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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22-2 Alternating current, generators, and motors GENERATORS AND ALTERNATING CURRENT In the previous section you learned that a current can be induced in a circuit either by changing the magnetic field strength or by moving the circuit loop in or out of the magnetic field. Another way to induce a current is to change the orientation of the loop with respect to the magnetic field. This second approach to inducing a current represents a practical means of generating electrical energy. In effect, the mechanical energy used to turn the loop is converted to electrical energy. A device that does this is called an electric generator. In most commercial power plants, mechanical energy is provided in the form of rotational motion. For example, in a hydroelectric plant, falling water directed against the blades of a turbine causes the turbine to turn; in a coal or natural-gasburning plant, energy produced by burning fuel is used to convert water to steam, and this steam is directed against the turbine blades to turn the turbine. Basically, a generator uses the turbine’s rotary motion to turn a wire loop in a magnetic field. A simple generator is shown in Figure 22-10. As the loop rotates, the effective area of the loop changes with time, inducing an emf and a current in an external circuit connected to the ends of the loop. A generator produces a continuously changing emf Consider a single loop of wire that is rotated with a constant angular frequency in a uniform magnetic field. The loop can be thought of as four conducting wires similar to the conducting wire discussed on page 795 of Section 22-1. In this example, the loop is rotating counterclockwise within a magnetic field directed to the left. Copyright © by Holt, Rinehart and Winston. All rights reserved. 22-2 SECTION OBJECTIVES • Calculate the maximum emf for an electric generator. • Calculate rms current and potential difference for ac circuits. • Describe how an electric motor relates to an electric generator. generator a device that uses induction to convert mechanical energy to electrical energy Figure 22-10 In a simple generator, the rotation of conducting loops (located on the left end of the axis) through a constant magnetic field induces an alternating current in the loops. Induction and Alternating Current 803 Section 22-2 STOP Misconception Alert Some students may think that no work is required to generate electricity. Stress that this is not the case; work is required to rotate a loop in a magnetic field. In a hydroelectric power plant, this work is generated by falling water. The water loses gravitational potential energy as it does work. Demonstration 4 Alternating current Purpose Illustrate the effects of changing cross-sectional area on induced emf, and show how generators create ac currents. Materials overhead projector, colored cellophane filter mounted in a frame (like a 35 mm slide) Procedure Tell students that the light from the overhead projector represents a uniform magnetic field and the filter represents a wire loop. The amount of colored light that reaches the screen represents the induced emf and thus the current. Slowly rotate the filter over the overhead projector, letting changing amounts of colored light reach the screen. Have students observe the changing intensity. Draw a graph of the changes in intensity over time on the board. (The graph is a sine curve.) Point out the analogy between the changing intensity and a changing current. 803
SECTION 22-2 STOP 804 Misconception Alert When trying to determine the direction of current in Figure 22-11, students might try using the right-hand rule for a currentcarrying wire in a magnetic field instead of the right-hand rule for charges moving in a magnetic field. Explain that the latter rule must be used because charges are not initially moving along the wire; rather, they move with the wire, in a direction perpendicular to the length of the wire. Thus, when applying the right-hand rule to find the direction of induced current, the thumb points along the direction in which the wire is moving, not along the wire, and the fingers point along the direction of the magnetic field. The resulting force on the charges, and thus the current, points out of the palm of the hand. Have students apply this form of the right-hand rule for segments a, b, c, and d in each case shown in Figure 22-11 to determine the direction of the current in each segment. Visual Strategy Figure 22-11 Have students compare Figure 22-11 with the analogy introduced in Demonstration 4. After asking students the following question, plot graphs of y = sin x and y = sin x on the chalkboard, and use the graphs to compare the two cases. The amount of colored light Q in Demonstration 4 varies much like the induced emf in Figure 22-11. How are these two cases different? The emf becomes negative, while light does not. A NSTA TOPIC: Alternating current GO TO: www.scilinks.org sciLINKS CODE: HF2222 Figure 22-11 For a rotating loop in a magnetic field, the induced emf is zero when the loop is perpendicular to the magnetic field, as in (a) and (c), and is at a maximum when the loop is parallel to the field, as in (b) and (d). 804 Chapter 22 When the area of the loop is perpendicular to the magnetic field lines, as shown in Figure 22-11(a), every segment of wire in the loop is moving parallel to the magnetic field lines. At this instant, the magnetic field does not exert force on the charges in any part of the wire, so the induced emf in each segment is therefore zero. As the loop rotates away from this position, segments a and c cross magnetic field lines, so the magnetic force on the charges in these segments, and thus the induced emf, increases. The magnetic force on the charges in segments b and d is directed outside of the wire, so the motion of these segments does not contribute to the emf or the current. The greatest magnetic force on the charges and the greatest induced emf occur at the instant when segments a and c move perpendicularly to the magnetic field lines, as in Figure 22-11(b). This occurs when the plane of the loop is parallel to the field lines. Because segment a moves downward through the field while segment c moves upward, their emfs are in opposite directions, but both produce a counterclockwise current. As the loop continues to rotate, segments a and c cross fewer lines, and the emf decreases. When the plane of the loop is perpendicular to the magnetic field, the motion of segments a and c is again parallel to the magnetic lines and the induced emf is again zero, as shown in Figure 22-11(c). Segments a and c now move in directions opposite those in which they moved from their positions in (a) to those in (b). As a result, the polarity of the induced emf and the direction of the current are reversed, as shown in Figure 22-11(d). Induced emf – 0 + B (a) (b) Induced emf – 0 + B (c) (d) b b a c c a d d Induced emf – – 0 + Induced emf 0 + B B b b a d c Copyright © by Holt, Rinehart and Winston. All rights reserved. a c d
Page 1 and 2: Chapter 22 Overview Section 22-1 in
Page 3 and 4: Section 22-1 Teaching Tip Remind st
Page 5 and 6: SECTION 22-1 Demonstration 1 Induce
Page 7 and 8: SECTION 22-1 Demonstration 2 Lenz
Page 9 and 10: SECTION 22-1 Teaching Tip Point out
Page 11: SECTION 22-1 Section Review ANSWERS
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

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