You probably know thatmagnetsattract specific metals and they have north and south poles. Opposite poles attract each other while like poles repel each other. Magnetic and electrical fields are related, and magnetism, along withgravityand strong and weakatomicforces, is one of thefour fundamental forces in the universe.
But none of those facts answers the most basic question: What exactly makes a magnet stick to certain metals? Or why don't they stick to other metals? Why do they attract or repel each other, depending on their positioning? And what makes neodymium magnets so much stronger than the ceramic magnets we played with as children?
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To understand the answers to these questions, it helps to have a basic definition of a magnet. Magnets are objects that producemagnetic fieldsand attract metals like iron, nickel andcobalt. The magnetic field's lines of force exit the magnet from its north pole and enter its south pole. Permanent or hard magnets create their own magnetic field all the time. Temporary or soft magnets produce magnetic fields while in the presence of a magnetic field and for a short while after exiting the field.Electromagnetsproduce magnetic fields only whenelectricitytravels through their wire coils.
Becauseelectrons and protons are tiny magnets, all materials have some sort of magnetic property. In most materials, however, the wayelectrons spin in opposite directionscancels out an atom's magnetic properties. Metals are the most common choices to manufacture magnets. Although some are made from simple metals, combinations of metals — called alloys — produce magnets of different strengths. For example:
- Ferritesorceramic magnets: These are like those used in refrigerator magnets and elementary-school science experiments. They contain iron oxide and other metals in a ceramic composite. A ceramic magnet known as lodestone, or magnetite, wasthe first magnetic material discoveredandoccurs naturally. Even though the ceramic magnet has been around for so long, they weren't commercially produced until 1952. Although they're common and keep their magnetism, they tend to have a weaker magnetic field (known as theenergy product) than other types of magnets.
- Alnico magnets: These weredeveloped in the 1930sand are made from aluminum, nickel and cobalt. They're stronger than ceramic magnets, but not as strong as the ones that incorporate a class of elements known asrare-earth metals.
- Neodymium magnets: These contain iron, boron and the rare-earth element neodymium, and as of this writing,they are the strongest商用磁体。他们第一次看起来d in the 1980s after scientists at theGeneral Motors Research Laboratoriesand theSumitomo Special Metals Companypublished their research.
- Samarium cobalt magnets: These weredeveloped by scientistsat the Dayton University Research University in the 1960s, and combine cobalt with the rare-earth element samarium. In the past few years, scientists have also discoveredmagnetic polymers, or plastic magnets. Some of these are flexible and moldable. However, some work only at extremely low temperatures, and others pick up only very lightweight materials, like iron filings.
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