Magnetic Fields
A magnet can attract or repel another magnet or a piece of soft iron without touching it. Magnetic forces are exerted even when empty space, air, or any nonmagnetic material such as cardboard separates the two. Furthermore, two magnets can exert a force on each other even when they are placed at an angle to each other. Scientists describe the space around a magnet as occupied by a magnetic field. They can “map” the field by observing the direction of the magnetic force at many points around the magnet.
For example, a small compass may be placed in different positions near a long bar magnet. Since the compass is free to swing around, it will point in the direction of the magnetic field at each position. By observing the way the compass points, the direction of the magnetic field at any position can be determined.
The compass is moved in short steps, always in the direction in which it points. After each move, the position of the compass is marked. If the marks are connected by lines, they are seen to start at one pole and move around to the other pole. These lines are called lines of force, or lines of flux, of the magnetic field. By convention, the magnitude of the magnetic field is described as positive when it goes from north to south, negative when it goes from south to north. The lines of force exist inside the magnet as well: they go from one pole to the other. One may also sprinkle tiny iron filings on a piece of stiff paper covering a flat bar magnet. If the paper is gently tapped while the iron filings are sprinkled, the filings jump around and arrange themselves along the lines of the magnetic field. Where the lines are close to one another, the field is strong; where the lines are far apart, the field is weak. The lines and the filings are densest at the poles of the magnet, where the magnetic field is strongest.
The shape of a magnetic field can be changed by changing the shape of a magnet. If a long bar magnet is bent into a horseshoe shape, the magnetic poles are brought closer together. Most of the magnetic field lines lie between the two tips of the horseshoe. Since the field is strongest where the lines are densest, this shape produces a highly concentrated field between the two tips of the horseshoe. Magnets can be made in other shapes to produce other magnetic field patterns.