units of electricity and magnetism
The ampere (A) is the base unit of electric current in the SI system. One ampere (A), or amp, is defined as the constant current that, if maintained in two straight parallel conductors of infinite length and negligible circular cross-section, and placed 1 meter apart in vacuum, would produce between these conductors a force of 2 × 10–7 newton per meter. It is named after the French physicist André Marie Ampère.
An ampere-hour (Ah) is the quantity of charge transported by an electric current in a given amount of time. It is the current time product at any level of current (Q = I × t). The coulomb is defined as 1 ampere-second, so 1 ampere-hour (Ah) would be 3600 A·s or 3600 C.
Most batteries are rated in Ah. A modern AA alkaline cell is rated at about 2.5–2.8 Ah (at the rate 100h rate). Battery capacities can be very sensitive to discharge rate and the characterizing rate will be given by the manufacturer. Nickel-cadmium and nickel metal-hydride cells, for example, are typically characterized at a 5h rate; sealed lead acid cells are typically characterized at a 20h rate. At different discharge rates, the Ah rating will be different.
The coulomb (C) is the derived unit of electric charge in the SI system of units. 1 coulomb is equal to the charge transferred by a current of one ampere in one second (that is, 1 C = 1 A s). The unit is named after the French physicist Charles Coulomb.
The farad (F) is the derived unit of electrical capacitance in the SI system of units. One farad (F) is the capacitance of an electric capacitor between the two plates of which there appears a difference of electric potential of one volt when it is charged by a quantity of electricity equal to one coulomb (F = C/V). The unit is named after the British scientist Michael Faraday.
One farad is an exceptionally large capacitance: the entire Earth, considered as a spherical conductor of radius 4,000 miles, has a calculated capacitance of only 0.0007 farad. Capacitances that are actually encountered are expressed in microfarads or picofarads.
The gilbert (Gb) is a now-obselete unit of magnetomotive force in the CGS system. It equals 10/4π ampere-turns.
The henry (H) is the derived unit of inductance in the SI system of units. One henry is equal to the inductance of a closed circuit in which an electromotive force of one volt is produced when the electric current in the circuit varies uniformly at a rate of one ampere per second. It is named after the American physicist Joseph Henry.
The neper (Np), named for the mathematician James Napier, is a unit used to express the ratio of quantities such as current or voltage. Thus the ratio N, in nepers, of current I1 and I2 is N = ln(I2/I1). The neper is thus the natural logarithmic equivalent (see logarithm) of the decibel. Power ratios, say of powers P1 and P2, can also be expressed in nepers; here N = ½ ln (P2/P1), and 1 neper = 8.686 dB.
The oersted (Oe) is the unit of magnetic field strength in CGS electromagnetic units. It is defined as the field at the center of a single-turn circular coil of radius 1 centimeter and carrying a current of 1/(2π) abamperes. It is named after the Danish physicist Hans Oersted.
The ohm (Ω) is the derived unit of electrical resistance, in the case of direct current, or impedance, in the case of alternating current, in the SI system of units. 1 ohm (Ω) is the resistance that produces a potential difference of one volt when a current of one ampere is flowing through it. It is named after the German physicist Georg Ohm.
The tesla (T) is the derived unit of magnetic flux density in the SI system of units. One tesla (T) is equal to one weber of magnetic flux per square meter, that is, 1 T = 1 Wb m–2. It is named after Nikola Tesla.
The volt (V) is the SI unit of electromotive force or electrical potential difference. The volt is defined as the potential difference across a conductor when a current of one ampere dissipates one watt of power. It is named after the Italian physicist Alessandro Volta.
The weber (Wb) is the derived unit of magnetic flux in the SI system of units. 1 weber is the flux that, linking a circuit of one turn, produces in it an electromotive force of 1 volt as it is reduced to zero at a uniform rate in 1 second. It is named after the German physicist Wilhelm Weber.