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
For more information and some helpful
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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.
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