Legge di Ampère

Ampere's Law states that 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. This relationship holds for steady currents and constant electric fields. It expresses the fundamental connection between magnetic fields and their current sources, unlike electrostatic fields which are conservative.

According to the right-hand thumb rule, curl your fingers along the loop direction and your thumb points to the positive current direction. Current flowing in the thumb's direction is positive; current opposite to it is negative. This convention determines the sign of the enclosed current used in Ampere's Law calculations.

Ampere's Law is particularly effective for highly symmetric current distributions, similar to how Gauss's Law works for symmetric charge distributions. For symmetric geometries like straight wires or solenoids, the law simplifies magnetic field calculations significantly. It allows direct computation without integrating complex field contributions from each current element.

For a straight current-carrying conductor, the magnetic field is directly proportional to the current and inversely proportional to the distance from the wire. The magnetic field forms concentric circles around the conductor. Using Ampere's Law with a circular Amperian loop, the circulation of the magnetic field equals the permeability times the enclosed current.

Ampere's Law is valid only when currents are steady and no magnetic materials or time-varying electric fields are present. These conditions ensure the magnetic field remains static and directly related to the current distribution. Violations of these conditions require more complex formulations to account for changing fields or material effects.

If the line integral equals zero, it does not mean the magnetic field is zero everywhere along the path. Instead, it indicates that the net current enclosed by the closed path is zero. Positive and negative currents may cancel, resulting in zero circulation even though the magnetic field exists at various points along the path.

The circulation of a magnetic field—the line integral along a closed path—equals the permeability of free space times the enclosed current. For an infinitely long straight wire, a small length element parallel to the magnetic field acts tangentially to the path. This relationship forms the mathematical foundation of Ampere's Law for determining magnetic field patterns.