10 Magnetism

Learning Objectives

After successful completion of this section, you will be able to demonstrate competency in the following areas:

  • Define: magnetic force (CLO1)(CLO2)(CLO3)
  • Explain the effects of magnetic fields on moving charge (CLO2)(CLO3)
  • Describe how electric motors work and the role of magnetism in their function (CLO2)(CLO3)

 

A shimmering curtain of green lights in the sky above a snow covered landscape. Stars are visible in the dusky sky beyond the lights.
Fig. 10.1 The magnificent spectacle of the Aurora Borealis, or northern lights, glows in the northern sky above Bear Lake near Eielson Air Force Base, Alaska. Shaped by the Earth’s magnetic field, this light is produced by radiation spewed from solar storms.

Introduction to Magnetism

One evening, an Alaskan sticks a note to his refrigerator with a small magnet. Through the kitchen window, the Aurora Borealis glows in the night sky. This grand spectacle is shaped by the same force that holds the note to the refrigerator.

People have been aware of magnets and magnetism for thousands of years. The earliest records date to well before the time of Christ, particularly in a region of Asia Minor called Magnesia (the name of this region is the source of words like magnetic). Magnetic rocks found in Magnesia, which is now part of western Turkey, stimulated interest during ancient times. A practical application for magnets was found later, when they were employed as navigational compasses. The use of magnets in compasses resulted not only in improved long-distance sailing, but also in the names of “north” and “south” being given to the two types of magnetic poles.

Today magnetism plays many important roles in our lives. Physicists’ understanding of magnetism has enabled the development of technologies that affect our everyday lives. The iPod in your purse or backpack, for example, wouldn’t have been possible without the applications of magnetism and electricity on a small scale.

All electric motors, with uses as diverse as powering refrigerators, starting cars, and moving elevators, contain magnets. Generators, whether producing hydroelectric power or running bicycle lights, use magnetic fields. Recycling facilities employ magnets to separate iron from other refuse. Hundreds of millions of dollars are spent annually on magnetic containment of fusion as a future energy source. Magnetic resonance imaging (MRI) has become an important diagnostic tool in the field of medicine, and the use of magnetism to explore brain activity is a subject of contemporary research and development. The list of applications also includes computer hard drives, tape recording, detection of inhaled asbestos, and levitation of high-speed trains. Magnetism is used to explain atomic energy levels, cosmic rays, and charged particles trapped in the Van Allen belts. Once again, we will find all these disparate phenomena are linked by a small number of underlying physical principles.

Magnets

Bar magnets, horseshoe magnets, and disc-shaped magnets attract and repel each other. Metal paperclips stick to some of the magnets.
Fig 10.2 Magnets come in various shapes, sizes, and strengths. All have both a north pole and a south pole. There is never an isolated pole (a monopole).

All magnets attract iron, such as that in a refrigerator door. However, magnets may attract or repel other magnets. Experimentation shows that all magnets have two poles. If freely suspended, one pole will point toward the north. The two poles are thus named the north magnetic pole and the south magnetic pole (or more properly, north-seeking and south-seeking poles, for the attractions in those directions).

It is a universal characteristic of all magnets that like poles repel and unlike poles attract. (Note the similarity with electrostatics: unlike charges attract and like charges repel.) Further experimentation shows that it is impossible to separate north and south poles in the manner that + and − charges can be separated.

 

A globe of the Earth with a bar magnet inside it. The south pole of the bar magnet inside the globe is at the south magnetic pole and is near, but not exactly on, the north geographic pole. The north pole of the bar magnet inside the globe is near the south geographic pole. Another bar magnet hangs beside the globe. The north pole of this magnet is pointing toward the north pole of the globe (or the south pole of the magnet inside the globe).
Fig. 10.3 One end of a bar magnet is suspended from a thread that points toward north. The magnet’s two poles are labeled N and S for north-seeking and south-seeking poles, respectively.

 

Misconception Alert

Earth acts like a very large bar magnet with its south-seeking pole near the geographic North Pole. That is why the north pole of your compass is attracted toward the geographic north pole of Earth—because the magnetic pole that is near the geographic North Pole is actually a south magnetic pole! Confusion arises because the geographic term “North Pole” has come to be used (incorrectly) for the magnetic pole that is near the North Pole. Thus, “north magnetic pole” is actually a misnomer—it should be called the south magnetic pole.

Two sets of bar magnets. The first set of magnets are oriented with the unlike poles adjacent to each other. Force arrows show that these magnets are pulling on each other. The second set of magnets is oriented with the like poles adjacent to each other. Force arrows show that these magnets are pushing each other away.
Fig. 10.4 Unlike poles attract, whereas like poles repel.

 

A bar magnet is split in half several times. The original magnet has a south pole and a north pole. Each time the magnet is split, each new half has both a south pole and a north pole.
Fig. 10.5 North and south poles always occur in pairs. Attempts to separate them result in more pairs of poles. If we continue to split the magnet, we will eventually get down to an iron atom with a north pole and a south pole—these, too, cannot be separated.

 

The fact that magnetic poles always occur in pairs of north and south is true from the very large scale—for example, sunspots always occur in pairs that are north and south magnetic poles—all the way down to the very small scale. Magnetic atoms have both a north pole and a south pole, as do many types of subatomic particles, such as electrons, protons, and neutrons.

 

Take-Home Experiment: Refrigerator Magnets

We know that like magnetic poles repel and unlike poles attract. See if you can show this for two refrigerator magnets. Will the magnets stick if you turn them over? Why do they stick to the door anyway? What can you say about the magnetic properties of the door next to the magnet? Do refrigerator magnets stick to metal or plastic spoons? Do they stick to all types of metal?

 

Ferromagnets

Only certain materials, such as iron, cobalt, nickel, and gadolinium, exhibit strong magnetic effects. Such materials are called ferromagnetic, after the Latin word for iron, ferrum. A group of materials made from the alloys of the rare earth elements are also used as strong and permanent magnets; a popular one is neodymium. Other materials exhibit weak magnetic effects, which are detectable only with sensitive instruments. Not only do ferromagnets respond strongly to magnets (the way iron is attracted to magnets), they can also be magnetized themselves—that is, they can be induced to be magnetic or made into permanent magnets.

 

An unmagnetized piece of iron is turned into a permanent magnet using heat and another magnet.
Fig 10.6 An unmagnetized piece of iron is placed between two magnets, heated, and then cooled, or simply tapped when cold. The iron becomes a permanent magnet with the poles aligned as shown: its south pole is adjacent to the north pole of the original magnet, and its north pole is adjacent to the south pole of the original magnet. Note that there are attractive forces between the magnets.

 

Electromagnets

Early in the 19th century, it was discovered that electrical currents cause magnetic effects. The first significant observation was by the Danish scientist Hans Christian Oersted (1777–1851), who found that a compass needle was deflected by a current-carrying wire. This was the first significant evidence that the movement of charges had any connection with magnets. Electromagnetism is the use of electric current to make magnets. Such a temporarily induced magnet is called an electromagnet. Electromagnets are employed for everything from a wrecking yard crane that lifts scrapped cars to controlling the beam of a 90-km-circumference particle accelerator to the magnets in medical imaging machines. (See Figure 10.7.)

 

M R I machine at a hospital.
Fig. 10.7 Instrument for magnetic resonance imaging (MRI). The device uses a superconducting cylindrical coil for the main magnetic field. The patient goes into this “tunnel” on the gurney.

 

Figure 10.8 shows the response of iron filings to a current-carrying coil and to a permanent bar magnet. The patterns are similar. In fact, electromagnets and ferromagnets have the same basic characteristics—for example, they have north and south poles that cannot be separated and for which like poles repel and unlike poles attract.

 

The arrangement of iron filings as they are affected by a metal coil that is carrying an electric current and a bar magnet. At the poles of the magnet, the filings are aligned radially to the poles. Between the poles, the filings are roughly parallel to the magnet. Thus, from one pole to the other, the filings have an arcuate arrangement. The density of filings is very high at the poles and relatively low on either side of the center of the magnet. The arrangement is similar around the current-carrying coil.
Fig. 10.8 Iron filings near (a) a current-carrying coil and (b) a magnet act like tiny compass needles, showing the shape of their fields. Their response to a current-carrying coil and a permanent magnet is seen to be very similar, especially near the ends of the coil and the magnet.

Combining a ferromagnet with an electromagnet can produce particularly strong magnetic effects. (See Figure 10.9.) Whenever strong magnetic effects are needed, such as lifting scrap metal, or in particle accelerators, electromagnets are enhanced by ferromagnetic materials.

 

The arrangement of iron filings as they are affected by a metal coil that is carrying an electric current and a bar magnet. At the poles of the magnet, the filings are aligned radially to the poles. Between the poles, the filings are roughly parallel to the magnet. Thus, from one pole to the other, the filings have an arcuate arrangement. The density of filings is very high at the poles and relatively low on either side of the center of the magnet. The arrangement is similar around the current-carrying coil.
Fig. 10.9 An electromagnet with a ferromagnetic core can produce very strong magnetic effects. Alignment of domains in the core produces a magnet, the poles of which are aligned with the electromagnet.

 

Figure 10.10  shows a few uses of combinations of electromagnets and ferromagnets. Ferromagnetic materials can act as memory devices, because the orientation of the magnetic fields of small domains can be reversed or erased. Magnetic information storage on videotapes and computer hard drives are among the most common applications. This property is vital in our digital world.

 

Three views into a computer disk showing the magnetic portions of the recording head and the tape.
Fig. 10.10 An electromagnet induces regions of permanent magnetism on a floppy disk coated with a ferromagnetic material. The information stored here is digital (a region is either magnetic or not); in other applications, it can be analog (with a varying strength), such as on audiotapes.

Current: Source of All Magnetism

An electromagnet creates magnetism with an electric current. But what about ferromagnets?  Figure 10.12 shows models of how electric currents create magnetism at the submicroscopic level. (Note that we cannot directly observe the paths of individual electrons about atoms, and so a model or visual image, consistent with all direct observations, is made. We can directly observe the electron’s orbital angular momentum, its spin momentum, and subsequent magnetic moments, all of which are explained with electric-current-creating subatomic magnetism.) Currents, including those associated with other submicroscopic particles like protons, allow us to explain ferromagnetism and all other magnetic effects. Ferromagnetism, for example, results from an internal cooperative alignment of electron spins, possible in some materials but not in others.

Crucial to the statement that electric current is the source of all magnetism is the fact that it is impossible to separate north and south magnetic poles. (This is far different from the case of positive and negative charges, which are easily separated.) A current loop always produces a magnetic dipole—that is, a magnetic field that acts like a north pole and south pole pair. Since isolated north and south magnetic poles, called magnetic monopoles, are not observed, currents are used to explain all magnetic effects. If magnetic monopoles did exist, then we would have to modify this underlying connection that all magnetism is due to electrical current. There is no known reason that magnetic monopoles should not exist—they are simply never observed—and so searches at the subnuclear level continue. If they do not exist, we would like to find out why not. If they do exist, we would like to see evidence of them.

 

Three diagrams illustrating magnetic field lines. Figure a shows a bar magnet with a number of compasses arranged along the magnet on either side. The needles of the compasses at the north pole of the magnet point away from the pole. The needles of the compasses at the south pole of the magnet point toward the pole. The needles of compasses in between the two poles point parallel to the magnet, toward the south pole. Figure b shows lines running from the north pole out and around to the south pole. Figure c shows lines as closed loops running from the north pole outside the magnet and around the south pole, and then up through the magnet to the north pole.
Figure 10.12 Magnetic field lines are defined to have the direction that a small compass will point when placed at a location. (a) If small compasses are used to map the magnetic field around a bar magnet, they will point in the directions shown: away from the north pole of the magnet, toward the south pole of the magnet. (Recall that the Earth’s north magnetic pole is really a south pole in terms of definitions of poles on a bar magnet.) (b) Connecting the arrows gives continuous magnetic field lines. The strength of the field is proportional to the closeness (or density) of the lines. (c) If the interior of the magnet could be probed, the field lines would be found to form continuous closed loops.

Link to Learning

Explore the interactions between a compass and bar magnet. Discover how you can use a battery and wire to make a magnet! Can you make it a stronger magnet? Can you make the magnetic field reverse? (Opens in a new window.)

Click to view content.

Magnetic Fields and Magnetic Field Lines

Einstein is said to have been fascinated by a compass as a child, perhaps musing on how the needle felt a force without direct physical contact. His ability to think deeply and clearly about action at a distance, particularly for gravitational, electric, and magnetic forces, later enabled him to create his revolutionary theory of relativity. Since magnetic forces act at a distance, we define a magnetic field to represent magnetic forces. The pictorial representation of magnetic field lines is very useful in visualizing the strength and direction of the magnetic field. As shown in Figure 10.12, the direction of magnetic field lines is defined to be the direction in which the north end of a compass needle points. The magnetic field is traditionally called the B-field.

 

Three diagrams illustrating magnetic field lines. Figure a shows a bar magnet with a number of compasses arranged along the magnet on either side. The needles of the compasses at the north pole of the magnet point away from the pole. The needles of the compasses at the south pole of the magnet point toward the pole. The needles of compasses in between the two poles point parallel to the magnet, toward the south pole. Figure b shows lines running from the north pole out and around to the south pole. Figure c shows lines as closed loops running from the north pole outside the magnet and around the south pole, and then up through the magnet to the north pole.
Figure 10.12 Magnetic field lines are defined to have the direction that a small compass will point when placed at a location. (a) If small compasses are used to map the magnetic field around a bar magnet, they will point in the directions shown: away from the north pole of the magnet, toward the south pole of the magnet. (Recall that the Earth’s north magnetic pole is really a south pole in terms of definitions of poles on a bar magnet.) (b) Connecting the arrows gives continuous magnetic field lines. The strength of the field is proportional to the closeness (or density) of the lines. (c) If the interior of the magnet could be probed, the field lines would be found to form continuous closed loops.

Small compasses used to test a magnetic field will not disturb it. (This is analogous to the way we tested electric fields with a small test charge. In both cases, the fields represent only the object creating them and not the probe testing them.) Figure 10.13 shows how the magnetic field appears for a current loop and a long straight wire, as could be explored with small compasses. A small compass placed in these fields will align itself parallel to the field line at its location, with its north pole pointing in the direction of B. Note the symbols used for field into and out of the paper.

 

Figure a: magnetic field of a circular current loop with a current moving counter-clockwise. The field lines are also roughly circular, running up through the center of the current loop, and back down outside the loop. Figure b: a straight wire with a current running straight up. The magnetic field lines circle the wire in a counter-clockwise direction. Figure c: a right hand with the thumb pointing up, parallel to a wire with the current running upward. The figures of the hand curl around the wire in the counter-clockwise direction to show the direction of the magnetic field when current is up. The symbol to represent magnetic field lines running out of the surface and toward the viewer—B out—is a circle with a sold circle inside. The symbol to represent magnetic field lines running into the surface and away from the viewer—B in—is represented with a circle with an x inside it. When the current is running straight up, B out is to the left and B in is to the right.
Fig. 10.13 Small compasses could be used to map the fields shown here. (a) The magnetic field of a circular current loop is similar to that of a bar magnet. (b) A long and straight wire creates a field with magnetic field lines forming circular loops. (c) When the wire is in the plane of the paper, the field is perpendicular to the paper. Note that the symbols used for the field pointing inward (like the tail of an arrow) and the field pointing outward (like the tip of an arrow).

Making Connections: Concept of a Field

A field is a way of mapping forces surrounding any object that can act on another object at a distance without apparent physical connection. The field represents the object generating it. Gravitational fields map gravitational forces, electric fields map electrical forces, and magnetic fields map magnetic forces.

Magnetic Field Rules

Extensive exploration of magnetic fields has revealed a number of hard-and-fast rules. We use magnetic field lines to represent the field (the lines are a pictorial tool, not a physical entity in and of themselves). The properties of magnetic field lines can be summarized by these rules:

  1. The direction of the magnetic field is tangent to the field line at any point in space. A small compass will point in the direction of the field line.
  2. The strength of the field is proportional to the closeness of the lines. It is exactly proportional to the number of lines per unit area perpendicular to the lines (called the areal density).
  3. Magnetic field lines can never cross, meaning that the field is unique at any point in space.
  4. Magnetic field lines are continuous, forming closed loops without beginning or end. They go from the north pole to the south pole.

The last property is related to the fact that the north and south poles cannot be separated. It is a distinct difference from electric field lines, which begin and end on the positive and negative charges. If magnetic monopoles existed, then magnetic field lines would begin and end on them.

Check Your Understanding

 

The content in this chapter  is adapted from multiple sources. The overall chapter structure is adapted from Exploring the Physical World and used under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License. The sections on introduction to magnetism, ferromagnets, electromagnets, current as the source of magnetism, and magnetic fields are adapted from College Physics 2e by OpenStax  and are used under a Creative Commons-Attribution 4.0 International License.  Other content and interactive activities are the original work of the authors of this text and are shared under a Creative Commons-Attribution 4.0 International License.

 

 

Media Attributions

definition

License

Icon for the Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License

Exploring the Physical World: An Updated and Expanded Introduction To the Physical Sciences Copyright © 2024 by LOUIS: The Louisiana Library Network is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

Share This Book