To find the polarity of a solenoid, you can use
the right-hand rule for current (or left-hand rule
for electrons since they go in the opposite
direction to the current). To do this, point your
right thumb in the direction that the current
travels in the coiled wire in the solenoid and
curl your fingers.* The direction your fingers
point will determine the direction of the magnetic
field created by the current.
Alternately the right-hand-grip rule is easier
to use with solenoids. If you wrap your fingers in
the direction of the current traveling in the
solenoid, your thumb will point in the direction
of the North Pole of the magnetic field**.
The solenoid itself is simply a coil of wire
with no directional components (e.g. a diode), so
it can simply be reversed in the circuit to
generate the magnetic field in the opposite
direction, or you can reverse the direction of the
current traveling through the solenoid.
If you would like to know more about solenoids
and magnetic fields, this website has a good
explanation with diagrams:
*For this method you must point your thumb
parallel to the wire, not the solenoid. Your thumb
will be pointing perpendicular to the axis of the
solenoid because of the way the wire wraps in the
**For this method, wrap your fingers around
the solenoid as though you were holding it in your
hand, with your finger tips pointing in the
direction of the current.
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