Sources and Laws of Magnetism in Physics

Magnetism has no monopoles, so a magnetic field can never begin or end anywhere. Every field line has to close on itself, which is why the list of sources on this page is nothing but moving charge. Take any current, break it into small pieces, and the Biot-Savart law tells you the field each piece contributes. For the shapes worth remembering, a straight wire, a loop, a solenoid or a toroid, Ampere's law gets the same answer in a single line by choosing a loop that follows the symmetry. The blocks end with what happens once a material is placed in the field, where the material's own tiny currents either help the field along or push back against it.