Induction and Alternating Current with teacher's notes

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$Holt Math Minnesota Test Prep Workbook for Grade 9$

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Consider a conducting wire pulled through a magnetic field, as shown on the left in Figure 22-2. You learned in Chapter 21 that positive charges moving with a velocity at an angle to the magnetic field will experience a magnetic force. According to the right-hand rule, this force will be directed perpendicular to both the magnetic field and the motion of the charges. For positive charges in the wire, the force is directed downward along the wire. For negative charges, the force is upward. This effect is equivalent to replacing the segment of wire and the magnetic field with a battery that has a potential difference, or emf, between its terminals, as shown on the right in Figure 22-2. As long as the conducting wire moves through the magnetic field, the emf will be maintained. The polarity of the induced emf—and thus the direction of the induced current in the circuit—depends on the direction in which the wire is moved through the magnetic field. For instance, if the wire in Figure 22-2 is moved to the right, the right-hand rule predicts that the positive charges will be pushed downward. If the wire is moved to the left, the positive charges will be pushed upward. The magnitude of the induced emf depends on the velocity with which the wire is moved through the magnetic field, the length of the wire, and the strength of the magnetic field. The angle between a magnetic field and a circuit affects induction To induce an emf in a wire loop, part of the loop must move through a magnetic field or the entire loop must pass into or out of the magnetic field. No emf is induced if the loop is static or the magnetic field is constant. The magnitude of the induced emf and current depend partly on how the loop is oriented to the magnetic field, as shown in Figure 22-3. The induced current is largest when the plane of the loop is perpendicular to the magnetic field, as in (a); it is less when the plane is tilted into the field, as in (b); and it is zero when the plane is parallel to the field, as in (c). The role of orientation on induced current can be understood in terms of the force exerted by a magnetic field on charges in the moving loop. The smaller the component of the magnetic field perpendicular to both the plane and the motion of the loop, the smaller the magnetic force on the charges in the loop. When the area of the loop is moved parallel to the magnetic field, there is no magnetic field component perpendicular to the plane of the loop and therefore no force that moves the charges through the circuit. v v v (a) (b) (c) Figure 22-3 The induced emf and current are largest when the plane of the loop is perpendicular to the magnetic field (a), smaller when the plane of the loop is tilted into the field (b), and zero when the plane of the loop and the magnetic field are parallel (c). Copyright © by Holt, Rinehart and Winston. All rights reserved. Induction and Alternating Current − + v B (out of page) = – + Figure 22-2 The separation of positive and negative moving charges by the magnetic force creates a potential difference (emf ) between the ends of the conductor. NSTA TOPIC: Magnetic fields GO TO: www.scilinks.org sciLINKS CODE: HF2221 795 SECTION 22-1 The Language of Physics The term emf originally stood for electromotive force. Today, emf is considered to be a potential difference rather than a force. Specifically, emf is a potential difference that sets charges in a circuit in motion. To avoid misconceptions, the term electromotive force is not used in this text. Visual Strategy Figure 22-3 Point out that any charge with a velocity component perpendicular to a magnetic field will experience a force that is perpendicular both to the magnetic field and to that velocity component. Why does a loop that is paral- Q lel to the field, and moving as shown in Figure 22-3(c), experience no current around the loop even though the loop is moving in the magnetic field? The electrons do experience a A magnetic force perpendicular to the field, but they cannot move in that direction because the loop is parallel to the field. 795
SECTION 22-1 Demonstration 1 Induced current Purpose Show students an example of an induced current. Materials flashlight bulb in holder, coil of wire, bar magnet (two may be required), connecting wires Procedure Connect the flashlight bulb to the coil of wire with the connecting wires. Tell students to observe the demonstration with the intent of explaining the energy conversions. Move the bar magnet into and out of the coil several times in rapid succession. Have students explain the energy conversions on the board or in their notebooks. The following energy conversions should be discussed: kinetic energy is converted to electrical energy (moving magnet generates a current) and electrical energy is converted to light (the current heats the light bulb’s filament). 796 In 1996, the space shuttle Columbia attempted to use a 20.7 km conducting tether to study Earth’s magnetic field in space. The plan was to drag the tether through the magnetic field, inducing an emf in the tether. The magnitude of the emf would directly vary with the strength of the magnetic field. Unfortunately, the tether broke before it was fully extended, so the experiment was abandoned. Table 22-1 Ways of inducing a current in a circuit 796 Chapter 22 Change in the number of magnetic field lines induces a current So far, you have learned that moving a circuit loop into or out of a magnetic field can induce an emf and a current in the circuit. Changing the size of the loop or the strength of the magnetic field also will induce an emf in the circuit. One way to predict whether a current will be induced in a given situation involves the concept of changes in magnetic field lines. For example, moving the circuit into the magnetic field causes some lines to move into the loop. Changing the size of the circuit loop or rotating the loop changes the number of field lines passing through the loop, as does changing the magnetic field’s strength. Table 22-1 summarizes these three ways of inducing a current. CHARACTERISTICS OF INDUCED CURRENT Suppose a bar magnet is pushed into a coil of wire. As the magnet moves into the coil, the strength of the magnetic field within the coil increases, and a current is induced in the circuit. This induced current in turn produces its own magnetic field, whose direction can be found by using the right-hand rule. If you were to apply this rule for several cases, you would notice that the induced magnetic field direction depends on the change in the applied field. As the magnet approaches, the magnetic field passing through the coil increases in strength. The induced current in the coil must be in a direction that produces a magnetic field that opposes the increasing strength of the approaching field. The induced magnetic field is therefore in the direction opposite that of the approaching magnetic field. Description Before After Circuit is moved into or out of magnetic field (either circuit or magnet moving). Circuit is rotated in the magnetic field (angle between area of circuit and magnetic field changes). Intensity of magnetic field is varied. v B B I v B I B I Copyright © by Holt, Rinehart and Winston. All rights reserved. B
Page 1 and 2: Chapter 22 Overview Section 22-1 in
Page 3: Section 22-1 Teaching Tip Remind st
Page 7 and 8: SECTION 22-1 Demonstration 2 Lenz
Page 9 and 10: SECTION 22-1 Teaching Tip Point out
Page 11 and 12: SECTION 22-1 Section Review ANSWERS
Page 13 and 14: SECTION 22-2 STOP 804 Misconception
Page 17 and 18: SECTION 22-2 Demonstration 6 Effect
Page 19 and 20: SECTION 22-2 PRACTICE GUIDE 22C Sol
Page 23 and 24: Section 22-3 Demonstration 8 Transf
Page 25 and 26: SECTION 22-3 Teaching Tip Some stud
Page 27 and 28: SECTION 22-3 PRACTICE GUIDE 22D Sol
Page 29 and 30: Chapter 22 Summary Teaching Tip Ask
Page 31 and 32: 22 REVIEW & ASSESS 12. −0.63 V 13
Page 33 and 34: 22 REVIEW & ASSESS 42. 1.03 × 10 5
Page 35 and 36: Chapter 22 Laboratory Exercise NOTE

Induction and Alternating Current with teacher's notes

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