La Ley de Ampère

Ampere's Law states that, for a constant electric field and steady current, the line integral of a magnetic field along a closed curve equals the permeability of free space times the net current passing through the loop.

According to the right-hand thumb rule, if the curl of the fingers is along the loop direction, the thumb points to the current's direction. Conventionally, the current is positive in the direction of the thumb and negative when it is opposite to it.

Consider a straight current carrying conductor, having a magnetic field concentric to the conductor.

Now consider a circular counter-clockwise Amperian loop around this conductor. An infinitesimally small length dl on this loop is parallel to the magnetic field at each point.

The line integral of the magnetic field on this loop equals the constant magnetic field times the circumference of the loop.

Using Ampere's Law, the magnetic field for a straight conductor is directly proportional to the current and inversely proportional to the circle's radius.