A single magnetic pole is called a magnetic monopole. They have never been found in Nature, although they do exist in the sense that the laws of physics make it possible for them to exist.
The mass of a particle with a magnetic charge can be estimated and it is found that they would be extremely heavy. So, they cannot be produced in normal reactions. However, early models of the Big Bang predicted that monopoles would be produced in such large quantities in the first fraction of the second after the Big Bang that this would cause the universe to collapse very soon after the Big Bang (due to the gravitational attraction of all this mass in the form of monopoles).
More recent models of the early universe predict that the universe underwent an extremely rapid expansion. This is called "inflation". Monopoles can still be formed in this scenario, but the rapid expamnsion of the universe would dilute them to such low levels that the density of monoples would be practically zero.
Now, without any magnetic monopoles you can still generate magnetic fileds. Magnetic fields can be generated by:
Changing electric fields:
Electric currents (i.e. a flow of electrically charged particles)
In a magnet most of the magnetic field is generated by the spin of the electrons.
The magnetic field lines can only start or end at a magnetic charge (monopole). So, if there aren't any magnetic charges, the field lines cannot start or end and they have to form closed loops. The field lines of a magnet start at one side, leave the magnet, curve back to the other side and return to the starting point inside the magnet.
The place were the field lines leave and enter the magnet are called the poles of the magnet (North pole and South pole), but they aren't real magnetic charges.
Electric fields can also be generated witout electric charges by changing magnetic fields. Also, theoretically, by a monopole current.
2) because the current can flow more easily as there is another path for the current.
3) If you touch two electrical wires at a different potential then, because your body has a resistance, a current will flow through your body.
4) The shock wave generated by the explosion of the gun powder will push on the cannonball for a longer time.