Magnets come in many different shapes, sizes and strengths. According to physical laws, any moving electric charge will produce a magnetic field. This applies to the electrons of the iron atoms in a permanent magnet or the electricity in the wire surrounding an electromagnet. In addition to these everyday magnets, you'll find that aluminium has magnetic properties, and water and gold create an odd kind of reverse magnetism.
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The latches in your bathroom medicine cabinet and the ear buds for your portable music player both use permanent magnets. When you combine certain metals, such as iron and nickel, and expose the resulting object to a strong magnetic field, it acquires a field of its own. A permanent magnet has distinct north and south poles. It will stick to an iron or steel surface. A permanent magnet, as with all magnets, exhibits attraction and repulsion to other magnets -- like poles repel, opposites attract. With care, a permanent magnet remains strong for many years, though heat and sharp impacts weaken it.
If you've watched a junkyard crane lift car bodies, you've seen a big electromagnet in action. An electromagnet consists of a coil of wire wound around a piece of iron. With no electricity flowing through it, the electromagnet has no magnetic field. With the current turned on, the magnet produces a field. The bigger the coil, and the more electricity it gets, the stronger the magnet becomes. If you reverse the flow of current through an electromagnet, its north and south poles also reverse.
You can't turn a piece of aluminium or copper into a permanent magnet. Only iron, nickel, and a few other metals have this ability. They do, however, produce weak magnetic fields in the presence of another magnet. Scientists call this property paramagnetism. When you remove the magnetic field, the field around these materials disappears. Recycling plants use this property to sort aluminium cans from plastic and other trash. A strong electromagnet sends the cans flying into a special collection bin.
A few materials produce an opposite magnetic field when placed near a magnet. Since its north pole will face the magnet's north pole, the two objects repel each other. The force produced by these diamagnetic materials is very weak, unless you have a very strong magnet. Scientists have levitated drops of water, pieces of graphite and even small frogs by diamagnetic repulsion with some of the world's strongest magnets.
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