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Magnetism from Electricity SECTION OBJECTIVES • • Describe the magnetic field produced by current in a straight conductor and in a solenoid. Use the right-hand rule to determine the direction of the magnetic field in a current-carrying wire. MAGNETIC FIELD OF A CURRENT-CARRYING WIRE Scientists in the late 1700s suspected that there was a relationship between electricity and magnetism, but no theory had been developed to guide their experiments. In 1820, Danish physicist Hans Christian Oersted devised a method to study this relationship. Following a lecture to his advanced class, Oersted demonstrated that when brought near a current-carrying wire, a compass needle is deflected from its usual north-south orientation. He published an account of this discovery in July 1820, and his work stimulated other scientists all over Europe to repeat the experiment. A long, straight, current-carrying wire has a cylindrical magnetic field Module (8 "Magnetic Field of a Wire" provides an interactive lesson with guided problem-solving practice to teach you about magnetic fields produced by current-carrying wires. Figure 5 (a) When the wire carries a strong current, the alignments of the iron filings show that the magnetic field induced by the current forms concentric circles around the wire, (b) Compasses can be used to show the direction of the magnetic field induced by the wire. 684 Chapter 19 The experiment shown in Figure 5(a) uses iron filings to show that a current-carrying conductor produces a magnetic field. In a similar experiment, several compass needles are placed in a horizontal plane near a long vertical wire, as illustrated in Figure 5(b). When no current is in the wire, all needles point in the same direction (that of Earth's magnetic field). However, when the wire carries a strong, steady current, all the needles deflect in directions tangent to concentric circles around the wire. This result points out the direction of B, the magnetic field induced by the current. When the current is reversed, the needles reverse direction. Ca) The right-hand rule can be used to determine the direction of the magnetic field Figure 6 You can use the righthand rule to find the direction of this magnetic field. These observations show that the direction of B is consistent with a simple rule for conventional current, known as the right-hand rule: If the wire is grasped in the right hand with the thumb in the direction of the current, as shown in Figure 6, the four fingers will curl in the direction of B. As shown in Figure 5(a), the lines of B form concentric circles about the wire. By symmetry, the magnitude of B is the same everywhere on a circular path centered on the wire and lying in a plane perpendicular to the wire. Experiments show that B is proportional to the current in the wire and inversely proportional to the distance from the wire. B MAGNETIC FIELD OF A CURRENT LOOP The right-hand rule can also be applied to find the direction of the magnetic field of a current-carrying loop, such as the loop represented in Figure 7(a). Regardless of where on the loop you apply the right-hand rule, the field within the loop points in the same direction—upward. Note that the field lines of the current-carrying loop resemble those of a bar magnet, as shown in Figure 7(b). If a long, straight wire is bent into a coil of several closely spaced loops, as shown on the next page in Figure 8, the resulting device is called a solenoid. solenoid a long, helically wound coil of insulated wire >f (a) (b) Ick lab Electromagnetism MATERIALS LIST "D-cell battery ] m length of insulated wire •"large nail Figure 7 (a) The magnetic field of a current loop is similar to (b) that of a bar magnet. Wind the wire around the nail, as shown below. Remove the insulation from the ends of the wire, and hold these ends against the metal terminals of the battery. Use the compass to determine whether the nail is magnetized. Next, flip the battery so that the direction of the current is reversed. Again, bring the compass toward the same part of the nail. Can you explain why the compass needle now points in a different direction? Bring paper clips near the nail while connected to the battery. What happens to the paper clips? How many can you pick up? compass metal paper clips Magnetism 685 (INKS. Developed and maintained by the National Science Teachers Association For a variety of links related to this chapter, go to www.scilinks.org Topic: Electromagnets SciLinks Code: HF60484 4 Solenoids produce a strong magnetic field by combining several loops A solenoid is important in many applications because it acts as a magnet when it carries a current. The magnetic field strength inside a solenoid increases with the current and is proportional to the number of coils per unit length. The rna°netic field of a solenoid can be increased by inserting an iron rod through the center of the coil; this device is often called an electromagnet. The magnetic field that is induced in the rod adds to the magnetic field of the solenoid, often creating a powerful magnet. Figure 8 shows the magnetic field lines of a solenoid. Note that the field lines inside the solenoid point in the same direction, are nearly parallel, are uniformly spaced, and are close together. This indicates that the field inside the solenoid is strong and nearly uniform. The field outside the solenoid is nonuniform and much weaker than the interior field. Solenoids are used in a wide variety of applications, from most of the appliances in your home to very high-precision medical equipment. Figure 8 The magnetic field inside a solenoid is strong and nearly uniform. Note that the field lines resemble those of a bar magnet, so a solenoid effectively has north and south poles. SECTION REVIEW 1. What is the shape of the magnetic field produced by a straight currentcarrying wire? 2. Why is the magnetic field inside a solenoid stronger than the magnetic field outside? 3. If electrons behave like magnets, then why aren't all atoms magnets? 4. Critical Thinking In some satellites, torque coils are replaced by devices called torque rods. In torque rods, a ferromagnetic material is inserted inside the coil. Why does a torque rod have a stronger magnetic field than a torque coil? 686 Chapter 19