#20 Divergence

This is called the Gauss's law for magnetism, and it is one of the Maxwell's equations that form the foundation of classical electrodynamics, classical optics and electric circuits. Gauss's law for magnetism states that there are no isolated magnetic charges but there are dipoles which generates the magnetic field. As you can check easily, when you have a magnet it always have a north pole and a south pole, and if you cut it apart, you'll have two magnets each of them with its own north and south pole. You can't have a north pole in one hand and a south pole in the other. That appropriate behaviour of magnetic charge make it possible for the compasses to work and to point the north pole of the Earth.

So, it is VERY important. Never EVER forget that divergence of B is zero, you'll never know when this information would save your ass.