Explain why iron fillings which are sprinkled on a sheet of cardboard placed over a bar magnet, take up a definite pattern when cardboard is slightly tapped.

The iron fillings which are sprinkled on a sheet of cardboard placed over a bar magnet, take up a definite pattern when cardboard is slightly tapped because each piece of iron fillings gets magnetized by magnetic induction and experiences a force due to the magnet. Therefore, they arrange themselves along curved lines and these curved lines are called magnetic field lines.