Question

Write an expression for the magnetic force per unit length between two parallel thin current carrying wires. Hence define one ampere.

Answer 1

`F/L = (mu_0 I_1 I_2)/(2 pi r)`    ...(I₁ is the current in first wire and I₂ is the current in second wire.)

Thus we define ampere as the current flowing in each conductor separated by a unit distance so that one conductor applies a force of 2 × 10⁻⁷ N on a unit length of another parallel conductor.