When you demonstrate the magnetic field of a bar magnet using iron filings, the filings form lines. But isn't the field a continuous plane? What makes the lines form and why do they spread apart at the sides of the magnet and converge at the poles? 1. The field is indeed smooth -until you put iron filings into it. These become tiny bar magnets, their poles opposite to the field. This twists them and aligns them with the field, then makes them join nose-to-tail in the way bar magnets like to join. But bar magnets don't like to be side by side, so they repel their neighbours and the strings of filings push each other apart. Another way to look at it is that by opposing the field that magnetises them, the iron filings reduce the local magnetic field, and end up being pushed to areas where the residual field is strongest.
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