Magnetic Field due to a Long Straight Wire

The magnetic field due to a long straight wire carrying a current I has magnitude given by

where r is the radial distance from the wire. The direction is azimuthal; that is, tangent to a circle centered on the wire in a sense given by the right hand rule.

Force Between Long Parallel Wires

The force between two long, parallel wires of length L carrying currents I1 and I2 and separated by a distance d has magnitude

The force is attractive if the currents are in the same direction and repulsive otherwise.