Magnetic Field and Polarity of a Coil:

There are two results occur while Bending a straight conductor into a loop:  (1) magnetic field lines become denser within the loop, and (2) all lines inside the loop are aiding in the similar direction.

When a conductor is shaped into various loops, it is considered to be a coil.  For determine the polarity of a coil, use the left-hand rule for coils in the given Figure.

Figure: Left-hand Rule for Coils

Adding an iron core within of a coil will raise the flux density.  The polarity of the iron core will be the similar as that of the coil.  The Current flow is from the negative side of the voltage source, by the coil, and back to the positive side of the source in the Figure.

Figure: Left-hand Rule to Find North Pole of an Electromagnet