Magnetic Lines of Force

1. Remove any labels from the plastic bottle.
2. Use the funnel to fill the bottle about one-fifth full of iron filings.
3. Wrap the top of the test tube with masking tape so the test tube will fit snugly into the mouth of the bottle, plugging the opening completely (see photo below).

   

4. Jam the tube into the mouth of the bottle.
5. Slide the cylindrical magnet into the test tube and put the bottle cap back on. (Click to enlarge the diagram of this assembly below.)
   

Turn the bottle on its side and rotate it. Watch what happens to the iron filings. They will form a three-dimensional pattern that traces out the magnetic field of the magnet.

Pay particular attention to what happens at the end of the magnet. Here, the iron filings stand up like a spiky punk haircut.

Shake the magnet out of the tube, and watch the filings collapse.

Each atom in a piece of iron is a magnet, with a north pole and a south pole. Most pieces of iron are not magnetic, because the atomic magnets all point in different directions.

When you bring a magnet near a piece of iron, the iron-atom magnets line up with the applied magnetic field: The north poles of the iron atoms all point in the same direction. Because the iron atoms line up, the piece of iron becomes a magnet and is attracted to the original magnet.

In a rod-shaped piece of iron, the atoms will tend to line up so that all the north poles face one end of the rod and all the south poles face the other end. Since iron filings are rod-shaped, the atoms line up pointing along the length of the rod, and the rods line up parallel to the direction of the applied magnetic field. The field of a cylindrical magnet comes out of the end of the magnet and then loops around next to the side. The iron filings stick out like a crew cut on the ends of the magnet but lie flat on the sides (click to enlarge diagram below).

Because the iron filings become magnets themselves, their presence slightly changes the shape of the magnetic field. Even so, this Snack gives an indication of the shape of the magnetic field in three dimensions.

Note that if you’ve sealed the plastic bottle really well by jamming the test tube into its mouth, the sides of the bottle will begin to collapse inward after a few hours—particularly if the inside of the bottle is damp. This happens because the iron filings will begin to rust. As the iron rusts, it combines with and removes oxygen from the air trapped in the bottle. To prevent the bottle’s collapse, simply punch a small hole in the plastic with a pushpin. Black sand from the beach—which is made of magnetite and does not rust—can also be used as a substitute for iron filings.

Cow magnets are strong, permanent magnets made out of alnico, an iron alloy containing aluminum, nickel, and cobalt. These magnets are available at most feed stores.

Ranchers feed these magnets to their cows. The magnet settles in the cow’s first stomach. When the cow accidentally eats bits of steel or iron, the magnet attracts the metal bits and holds them in its first stomach. If the sharp pieces of metal were to pass through the cow's digestive system, the animal would suffer what ranchers call “hardware disease.”