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Chapter 8 Electromagnetism and EM Waves (Section 1). Metal Detectors. Metal detectors are the first line of defense against persons trying to smuggle weapons onto passenger planes or into schools, government buildings, and many other places. Metal Detectors.
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Metal Detectors • Metal detectors are the first line of defense against persons trying to smuggle weapons onto passenger planes or into schools, government buildings, and many other places.
Metal Detectors Metal detectors probe your clothing and body without physically touching you, looking for metal that could be part of a gun, a knife, or other dangerous object. A device that can find hidden items on a person walking through an arch seems like something from science fiction. But, in today’s world, it is routine.
Metal Detectors How do these devices work their magic? Although metal detectors operate on electricity, it is magnetism that probes you. Brief magnetic pulses are sent around and through you, typically at a rate of about 100 times a second. The device carefully monitors how swiftly each magnetic pulse dies out.
Metal Detectors Any metal object encountered by a pulse is induced to produce its own magnetic pulse, which affects how rapidly the total pulse dies out. Sophisticated electronics in the metal detectors sense this change and signal that metal is present. They detect iron and other metals that ordinary magnets attract as well as metals such as aluminum and gold that do not respond to magnets.
Metal Detectors This explanation might raise some questions in your mind. How are the magnetic pulses produced? How does the metal detector monitor how the pulses die out? Why do they cause nonmagnetic metals such as aluminum to produce magnetic pulses? The answers lie in the key concepts presented in this chapter, the fundamental ways in which electricity and magnetism interact with each other.
Metal Detectors Magnetism and its useful interrelationship with electricity are the subjects of this chapter. First, the properties of permanent magnets and Earth’s magnetic field are described. Next, we demonstrate how electric fields and magnetic fields intertwine whenever motion or change is involved.
Metal Detectors These concepts are used to explain how many common electrical devices operate. They also suggest the existence of electromagnetic (EM) waves. The properties and uses of the different types of EM waves are the main topics of the latter half of this chapter.
8.1 Magnetism Magnetism was first observed in a naturally occurring ore called lodestone. Lodestones were fairly common around Magnesia, an ancient city in Asia Minor. Small pieces of iron, nickel, and certain other metals are attracted by lodestones, much as pieces of paper are attracted by charged plastic.
8.1 Magnetism The Chinese were probably the first to discover that a piece of lodestone will orient itself north and south if suspended by a thread or floated on water on a piece of wood. The compass revolutionized navigation because it allowed mariners to determine the direction of north even in cloudy weather. It was also one of the few useful applications of magnetism up to the 19th century.
8.1 Magnetism Now magnets are made into a variety of sizes and shapes out of special alloys that exhibit much stronger magnetism than lodestone. All simple magnets exhibit the same compass effect—one end or part of it is attracted to the north, and the opposite end or part is attracted to the south. The north-seeking part of a magnet is called its north pole, and the south-seeking part is its south pole.
8.1 Magnetism All magnets have both poles. If a magnet is broken into pieces, each part will have its own north and south poles. The south pole of one magnet exerts a mutually attractive force on the north pole of a second magnet. The south poles of two magnets repel each other, as do the north poles. Simply put: like poles repel, unlike poles attract (just as with electric charges).
8.1 Magnetism Metals that are strongly attracted by magnets are said to be ferromagnetic. Such materials have magnetism induced in them when they are near a magnet. If a piece of iron is brought near the south pole of a magnet, the part of the iron nearest the magnet has a north pole induced in it, and the part farthest away has a south pole induced in it.
8.1 Magnetism Once the iron is removed from the vicinity of the magnet, it loses most of the induced magnetism. Some ferromagnetic metals actually retain the magnetism induced in them—they become permanent magnets. Common household magnets and compass needles are made of such metals. Ferromagnetism is also the basis of magnetic data recording, but more on this later.
8.1 Magnetism As with gravitation and electrostatics, it is useful to employ the concept of a field to represent the effect of a magnet on the space around it. A magnetic field is produced by a magnet and acts as the agent of the magnetic force. The poles of a second magnet experience forces when in the magnetic field: Its north pole has a force in the same direction as the magnetic field, but its south pole has a force in the opposite direction.
8.1 Magnetism A compass can be thought of as a “magnetic field detector” because its needle will always try to align itself with a magnetic field.
8.1 Magnetism The shape of the magnetic field produced by a magnet can be “mapped” by noting the orientation of a compass at various places nearby. Magnetic field lines can be drawn to show the shape of the field. The direction of a field line at a particular place is the direction that the north pole of a compass needle at that location points.
8.1 Magnetism Because magnets respond to magnetic fields, the fact that compass needles point north indicates that Earth itself has a magnetic field. The shape of Earth’s field has been mapped carefully over the course of many centuries because of the importance of compasses in navigation.
8.1 Magnetism Earth’s magnetic field has the same general shape as the field around a bar magnet, with its poles tilted about 11 with respect to the axis of rotation.
8.1 Magnetism The direction of “true north” shown on maps is determined by the orientation of Earth’s axis of rotation. The axis is aligned closely with Polaris, the North Star. Because of the tilt of Earth’s “magnetic axis,” at most places on Earth compasses do not point to true north.
8.1 Magnetism For example, in the western two-thirds of the United States, compasses point to the right (east) of true north, whereas in New England compasses point to the left (west) of true north.
8.1 Magnetism The difference, in degrees, between the direction of a compass and the direction of true north varies from place to place and is referred to as the magnetic declination. In parts of Alaska, the magnetic declination is as high as 25 east. This must be taken into account when navigating with a compass.
8.1 Magnetism Earth’s field is responsible for the magnetism in lodestone. This naturally occurring ferromagnetic ore is weakly magnetized by Earth’s magnetic field. Another thing to note about Earth’s magnetic field: Earth’s north magnetic pole is at (near) its south geographic pole, and vice versa. Why?
8.1 Magnetism 1. The north pole of a magnet is attracted to the south pole of a second magnet. 2. The north pole of a compass needle points to the north. Therefore, a compass’s north pole points at the Earth’s south magnetic pole. This is not a physical contradiction: It is a result of naming the poles of a magnet after directions instead of, say, + and –, or A and B.
8.1 Magnetism Some organisms use Earth’s magnetic field to aid navigation. Although the biological mechanisms that they employ have not yet been fully identified, certain species of fish, frogs, turtles, birds, newts, and whales are able to sense the strength of Earth’s field or its direction (or both).
8.1 Magnetism The strength of the field allows the animal to determine its approximate latitude (how far north or south it is) because Earth’s magnetic field is stronger near the magnetic poles. Some migratory species travel thousands of miles before returning home, guided—at least in part—by sensing Earth’s magnetic field.
8.1 Magnetism Superconductors, so named because of their ability to carry electric current with zero resistance, react to magnetic fields in a rather startling fashion. In the superconducting state, the material will expel any magnetic field from its interior. This phenomenon, known as the Meissnereffect, is why strong magnets are levitated when placed over a superconductor.
8.1 Magnetism When trying to determine whether a material is in the superconducting state, it is easier to test for the presence of the Meissner effect than it is to see if the resistance is exactly zero. You have probably noticed that magnetism and electrostatics are very similar: There are two kinds of poles and two kinds of charges. Like poles repel, as do like charges. There are magnetic fields and electric fields. However, there are some important differences.
8.1 Magnetism Each kind of charge can exist separately, whereas magnetic poles always come in pairs. Modern theory indicates the possible existence of a particular type of subatomic “elementary particle” that has a single magnetic pole, which has not been found. Furthermore, all conventional matter contains positive and negative charges (protons and electrons) and can exhibit electrostatic effects by being “charged.” But, with the exception of ferromagnetic materials, most matter shows very little response to magnetic fields.
8.1 Magnetism We should also point out that the electrostatic and magnetic effects described so far are completely independent. Magnets have no effect on pieces of charged plastic, for instance, and vice versa. This is the case as long as there is no motion of the objects or changes in the strengths of the electric and magnetic fields. A number of fascinating and useful interactions between electricity and magnetism take place when motion or change in field strength occurs.
