Beatrice Lang
The ampere, often shortened to amp, is the unit of electric current in the International System of Units (SI). It is named after French mathematician and physicist André-Marie Ampère, considered the father of electromagnetism alongside Danish physicist Hans Christian Ørsted. One ampere is equal to one coulomb (approximately 6.24 x 10^18 elementary charges) moving past a point per second. The ampere is used by both scientists and technologists and is one of the seven SI base units, representing the fundamental dimension of electric current.
| Characteristics | Values |
|---|---|
| Definition | The ampere is the unit of electric current in the International System of Units (SI). |
| Symbol | A |
| Named After | French mathematician and physicist André-Marie Ampère (1775–1836), considered the father of electromagnetism along with Danish physicist Hans Christian Ørsted. |
| Value | One ampere is equal to one coulomb (approximately 6.24 x 10^18 elementary charges) moving past a point per second. |
| SI Definition (2019 revision) | The ampere is defined by taking the fixed numerical value of the elementary charge e to be 1.602176634×10^-19 C. |
| Previous Definition | The ampere was previously defined as the current passing through two parallel wires one metre apart that produces a magnetic force of 2×10^-7 newtons per metre. |
| Expression in Terms of Other Units | The ampere can be expressed in terms of power and voltage using the relationship I = P/V, and thus 1 A = 1 W/V. |
| Measurement | The ampere can be measured using a multimeter or an ammeter. |
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What You'll Learn
Ampere definition
The ampere, often shortened to amp, is the unit of electric current in the International System of Units (SI). One ampere is equal to one coulomb (approximately 6.24 x 10^18 elementary charges) moving past a point per second. The ampere is named after 19th-century French mathematician and physicist André-Marie Ampère (1775–1836), considered the father of electromagnetism alongside Danish physicist Hans Christian Ørsted.
The ampere was originally defined as one-tenth of the unit of electric current in the centimetre–gram–second system of units. This unit, now known as the abampere, was defined as 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 realisation of the ampere, defined as the current that would deposit 0.001118 grams of silver per second from a silver nitrate solution. Later, more accurate measurements revealed that this current is 0.99985 A. The 2019 revision of the SI defined the ampere by taking the fixed numerical value of the elementary charge e to be 1.602176634×10^-19 C, which is equal to A⋅s, where the second is defined in terms of ∆νCs, the unperturbed ground state hyperfine transition frequency of the caesium-133 atom.
In practical terms, the ampere can be maintained via Ohm's law from the units of electromotive force and resistance, the volt and the ohm. This is because the latter two can be tied to physical phenomena that are relatively easy to reproduce, the Josephson effect and the quantum Hall effect, respectively. Current can be measured by a multimeter, a device that can measure electrical voltage, current, and resistance.
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Historical definitions
The ampere, often shortened to amp, is the unit of electric current in the International System of Units (SI). It is named after 19th-century French mathematician and physicist André-Marie Ampère (1775–1836), considered the father of electromagnetism alongside Danish physicist Hans Christian Ørsted.
Prior to the 2019 redefinition, the ampere was defined as the constant current that, if maintained in two parallel wires 1 metre apart in a vacuum, would produce a magnetic force of 2×10−7 newtons per metre. This definition was chosen by the CIPM and referenced Ampère's force law, which states that there is an attractive or repulsive force between two parallel wires carrying an electric current.
The 2019 revision of the SI defined the ampere by taking the fixed numerical value of the elementary charge e to be 1.602176634×10−19 coulombs, which is equal to A⋅s. This definition was agreed upon by the General Conference on Weights and Measures (CGPM) and represented a radical change in the field of measurement science.
The new definition has brought challenges for measurement science as the elementary charge e is extremely small, making it technically demanding to measure individual electrons. However, it also has advantages, as the use of natural constants to define a unit makes it universally measurable and assures the long-term stability of measurement standards.
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Practical measurement
The ampere, or amp, is the unit of electric current in the International System of Units (SI). One ampere is equal to one coulomb (approximately 6.24 x 10^18 elementary charges) moving past a point per second. In practical terms, measuring the amperage of an electrical circuit can be done using a multimeter or ammeter/clamp meter. This is a device that can measure electrical voltage, current, and resistance.
Before attempting to measure amperage, it is important to prioritise safety. Put on rubber gloves and ensure you are not working on or near a metal surface to avoid the risk of electric shock. It is also important to check that the multimeter can handle the maximum current of the circuit. The power source should have a label indicating the maximum discharge amperage. If the multimeter cannot handle the maximum current, AC/DC current clamps can be used to connect to the multimeter for measuring a wider range of amperage values.
To measure amperage using a multimeter, first, configure the device by pushing the black probe into the "COM" socket and the red probe into the "A" socket. The "A" socket is specifically for measuring amperage, while the COM" socket stands for "common". Select AC or DC amperage on the meter, depending on the electrical system being tested. Ensure the multimeter is set to the range of the amperage being tested.
Once the multimeter is configured, turn off the power and break the circuit being tested. Attach the red probe to the negative terminal of the power supply, and touch the black probe to the negative wire. Turn on the circuit and leave the probes attached for about a minute before checking the amperage reading on the display. This initial reading may be accurate, but for the most precise measurement, leave the probes on the circuit for at least 60 seconds to ensure the current is steady.
It is also possible to measure amperage using an ammeter or clamp meter. This tool can be clamped onto a wire to measure the current, using strong iron clamps to hold the conductor tightly and concentrate the magnetic field. When current flows through the conductor, the magnetic field passes through the meter and generates a voltage, which is converted into a digital reading.
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SI unit of charge
The ampere, often shortened to amp, is the SI unit of electric current. It is named after French mathematician and physicist André-Marie Ampère, considered the father of electromagnetism. One ampere is equal to one coulomb (C) of electric charge moving past a point per second.
The coulomb is the SI unit of electric charge. One coulomb is defined as the quantity of electricity carried in one second by a current of one ampere. Conversely, a current of one ampere is one coulomb of charge going past a given point per second. One coulomb is equal to approximately 6.24 x 10^18 elementary charges.
The ampere was originally defined as one-tenth of the unit of electric current in the centimetre–gram–second system of units. That unit, now known as the abampere, was defined as 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.
In 2018, the General Conference on Weights and Measures (CGPM) agreed that on May 20, 2019, the ampere would be defined such that the elementary charge would be equal to 1.602176634 x 10^-19 coulombs. This means an ampere is an electric current equivalent to 10^19 elementary charges moving every 1.602176634 seconds. Prior to this redefinition, the ampere was defined as the current passing through two parallel wires one metre apart that produces a magnetic force of 2 x 10^-7 newtons per metre.
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Ampere in context
The ampere, often shortened to amp, is the unit of electric current in the International System of Units (SI). It is named after the French mathematician and physicist André-Marie Ampère (1775-1836), considered the father of electromagnetism alongside the Danish physicist Hans Christian Ørsted.
The ampere is defined as one coulomb (C) of electrical charge moving past a point per second. In other words, it is a measure of the rate of electron flow or current in an electrical conductor. One ampere of current represents one coulomb of electrical charge (approximately 6.24 x 10^18 charge carriers) moving past a specific point in one second.
The ampere was originally defined as one-tenth of the unit of electric current in the centimetre-gram-second system of units. This unit, now known as the abampere, was defined as 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 2019 revision of the SI defined the ampere by taking the fixed numerical value of the elementary charge e to be 1.602176634 x 10^-19 C. This means that an ampere is an electric current equivalent to 10^19 elementary charges moving every 1.602176634 seconds. Prior to this redefinition, the ampere was defined as the current passing through two parallel wires one metre apart that produces a magnetic force of 2 x 10^-7 newtons per metre.
Ampere is one of the main units used for measuring electricity, alongside volts, watts, and ohms. It is an important concept in electrical engineering and is used to calculate wattage, which is the product of voltage and amperage.
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Frequently asked questions
An ampere, often shortened to amp, is the unit of electric current in the International System of Units (SI). One ampere is equal to one coulomb (approximately 6.24 x 10^18 elementary charges) moving past a point per second.
The ampere is used to quantify the current flowing in a system. It is also used to define other SI units, such as the volt and watt.
The ampere is named after the 19th-century French physicist André-Marie Ampère, who is considered the father of electromagnetism. The unit was originally defined as the current passing through two parallel wires placed one metre apart that produces a magnetic force of 2 x 10^-7 newtons per metre. In 2019, the SI definition was revised to fix the elementary charge e to be exactly 1.602176634 x 10^-19 coulombs.
