In many cases (generators), AC current is produced by rotating magnets around a coil. As the poles of the magnets change, the direction of the magnetic field changes, and the direction of the current produced in the coil also changes. The direction of the current changes because the direction of the magnetic flux relative to the plane of the coils changes. In other words, there is a directionality to the orientation of the two poles of the magnet such that "north-to-south" does NOT equal "south-to-north".

The magnitude of the flux (how much magnetic field goes through a plan perpendicular to it) would be the same (for example, 2 teslas), but the direction is not the same. The concept is analogous to the positive and negative sides of a number line, and we call such quantities "vectors" - quantities that must be specified in terms of both magnitude AND direction. Because the direction of the magnetic field changes, the resulting direction of the current also changes. To make this a little clearer, it would not matter whether the north pole of the magnet starts on the left or right side of the coil of wires. The rotation of the magnet would still produce alternating currents.

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It is because a static magnetic field generated by the inductor coil does not create a voltage in the target coil. By flipping the current in the inductor coil on and off, you create alternating voltages in the target coil by virtue of the magnetic field appearing and disappearing, i.e. current on, magnetic field on, creates voltage in direction of magnetic field; current off, magnetic field off, creates voltage opposite the direction of formerly existing magnetic field.