Parallel wires carrying current in the same direction attract, and in the opposite direction repel. In cgs, the Biot-Savart law gives

where r is the radius of a loop around the first wire, B is the radial magnetic field strength, c is the speed of light, and is the current. The Lorentz force for a charge q traveling at speed v in a magnetic field of strength B is given by

so the differential force is given by

and the force per unit length of wire is

In MKS the Biot-Savart law gives

where is the permeability of free space, and the Lorentz force is

so

and the force per unit length of wire is