Chapter Electromagnetic Induction

In Chapter 13 you learned that an electric current produces a magnetic field. It 14 seems reasonable to expect that the reverse might also be true—that a magnetic field can induce an electric current. How can this be achieved? What properties of the magnetic field will induce an electric current? Is it sufficient to have a strong magnetic field near a conductor for an electric current to be produced? By the end of this chapter, you will be able to answer these questions. You will also In this chapter, you will be understand how the principles of electromagnetic induction apply to huge elec- able to tric generators like those shown in Figure 1. André Marie Ampère conducted experiments and observed the effects of • investigate and understand the electromagnetic induction, but it was not his emphasis at the time and so he principles of electromagnetic induction made little of it. The French physicist François Arago (1786–1853) observed that • use and understand Lenz’s law in a compass that was disturbed would settle down quickly if a copper base was various electromagnetic induction beneath it (Figure 2). Also, if a copper base was rotated beneath a compass he phenomena found that the needle would rotate with the disk. These events seemed inexpli- • apply the principles of electro- cable to Arago at the time and it wasn’t until Faraday’s discovery that it was magnetic induction to the opera- explained. tion and use of electric generators It is believed that the American physicist Joseph Henry (1797–1878) first dis- • investigate and solve problems involving transformers and the covered the principles of electromagnetic induction, although he did not publish efficient distribution of electrical his findings until after Michael Faraday announced his discoveries. As a result, energy Faraday has been credited for the discovery of the principles of electromagnetic • investigate careers in electricity induction even though Henry’s experiments were somewhat more sophisticated and magnetism using a variety of and contained information on self-induction that Faraday missed. resources • analyze and understand the method of magnetic storage of Reflect on information and investigate the yourLearning various uses of this method of storage 1. Why is it reasonable to think that a magnetic field might induce an electric current? 2. Why do you think some scientists, like Ampère, sometimes make impor- tant observations but fail to investigate them in detail? 3. (a) Why is Joseph Henry not given credit for the explanation of electro- magnetic induction? Do you think this is fair? (b) Give several reasons why the scientific community gives credit to the person who publishes findings first and not to the one who discovers them first. Throughout this chapter, note any changes in your ideas as you learn new con- cepts and develop your skills.

506 Chapter 14 Try This Activity A Demonstration of Electromagnetic Induction Often, scientists will make certain observations when doing experiments and miss the significance of what they have seen, as Ampère did. There are many possible reasons for this. Your teacher may demonstrate or allow you to investigate the following electromagnetic phenomena, and it is your task to make detailed observations of what has occurred. • Place a soft iron bar inside a coil with many loops, as shown in Figure 3. Connect the coil to a galvanometer or milliammeter. • Place the N-pole of a bar magnet in contact with the soft iron bar and observe what happens to the galvanometer (Figure 4(a)). Leave the N-pole in contact for a few seconds before removing it. • Remove the N-pole of the bar magnet from the soft iron bar and observe what happens to the galvanometer (Figure 4(b)). • Repeat the above procedure using the S-pole. (a) Describe what happens when the N-pole is moved toward the Figure 1 soft iron bar, while it is left in contact with the soft iron bar, and when it is moved away from the soft iron bar. (b) How did your observations change when the S-pole was used? (c) Can these observations be explained by what you know about electromagnetism or is further study required?

(a) (b) S

S

copper base

Figure 2 A compass needle will settle down more N readily when it has a copper base. The N compass needle will rotate with the copper disk if it is turned.

G G soft iron core

coil coil N S

G Figure 4 Figure 3