For a long straight wire the magnetic field has a magnitude:

B

=

mo I 2pr

where r represents the distance from the wire to the point where the field is being determined.

m_o is a constant known as the permeability of free space and has a value:

m_o = 4p x 10^-7 T m/A

For an infinitely long straight wire, the magnetic field lines are circles centered on the wire. The direction is given by a right-hand rule:

With your right hand, point your thumb in the direction of the current. When you curl your fingers they curl in the direction of the magnetic field.