Suppose we have a thick slab with a current density J pointing strictly in x direction...see below link for figure: http://teacher.pas.rochester.edu/PHY217/LectureNotes/Chapter5/LectureNotesChapter5056.jpg what is direction of magnetic field inside? It is easy for me to see that outside, above the slab, it points to the left, and below it, the field points to the right. I get this by visualizing the J as a bunch of current carrying wires all carrying current in +x direction. But using this idea, I think that insidem the field should cancel, although I know that Ampere's law clearly says that there must be a field since there is a net current enclosed... If someone could walk me through understanding the direction of the field inside, I would really appreciate it!