120 V AC

Most United States domestic power suppliers run at 120 V.

Household circuit breakers typically provide a maximum of 15 A or 20 A of current to a given set of outlets.

22-inch/56-centimeter Portable Television (35 W): 290 mA

Tungsten light bulb (60–100 W): 500–830 mA

Toaster, kettle (2 kW): 16.6 A

Immersion heater (4.6 kW): 38.3 A

European domestic supply – 230 V AC

Most European domestic power supplies run at 230 V, so the current drawn for a particular appliance will be less than for an equivalent United States appliance. This means that lighter (and cheaper) cabling can be used.

The current drawn by a number of typical appliances are:

22-inch/56-centimeter Portable Television (35 W): 150 mA

Tungsten light bulb (60–100 W): 250–450 mA

Toaster, kettle (2 kW): 9 A

Immersion heater (4.6 kW): 20 A

See also

Ammeter

Ampacity (current-carrying capacity)

Electric current

Electric shock

Hydraulic analogy

Magnetic constant

References

External links

The NIST Reference on Constants, Units, and Uncertainty

A short history of the SI units in electricity

NIST ''Definition of ampere and μ₀''

Category:SI base units Category:Units of electric current

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Coordinates	53°42′25″N16°23′39″N
name	André-Marie Ampère
birth date	January 20, 1775
birth place	Parish of St. Nizier, Lyon, France
death date	June 10, 1836
death place	Marseille, France
residence	France
nationality	French
field	Physics
work institutions	Bourg-en-Bresse École Polytechnique
known for	Ampere's Law
signature	André-Marie Ampère signature.svg }}

André-Marie Ampère (20 January 1775 – 10 June 1836) was a French physicist and mathematician who is generally regarded as one of the main discoverers of electromagnetism. The SI unit of measurement of electric current, the ampere, is named after him.

Early days

Ampère was born in Lyon, France on 20 January 1775. He spent his childhood and adolescence at the family property at Poleymieux-au-Mont-d'Or near Lyon. His father began to teach him Latin, until he discovered the boy's preference and aptitude for mathematical studies. The young Ampère, however, soon resumed his Latin lessons, to enable him to master the works of Euler and Bernoulli. In later life Ampère claimed that he knew as much about mathematics and science when he was eighteen as ever he knew; but, a polymath, his reading embraced history, travels, poetry, philosophy, and the natural sciences.

During the French Revolution, Ampere's father stayed at Lyon expecting to be safer there. Nevertheless, after the revolutionaries had taken the city he was captured and executed. This death was a great shock to Ampère.

In 1796 Ampère met Julie Carron, and in 1799 they were married. From about 1796, Ampère gave private lessons at Lyon in mathematics, chemistry, and languages. In 1801 he moved to Bourg-en-Bresse, as professor of physics and chemistry, leaving his ailing wife and his infant son (Jean-Jacques Ampère) at Lyon. Her death, in July 1803, troubled Ampère for the rest of his life. Also in 1804, Ampère was appointed professor of mathematics at the University of Lyon.

Ampère claimed that "at eighteen years he found three culminating points in his life, his First Communion, the reading of Antoine Leonard Thomas's "Eulogy of Descartes", and the Taking of the Bastille. On the day of his wife's death he wrote two verses from the Psalms, and the prayer, 'O Lord, God of Mercy, unite me in Heaven with those whom you have permitted me to love on earth.' Serious doubts harassed him at times, and made him very unhappy. Then he would take refuge in the reading of the Bible and the Fathers of the Church."

For a time he took into his family the young student Antoine-Frédéric Ozanam (1813-1853), one of the founders of the Conference of Charity, later known as the Society of Saint Vincent de Paul. Through Ampère, Ozanam had contact with leaders of the neo-Catholic movement, such as François-René de Chateaubriand, Jean-Baptiste Henri Lacordaire, and Charles Forbes René de Montalembert. Ozanam was beatified by Pope John Paul II in 1997.

Physics and further studies

Jean Baptiste Joseph Delambre's recommendation obtained for Ampère the Lyon appointment, and afterwards (1805) a minor position in the polytechnic school at Paris, where he was appointed professor of mathematics in 1809. Here Ampère continued to pursue his scientific research and his diverse studies with unabated diligence. He was admitted as a member of the Institute in 1814.

Ampère's fame mainly rests on his establishing the relations between electricity and magnetism, and in developing the science of electromagnetism, or, as he called it, electrodynamics. On 11 September 1820 he heard of H. C. Ørsted's discovery that a magnetic needle is acted on by a voltaic current. Only a week later, on 18 September, Ampère presented a paper to the Academy containing a much more complete exposition of that and kindred phenomena. On the same day, Ampère also demonstrated before the Academy that parallel wires carrying currents attract or repel each other, depending on whether currents are in the same (attraction) or in opposite directions (repulsion). This laid the foundation of electrodynamics.

The topic of electromagnetism thus begun, Ampère developed a mathematical theory which not only described the electromagnetic phenomena already observed, but also predicted many new ones.

During 1828, he was elected a foreign member of the Royal Swedish Academy of Science.

Writings

''Considerations sur la théorie mathématique du jeu'', Perisse, Lyon Paris 1802, online lesen im Internet-Archiv

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Last years

Ampère died at Marseille and was buried in the Cimetière de Montmartre, Paris. The great amiability and childlike simplicity of his character are well brought out in his ''Journal et correspondence'' (Paris, 1872).

Ampère's final work, published posthumously, was ''Essai sur la philosophie des sciences, ou exposition analytique d'une classification naturelle de toutes les connaissances humaines ("Essay on the philosophy of science or analytical exposition on the natural classification of human knowledge")''.

References

External links

Ampère and the history of electricity - a French-language, edited by CNRS, site with Ampère's correspondence (full text and critical edition with links to manuscripts pictures, more than 1000 letters), an Ampère bibliography, experiments, and 3D simulations

Ampère Museum - a French-language site from the museum in Poleymieux-au-Mont-d'or, near Lyon, France

Catholic Encyclopedia on André Marie Ampère

André-Marie Ampère: The Founder of Electromagnetism - Background information and related experiments

Category:1775 births Category:1836 deaths Category:People from Lyon Category:19th-century French people Category:Electrostatics Category:French physicists Category:People associated with electricity Category:Gentleman scientists Category:French Roman Catholics Category:Collège de France faculty Category:Foreign Members of the Royal Society Category:Fellows of the Royal Society of Edinburgh Category:Members of the French Academy of Sciences Category:Members of the Royal Swedish Academy of Sciences Category:Burials at Montmartre Cemetery, Paris

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