安培定律：解决问题

For a thick conductor with uniform current, use an Amperian loop with radius smaller than the conductor radius. The line integral of the magnetic field equals the product of magnetic field and loop circumference. The enclosed current equals current density times the enclosed area. Applying Ampere's Law yields a magnetic field that increases linearly from the conductor's center to its surface.

Outside a thick conductor, use an Amperian loop with radius greater than the conductor radius. The enclosed current equals the total current flowing through the conductor. Applying Ampere's Law shows the magnetic field is inversely proportional to the loop radius, meaning field strength decreases as distance from the conductor increases.

Cylindrical symmetry ensures that magnetic field lines form concentric circles around the conductor axis, making them either constant or zero along a symmetric Amperian path. This symmetry allows you to choose an integration path where the magnetic field is tangential or perpendicular, simplifying the line integral calculation and enabling direct application of Ampere's Law.

Curl your right hand's fingers along the integration path direction. Your thumb points in the positive current direction. If enclosed current is positive and the magnetic field is tangential to the path, the field direction follows the integration direction. If enclosed current is negative, the magnetic field direction is opposite to the integration direction.

Use Biot-Savart Law when the current distribution lacks symmetry. Ampere's Law requires identifying symmetric current distributions and choosing symmetric integration paths where the magnetic field is constant or zero. For non-symmetric current distributions, Biot-Savart Law provides a more general approach to calculating magnetic fields without relying on symmetry.

At the conductor's surface, the magnetic field reaches its maximum value. Inside the conductor, the field increases linearly from the center to this maximum at the surface. Outside the conductor, the field magnitude drops inversely with distance. This transition occurs because the enclosed current changes from increasing linearly inside to remaining constant outside.

When the magnetic field is tangential to the Amperian path and constant along that portion, the line integral reduces to the product of the constant magnetic field magnitude and the path length for that section. For regions where the magnetic field is perpendicular to the path or has zero magnitude, the line integral contribution is zero, allowing you to focus only on tangential segments.