Magnetism and Magnets

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MAGNETISM AND MAGNETS

Magnetism is the phenomenon in which iron is attracted to a natural material called lodestone, the properties of which are similar to a magnet.

LODESTONE AND MAGNETS

Lodestone is a crystalline oxide of iron called magnetite. Until the 1800s, lodestones and the earth were the only sources of magnetism. Iron-based materials are attracted to a lodestone as well as to any other magnet. It is the attraction between a magnet and the iron in a refrigerator door that pins a photograph to the door. Bringing an iron-based material in contact with a magnet or lodestone will make a magnet of the material. Unfold a paperclip and rub it with a magnet. The clip becomes magnetized, with the magnetic properties concentrated near the ends of the clip. These end regions are called poles. Magnetize two unfolded paperclips and, in one parallel orientation, the ends or poles attract each other. Change the orientation of one clip and the poles repel each other. Attach a thread to the middle of one of the clips, suspend it, and one pole will point in the general direction of geographic north. The one pointing north is called an N-pole; the opposite pole is an S-pole. We observe, and say, "unlike poles attract, like poles repel." It is an instructive experiment to cut the magnetized clip into two pieces in an effort to isolate a pole. Interestingly, each of the two pieces has an N-pole and an S-pole. Regardless of how many times a clip is cut in two, magnetic poles always occur in pairs of N-poles and S-poles.

MAGNETIC FIELD

A magnet does not materially change the space around it. Yet, if the magnet were not there, another magnet would not experience a force when brought into the space. Magnetically, the space around the magnet is altered and the modification is thought of as producing a magnetic field. When another magnet is brought into the magnetic field and experiences a force, the magnetic field is the mechanism for exerting the force. The concept of a field applies to gravitational and electric forces as well, and is an extremely important aspect of many energy applications.

Magnetic fields are measured in units of teslas, symbol T. A strong permanent magnet, such as might be found in a physics laboratory, produces a magnetic field of about 0.3 T. Such a magnet is capable of lifting several kilograms of iron. For comparison, the magnetic fields produced by other systems include

10⁸T: neutron stars
10³T: short bursts of electric current
10¹T: strong laboratory superconducting magne
10^-5T: Earth's magnetic field
10^-9T: interplanetary magnetic fields
10^-12T: magnetic field associated with the human body

In 1819, Hans Christian Oersted, professor of physics at Copenhagen University, discovered that a magnet experiences a force when in the vicinity of a wire carrying an electric current. The fact that the magnet experiences a force is evidence that the electric current produces a magnetic field, which eventually led to the development of innumerable devices—electric motors, electric generators, speakers for hifidelity amplifiers, and electromagnets, to name a few—based on this principle.

ENERGY APPLICATIONS

Electric motors

Electric motors are found in nearly every room of a typical house. They power everything from the washing machine, refrigerator, and vacuum cleaner, to the hair dryer, fan, garage door opener, and disk drive in a computer. In an automobile, motors adjust seats, raise an antenna, operate windshield wipers, adjust mirrors, run fans, start the engine, and someday may replace the internal combustion engine under the hood. There is probably no device more useful for doing work than electric motors, and their pervasiveness will surely grow.

Fundamentally, an electric motor converts electric energy to rotational energy. Rotation results from magnetic forces between a rotating part (the rotor) and a stationary part (the stator). There are many designs. In the simplest, the rotor is an electromagnet that rotates between the poles of a permanent magnet (Figure 1). The N-poles and S-poles of the electromagnet are determined by the direction of current flow. In the illustration, attraction and repulsion between poles on the rotor and stator cause the electromagnet to rotate clockwise. When the unlike poles approach each other, the direction of current flow is reversed, causing the poles on the electromagnet to change. The alternating magnetic attraction and repulsion between poles keeps the electromagnet rotating in the same direction.

Electric Generators

An electric generator for operating lights is a common sight on many bicycles. The generator has a coil of wire that rotates between the poles of a magnet and looks very much like a motor. Whereas a motor converts electric energy into rotational energy, a generator converts mechanical energy to electric energy. On a bicycle, the tire rubs against a wheel attached to the rotating coil. Some agent, in this case the cyclist pedaling, does the work to turn the coil with the reward being electric energy. In a large electric power plant an electric generator working on the same physical principle is driven by a large steam turbine.

Electric generators are based on the principle that an electric charge experiences a force when it moves in a magnetic field. Electrons in the metallic wires of the rotor of a generator experience a force when the coil is rotated in a magnetic field. An electric current is produced in a light bulb, for example, when it is connected to the open ends of the wires making up the coils of the rotor.

Magnetic Levitation of Vehicles

Whereas electric motors utilize both attraction and repulsion of magnetic fields, there are energy applications that rely only on attraction or repulsion. Magnetic levitation of vehicles is a good example. A vehicle riding on a track or roadway experiences frictional forces that oppose the movement. Any scheme that can reduce the frictional forces offers improved energy economy. Magnetic levitation involves magnetic forces that hold a vehicle above a roadbed so that the vehicle appears to float on a cushion of air. It does not. It floats on a "magnet" cushion. One method of suspension capitalizes on the idea that like poles repel. The poles are produced by electromagnets rather than using permanent magnets. Another scheme is based on the principle that a metal experiences a force when in the magnetic field of an electromagnet that is energized by an electric current that changes rapidly with time.

Once a vehicle is levitated, it is not in material contact with a track, so cannot be propelled by wheels. The propulsion system, like the levitating system, is based on magnetic principles that are identical to the principles in an ordinary electric motor. The road bed is appropriately configured so that the magnetic field exerts a force on a current-carrying coil secured to the vehicle. In a real sense, the vehicle and magnetic road bed constitute a motor, albeit a linear motor (as opposed to the rotational motor shown in Figure 1). This is called a linear indication motor (LIM). The National Aviation and Space Administration (NASA) believes that this scheme could also be used to launch spacecraft into orbit. A magnetically levitated space vehicle accelerated to a speed of about 600 miles/hour would be catapulted from the ground. Once aloft, a rocket engine would take over and propel the spacecraft into orbit.

Magnetic controls

A magnetic field due to an electric current can be turned on and off simply by turning the current on and off. A piece of iron attached to the end of a spring having the other end fixed can be moved with a magnetic field and returned to its initial position by the spring. The iron piece can then be used to actuate a switch or move a lever on a valve. Applications of this principle include electrically controlled valves in a washing machine and an electrically controlled switch for the starter in an automobile.

Superconducting Magnets

The magnetic field produced by an electric current is proportional to the current; doubling the current doubles the magnetic field. It would seem that an experimenter could achieve any desired magnetic field by creating the necessary electric current in a coil of wire. But wires like those in an electric toaster offer resistance to electric currents, resulting in the production of heat. If the heat is not removed, the wires will melt. Solving this problem is the driving force behind the effort to develop superconducting materials that offer zero electrical resistance to electric current.

Until 1987, achieving zero resistance required cooling electrical conductors to around the temperature of liquid helium (4.2 K or -268.8°C). Nevertheless, practical electromagnets using superconducting wires cooled to around 4 K have been used in the laboratory for several decades. A new class of superconducting materials requiring cooling to around 77 K (-196^oC) was discovered in 1987. These materials are difficult to fabricate, but are very attractive because of their substantially higher superconducting temperature. Superconducting materials could be used for wires for transmission lines bringing electricity from an electric power plant to a city. Electromagnets made from superconducting wires can carry much more electric current, and so generate much stronger magnetic fields.

Magnetic Materials

Because magnetic poles occur in N and S pairs, a magnet is referred to as a magnetic dipole. The net magnetic field produced by two magnetic dipoles depends on their orientations. If the N-poles of each point in opposite direction, their contributions to the net magnetic field tend to cancel. If the N-poles of each point in the same direction, their contributions to the net magnetic field tend to add, and the net magnetic field is larger than that of one alone. The more magnets there are with their N-poles pointing in the same direction, the greater is the net magnetic field. In a real sense, iron atoms are like tiny magnetic dipoles. In an unmagnetized piece of iron, the tiny atomic magnetic dipoles are randomly oriented and there is no net magnetic field produced by the iron. But if the dipoles are given a preferred orientation, a net magnetic field around the iron results. The preferred orientation can be achieved by putting the iron in a magnetic field caused by a magnet or an electric current. This is a way of producing a permanent magnet.

Magnetic Recording

The binary system of numbers requires only two numbers, usually designated 0 and 1. Any device having two distinct states can be used to represent the two numbers. A finger pointed up could represent 1; pointing down it could represent 0. Zero and one can be represented with the N-poles of a magnetic dipole pointing in opposite directions. Information is recorded on a floppy disk or magnetic tape in binary fashion. The disk or tape is coated with a magnetizable material. Tiny local areas are magnetized with either the N-pole up or down to represent 0 or 1 . The device that reads the information detects the orientations and translates the information.

Magnetic Resonance Imaging

Magnetic Resonance Imaging (MRI) is a revolutionary diagnostic tool for producing images of the interior of a human body. It works because a proton, the sole constituent of a hydrogen atom, behaves like a tiny magnetic dipole. There is a copious source of hydrogen atoms in the body because many components contain water, and every water molecule has two hydrogen atoms. Normally, the atomic dipoles are oriented randomly, but they align when in a strong magnetic field. When stimulated by a short burst of radio frequency electromagnetic radiation, the atomic dipoles are deflected away from the direction of the magnetic field. Following the short burst of radiation, the dipoles rotate (precess) in ever-decreasing circles around the direction of the magnetic field, and in doing so emit a detectable signal before returning to a state of equilibrium. The detected signal can be converted into an image using computer technology. The technique has revolutionized the diagnosis of problems associated with muscles and joints in the human body.

Joseph Priest