Define $1 \ A$ (Ampere) using the force between two current-carrying parallel wires.
(N/A) The force per unit length between two parallel wires carrying currents $I_1$ and $I_2$ separated by a distance $d$ is given by the formula: $F/L = \frac{\mu_0 I_1 I_2}{2 \pi d}$.
If $I_1 = I_2 = 1 \ A$ and $d = 1 \ m$,then the force per unit length is $F/L = \frac{4 \pi \times 10^{-7} \times 1 \times 1}{2 \pi \times 1} = 2 \times 10^{-7} \ N/m$.
Therefore,$1 \ A$ is defined as the constant current which,if maintained in two straight parallel conductors of infinite length and negligible circular cross-section,placed $1 \ m$ apart in a vacuum,would produce between these conductors a force equal to $2 \times 10^{-7} \ N$ per meter of length.
If $I_1 = I_2 = 1 \ A$ and $d = 1 \ m$,then the force per unit length is $F/L = \frac{4 \pi \times 10^{-7} \times 1 \times 1}{2 \pi \times 1} = 2 \times 10^{-7} \ N/m$.
Therefore,$1 \ A$ is defined as the constant current which,if maintained in two straight parallel conductors of infinite length and negligible circular cross-section,placed $1 \ m$ apart in a vacuum,would produce between these conductors a force equal to $2 \times 10^{-7} \ N$ per meter of length.
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