O que é um/uma Ampere (A)?
Formal Definition
The ampere (symbol: A) is the SI base unit of electric current. Since 20 May 2019, it has been defined by fixing the numerical value of the elementary charge e to exactly 1.602176634 × 10⁻¹⁹ coulombs (C), where 1 C = 1 A·s. This means one ampere is the electric current corresponding to the flow of 1/(1.602176634 × 10⁻¹⁹) elementary charges per second, which is approximately 6.241509 × 10¹⁸ electrons passing a given point per second.
The ampere is one of the seven SI base units and the only electrical unit among them. All other electrical units — the volt, ohm, watt, coulomb, farad, henry, siemens, and weber — are derived from the ampere in combination with the meter, kilogram, and second. Ohm's law (V = IR) relates the ampere to the volt and ohm, while electrical power (P = IV) relates it to the watt.
Physical Meaning
Electric current, measured in amperes, represents the rate of flow of electric charge through a conductor. In metallic conductors, current is carried by electrons moving through the crystal lattice of the metal. In electrolytes and plasmas, current is carried by ions. In semiconductors, current is carried by both electrons and "holes" (vacancies in the electron structure that behave as positive charge carriers). The direction of conventional current is defined as the direction in which positive charges would flow, which is opposite to the actual direction of electron flow in metals.
Etymology
Named After Andre-Marie Ampere
The ampere is named after Andre-Marie Ampere (1775-1836), the French physicist and mathematician who is considered one of the founders of the science of electromagnetism, which he termed "electrodynamics." Born in Lyon, France, Ampere was a prodigious intellect who taught himself mathematics from the books in his father's library. Despite the execution of his father during the French Revolution and other personal tragedies, Ampere made groundbreaking contributions to physics and mathematics.
Ampere's Key Contributions
Ampere's most significant achievement was his mathematical formulation of the relationship between electric current and the magnetic field it produces, published in his 1826 memoir "Memoire sur la theorie mathematique des phenomenes electrodynamiques uniquement deduite de l'experience." He demonstrated that two parallel wires carrying electric currents exert forces on each other — attracting when the currents flow in the same direction and repelling when they flow in opposite directions. This discovery formed the basis of the original definition of the ampere (prior to 2019) and is expressed mathematically as Ampere's force law.
James Clerk Maxwell called Ampere "the Newton of electricity" for his foundational contributions. Ampere's circuital law — one of Maxwell's equations — relates the magnetic field around a closed loop to the electric current passing through the loop. The unit of electric current was named in his honor at the 1881 International Electrical Congress in Paris.
Precise Definition
The 2019 Redefinition
Since 20 May 2019, the ampere is defined by fixing the numerical value of the elementary charge e at exactly 1.602176634 × 10⁻¹⁹ C. Combined with the definition of the second (based on the cesium-133 hyperfine transition), this completely specifies the ampere. One ampere equals exactly 1/(1.602176634 × 10⁻¹⁹) elementary charges per second.
Previous Definition (1948-2019)
The previous definition, adopted in 1948 by the 9th General Conference on Weights and Measures, defined the ampere as "the constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed one meter apart in vacuum, would produce between these conductors a force equal to 2 × 10⁻⁷ newtons per meter of length." This definition was conceptually clear but practically difficult to realize, as it required idealized infinite conductors.
Practical Realization
The ampere is realized in practice through quantum electrical standards. The Josephson effect (discovered by Brian Josephson in 1962) allows voltage to be measured with extreme precision using Josephson junctions, which produce voltage steps of exactly hf/(2e), where h is the Planck constant, f is frequency, and e is the elementary charge. The quantum Hall effect (discovered by Klaus von Klitzing in 1980) provides a resistance standard equal to h/e². By combining Josephson voltage standards with quantum Hall resistance standards, current can be measured with a relative uncertainty on the order of 10⁻⁸.
História
Early Electrical Measurements
The history of the ampere is inseparable from the broader history of understanding electric current. In 1800, Alessandro Volta invented the voltaic pile — the first true battery — which provided a continuous source of electric current for the first time. This invention enabled a surge of electrical experiments by scientists including Humphry Davy, Hans Christian Oersted, and Andre-Marie Ampere. In 1820, Oersted's discovery that an electric current deflects a compass needle revealed the fundamental connection between electricity and magnetism.
Ampere's Electrodynamics
Within weeks of learning of Oersted's discovery, Ampere conducted a series of brilliant experiments demonstrating the forces between current-carrying wires. He showed that parallel wires carrying currents in the same direction attract each other, while antiparallel currents repel. He formulated mathematical laws describing these forces and proposed that magnetism itself is caused by circulating electric currents within matter — a hypothesis later confirmed by the electron theory of magnetism.
Standardization
The first international standard for the ampere was established at the International Electrical Congress in Chicago in 1893, defined electrochemically as the current that deposits 1.118 milligrams of silver per second from a silver nitrate solution (the "silver ampere"). This practical definition was replaced in 1948 by the force-based definition using parallel conductors, and in 2019 by the current definition based on the elementary charge.
The 2019 Revision
The 2019 redefinition of the ampere was part of a broader revision of the SI that also redefined the kilogram, kelvin, and mole. The new definition eliminated the dependence on the meter and kilogram (which appeared in the force-based definition) and instead anchored the ampere directly to a fundamental constant of nature — the elementary charge. This was made possible by decades of advances in single-electron transport experiments and quantum electrical metrology.
Uso atual
In Electrical Engineering
The ampere is the fundamental unit for specifying and measuring electric current in all electrical engineering applications. Circuit breakers and fuses are rated in amperes — a typical household circuit breaker is rated at 15 or 20 amperes in North America and 16 or 32 amperes in Europe. Wire gauge standards specify the maximum current-carrying capacity (ampacity) of conductors in amperes, and electrical panels are rated by their total amperage capacity (commonly 100, 200, or 400 amperes for residential service).
In Electronics
In electronics, currents are typically measured in milliamperes (mA) or microamperes (μA). A smartphone draws approximately 100 to 500 mA during active use. An LED requires 10 to 20 mA. Integrated circuits operate with currents ranging from microamperes (for low-power sensors) to amperes (for high-performance processors). Battery capacity is rated in ampere-hours (Ah) or milliampere-hours (mAh) — a typical smartphone battery has a capacity of 3,000 to 5,000 mAh.
In Power Systems
In power generation and distribution, current is measured in amperes and kiloamperes. High-voltage transmission lines carry currents of hundreds to thousands of amperes. Large industrial motors draw tens to hundreds of amperes. Electric vehicle fast chargers can deliver 250 to 500 amperes at DC voltages, enabling rapid battery charging.
Everyday Use
Home Electrical Systems
Every home's electrical system is fundamentally described in terms of amperes. The main electrical panel in a modern house is typically rated at 100 to 200 amperes. Individual circuits are protected by breakers rated at 15 or 20 amperes for general outlets, 20 amperes for kitchen and bathroom circuits, and 30 to 50 amperes for high-demand appliances like electric ranges and clothes dryers. Understanding amperage is essential for safe electrical use — overloading a circuit by drawing more amperes than its rating can cause overheating and fire.
Charging Devices
Battery charging is described in terms of amperes. A standard USB charger supplies 0.5 to 2.4 amperes. USB-C Power Delivery chargers can supply up to 5 amperes at 20 volts (100 watts). Fast-charging standards for smartphones (Qualcomm Quick Charge, USB PD) increase current and voltage to reduce charging times. Electric vehicle chargers are rated by their amperage: a Level 1 charger (standard household outlet) provides about 12 amperes, a Level 2 charger provides 16 to 80 amperes, and DC fast chargers can deliver over 300 amperes.
Safety Awareness
Understanding amperage is important for electrical safety. A current of about 10 milliamperes (0.01 A) through the human body causes painful muscle contraction, 100 milliamperes (0.1 A) can cause fatal ventricular fibrillation, and higher currents cause severe burns. This is why electrical safety focuses on both voltage (which drives current through resistance) and grounding (which provides a safe path for fault currents).
In Science & Industry
In Electromagnetism
The ampere is central to electromagnetic theory. Ampere's circuital law states that the integral of the magnetic field around a closed loop is proportional to the electric current threading the loop. The Biot-Savart law gives the magnetic field produced by a current element. Maxwell's equations, which unify electricity, magnetism, and optics, express electromagnetic quantities in terms of amperes, volts, and their derivatives.
In Quantum Metrology
The quantum electrical standards that realize the ampere are among the most precise measurement tools in all of science. The Josephson effect provides a voltage standard with a relative uncertainty below 10⁻⁹. The quantum Hall effect provides a resistance standard with a similar level of precision. Single-electron transport devices (single-electron tunneling transistors and quantum dots) can count individual electrons, providing a direct realization of the ampere as a flow of elementary charges.
In Electrochemistry
In electrochemistry, Faraday's laws of electrolysis relate the amount of substance deposited at an electrode to the total charge passed (current × time) in ampere-seconds (coulombs). One faraday of charge (96,485 coulombs) deposits one mole of a monovalent substance. These relationships are fundamental to electroplating, electrolytic refining of metals, battery chemistry, and fuel cell operation.
Multiples & Submultiples
| Name | Symbol | Factor |
|---|---|---|
| Nanoampere | nA | 0.000000001 |
| Microampere | μA | 0.000001 |
| Milliampere | mA | 0.001 |
| Ampere | A | 1 |
| Kiloampere | kA | 1000 |
Interesting Facts
A single lightning bolt carries a peak current of approximately 20,000 to 200,000 amperes, though the pulse lasts only a few microseconds. The total charge transferred in a typical lightning stroke is about 5 coulombs.
The human brain operates on approximately 12 watts of power, corresponding to currents of milliamperes flowing through billions of neurons. Each neuron fires at voltages of about 70 millivolts.
Andre-Marie Ampere was so absent-minded that he once forgot he had been invited to dinner with Emperor Napoleon. He reportedly used the back of a cab as a blackboard to work out mathematical equations.
The original 1948 definition of the ampere involved "two straight parallel conductors of infinite length" — a condition impossible to achieve physically. This made the definition theoretically elegant but practically unrealizable.
A typical household in the United States uses about 30 amperes of peak current during periods of heavy electrical use. The main breaker panel is typically rated for 200 amperes to provide ample headroom.
The world's most powerful electromagnets, used in fusion research (such as ITER), carry currents of approximately 68,000 amperes through superconducting coils cooled to near absolute zero.
It takes approximately 6.24 × 10¹⁸ electrons flowing past a point per second to constitute one ampere of current. Despite this enormous number, individual electrons in a wire move surprisingly slowly — typically only a fraction of a millimeter per second (drift velocity).
The ampere-hour (Ah) is the standard unit for battery capacity. A 100 Ah battery can theoretically supply 1 ampere for 100 hours, or 100 amperes for 1 hour, or any equivalent combination.