The poles of a magnet

Spread some iron filings (tiny pieces of iron) on a sheet of paper and put a bar magnet over them. Do the filings stick all over the magnet equally? No. Most of them crowd near the two ends. Only a very few stick in the middle.

These two ends are the magnet's poles: the North pole and the South pole. This is where the magnet attracts most strongly. Most iron filings stick to the poles of a magnet of any shape, not only a bar magnet.

So if you roll a bar magnet over a heap of steel U-clips, many clips stick at one end, very few in the middle and many at the other end. Your book's answer: 10 at one end, 2 in the middle, 10 at the other end.

Poles always come in pairs. Break a magnet in two and a new pole appears at the break, so each piece still has a North pole and a South pole. Break it again and again: even the smallest piece has both. A single North pole or a single South pole on its own cannot exist. North and South are opposites, but they always stay together, like Tom and Jerry in one house. Whatever the shape (bar, disc, cylinder, ring or sphere), the poles come in pairs.