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How do we decide the polarity of a solenoid carrying current?
Answer 1:

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:

solenoid

References:
http://en.wikipedia.org/wiki/Right_hand_grip_rule#Direction_associated_with_a_rotation

*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 coil.

**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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