Magnets are attracted to some, but not all, metals. Iron, nickel, and alloys containing either or both of these elements are known as ferromagnetic materials. They “stick” to magnets. They can also be made into permanent magnets. When a magnet is brought near a piece of ferromagnetic material, the atoms in the material become lined up, so that the material is temporarily magnetized. This produces a magnetic force between the atoms of the ferromagnetic substance and those in the magnet.
Attraction and Repulsion
If a magnet is brought near another magnet, the force can be repulsive or attractive, depending on the way the magnets are oriented. The force gets stronger as the magnets are brought near each other. Some magnets are so strong that no human being can pull them apart if they get stuck together, and no person can bring them all the way together against their mutual repulsive force.
The tremendous forces produced by electromagnets are of use in industry. A large electromagnet can be used to carry heavy pieces of scrap iron from place to place. Other electromagnets can provide sufficient repulsion to suspend one object above another. This phenomenon is called magnetic levitation. It is the basis for low-friction, high-speed commuter trains now in use in some metropolitan areas.
Charge in Motion
Whenever the atoms in a ferromagnetic material are aligned, a magnetic field exists. A magnetic field can also be caused by the motion of electric charge carriers, either in a wire or in free space. The magnetic field around a permanent magnet arises from the same cause as the field around a wire that carries an electric current. The responsible factor in either case is the motion of electrically charged particles. In a wire, electrons move along the conductor, being passed from atom to atom. In a permanent magnet, the movement of orbiting electrons occurs in such a manner that an effective electrical current is produced.
Magnetic fields are also generated by the motion of charged particles through space. The sun is constantly ejecting protons and helium nuclei. These particles carry a positive electric charge. Because of this, and the fact that they are in motion, they are surrounded by tiny magnetic fields. When the particles approach the earth and their magnetic fields interact with the geomagnetic field, the particles are accelerated toward the geomagnetic poles. When there is a solar flare, the sun ejects far more charged particles than normal. When these approach the geomagnetic poles, the result is considerable disruption of the geomagnetic field. This type of event is called a geomagnetic storm. It causes changes in the earth’s ionosphere, affecting long distance radio communications at certain frequencies. If the fluctuations are intense enough, even wire communications and electric power transmission can be interfered with. Aurora (northern or southern lights) are frequently observed at night during these events.
1. The pattern of magnetic flux lines (dashed curves) around a bar magnet (rectangle). The N and S represent north and south magnetic poles, respectively.
2.The pattern of magnetic flux lines (dashed curves) around a straight, current carrying wire can be seen when the wire passes through a horizontal sheet of paper sprinkled with iron filings.
Have you seen the well-known experiment in which iron filings are placed on a horizontal sheet of paper, and then a magnet is placed underneath the paper? The filings arrange themselves in a pattern that shows, roughly, the shape of the magnetic field in the vicinity of the magnet. A bar magnet has a field with a characteristic form (Figure 1). Another popular experiment involves passing a current-carrying wire through a horizontal sheet of paper at a right angle, as shown in (Figure 2). The iron filings become grouped along circles centered at the point where the wire passes through the paper.
The intensity of a magnetic field is determined according to the number of flux lines passing through a certain cross section, such as a square centimeter or a square meter. The lines don’t exist as real objects, but it is intuitively appealing to imagine them that way. The iron filings on the paper really do bunch themselves into lines (curves, actually) when there is a magnetic field of sufficient strength to make them move. Sometimes lines of flux are called lines of force. But technically, this is a misnomer.
A magnetic field has a specific direction, as well as a specific intensity, at any given point in space near a current-carrying wire or a permanent magnet. The flux lines run parallel with the direction of the field. A magnetic field is considered to begin at the north magnetic pole, and to terminate at the south magnetic pole. In the case of a permanent magnet, it is obvious where the magnetic poles are. In the case of a current-carrying wire, the magnetic field goes in endless circles around the wire.
A charged electric particle, such as a proton or electron, hovering all by itself in space, constitutes an electric monopole. The electric lines of flux around an isolated, charged particle in free space are straight, and they “run off to infinity” (following figure). A positive electric charge does not have to be mated with a negative electric charge.
Electric flux lines (dashed lines) around an electrically charged object. This example shows a positive charge. The pattern of flux lines for a negative charge is identical.
A magnetic field is different. All magnetic flux lines, at least in ordinary real-world situations, are closed loops. With permanent magnets, there is a starting point (the north pole) and an ending point (the south pole). Around a straight, current-carrying wire, the loops are closed circles, even though the starting and ending points are not obvious. A pair of magnetic poles is called a magnetic dipole.
At first you might think that the magnetic field around a current-carrying wire is caused by a monopole, or that there aren’t any poles at all, because the concentric circles don’t actually converge anywhere. But you can envision a half plane, with the edge along the line of the wire, as a magnetic dipole. Then the lines of flux go around once in a 360° circle from the “north face” of the half plane to the “south face.”
The greatest flux density, or field strength, around a bar magnet is near the poles, where the lines converge. Around a current-carrying wire, the greatest field strength is near the wire.