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The force per unit length is: = (,,). The direction of the force is along the y-axis, representing wire 1 getting pulled towards wire 2 if the currents are parallel, as expected. The magnitude of the force per unit length agrees with the expression for F m L {\displaystyle {\frac {F_{m}}{L}}} shown above.
This force is used in the formal definition of the ampere. The SI unit of charge, the coulomb, was then defined as "the quantity of electricity carried in 1 second by a current of 1 ampere". [13]: 144 Conversely, a current of one ampere is one coulomb of charge going past a given point per second:
Symbol [1] Name of quantity Unit name Symbol Base units E energy: joule: J = C⋅V = W⋅s kg⋅m 2 ⋅s −2: Q electric charge: coulomb: C A⋅s I electric current: ampere
Ampere balance, an electromechanical apparatus for precise measurement of the ampere; Ampère's circuital law, a rule relating the current in a conductor to the magnetic field around it; Ampère's force law, the force of attraction or repulsion between two current-carrying wires
"The ampere, symbol A, is the SI unit of electric current. It is defined by taking the fixed numerical value of the elementary charge e to be 1.602 176 634 × 10 −19 when expressed in the unit C, which is equal to A s, where the second is defined in terms of ∆ν Cs." [1]
Furthermore, the same coherent SI unit may be a base unit in one context, but a coherent derived unit in another. For example, the ampere is a base unit when it is a unit of electric current, but a coherent derived unit when it is a unit of magnetomotive force. [1]: 140
The SI unit of mmf is the ampere, the same as the unit of current [3] (analogously the units of emf and voltage are both the volt). Informally, and frequently, this unit is stated as the ampere-turn to avoid confusion with current. This was the unit name in the MKS system. Occasionally, the cgs system unit of the gilbert may also be encountered.
In the International System of Units (SI), electric current is expressed in units of ampere (sometimes called an "amp", symbol A), which is equivalent to one coulomb per second. The ampere is an SI base unit and electric current is a base quantity in the International System of Quantities (ISQ).