21.1 Magnets and Magnetic Fields
Reading Focus
Key Concepts
How do magnetic poles interact?- How can a magnetic field affect a magnet that enters the field?
- Why are some materials magnetic while others are not?
Vocabulary
magnetic force
magnetic pole
magnetic field
magnetosphere
magnetic domain
ferromagnetic material
Reading Strategy
Using Prior Knowledge Copy the diagram below and add what you already know about magnets. After you read, revise the diagram based on what you learned.
Ancient Greeks observed that magnetite, or lodestone, attracts iron. Some time before 200 A.D., the Chinese sculpted magnetite into spoonshaped compasses. They called these stones “south pointers.” By 1150 A.D., Chinese navigators used compasses with magnetized iron needles. But properties of magnets were not well explained until 1600. In that year, the English physician William Gilbert published De Magnete.
Magnetic Forces
You can explore properties of magnets on your own. Either side of a magnet sticks to a refrigerator. Yet if you push two magnets together, they may attract or repel. Magnetic force is the force a magnet exerts on another magnet, on iron or a similar metal, or on moving charges. Recall that magnetic force is one aspect of electromagnetic force.
Magnetic forces, like electric forces, act over a distance. Look at the suspended magnets in Figure 1. If you push down on the top two magnets, you can feel the magnets repel. Push harder, and the force increases. Magnetic force, like electric force, varies with distance.
Figure 1 The green magnet and lower red magnet attract each other. The lower red magnet and the yellow magnet repel each other. Predicting What would happen if the upper red magnet on the pencil were flipped over?
Gilbert used a compass to map forces around a magnetite sphere. He discovered that the force is strongest at the poles. All magnets have two magnetic poles, regions where the magnet's force is strongest. One end of a magnet is its north pole; the other end is its south pole. The direction of magnetic force between two magnets depends on how the poles face. Like magnetic poles repel one another, and opposite magnetic poles attract one another.
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