Induction and Alternating Current with teacher's notes

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$Holt Math Minnesota Test Prep Workbook for Grade 9$

$Math Skills: More Practice$

Slip rings Brush N Brush AC Generator DC Generator Figure 22-15 A simple dc generator (shown on the right) employs the same design as an ac generator (shown on the left). A split slip ring converts alternating current to direct current. Alternating current can be converted to direct current The conducting loop in an ac generator must be free to rotate through the magnetic field. Yet it must also be part of an electric circuit at all times. To accomplish this, the ends of the loop are connected to conducting rings, called slip rings, that rotate with the loop. Connections to the external circuit are made by stationary graphite strips, called brushes, that make continuous contact with the slip rings. Because the current changes direction in the loop, the output current through the brushes alternates direction as well. By varying this arrangement slightly, an ac generator can be converted to a dc generator. Note in Figure 22-15 that the components of a dc generator are essentially the same as those of the ac generator except that the contacts to the rotating loop are made by a single split slip ring, called a commutator. At the point in the loop’s rotation when the current has dropped to zero and is about to change direction, each half of the commutator comes into contact with the brush that was previously in contact with the other half of the commutator. The reversed current in the loop changes directions again so that the output current has the same direction as it originally had, although it still changes from a maximum value to zero. A plot of this pulsating direct current is shown in Figure 22-16. A steady direct current can be produced by using many loops and commutators distributed around the rotation axis of the dc generator. The sinusoidal currents from each loop overlap. The superposition of the currents produces a direct current output that is almost entirely free of fluctuations. Copyright © by Holt, Rinehart and Winston. All rights reserved. S Brush N Commutator Brush Current Induction and Alternating Current S NSTA TOPIC: Electrical safety GO TO: www.scilinks.org sciLINKS CODE: HF2223 Time Figure 22-16 The output current for a dc generator is a sine wave with the negative parts of the curve made positive. 811 SECTION 22-2 Teaching Tip Inform students that converting from ac to dc may also be accomplished electronically. This is done, for example, by ac-to-dc power converters, such as those used for portable electronic games or CD players. Teaching Tip Point out that, as shown by the graph of direct current in Figure 22-16, the direct current produced by a generator alternates over time but does not change polarity, as does alternating current. The direct current generated by a battery, on the other hand, is steady and does not fluctuate. (See Figure 19-5 on page 699.) Generators can produce a steady direct current similar to that produced by batteries if many loops and commutators are used. 811
SECTION 22-2 Teaching Tip Point out that generators and motors have opposite functions. Generators convert mechanical energy to electrical energy, while motors convert electrical energy to mechanical energy. Demonstration 7 Electric motor Purpose Illustrate the similarity between generators and motors. Materials hand-operated generator, capacitor (1 F), 100 Ω resistor, connecting wires Procedure Connect the capacitor to the hand-operated generator. Charge the capacitor with the generator. Do not apply a large potential difference. Release the handle, and watch the generator become a motor. Optional: An interesting comparison can be made between the responses of the generator to different loads. Repeat the demonstration with and without the resistor attached. 812 back emf the emf induced in a motor’s coil that tends to reduce the current powering the motor 812 Chapter 22 MOTORS Figure 22-17 In a motor, the current in the coil interacts with the magnetic field, causing the coil and the shaft on which the coil is mounted to turn. Motors are devices that convert electrical energy to mechanical energy. Instead of a current being generated by a rotating loop in a magnetic field, a current is supplied to the loop by an emf source and the magnetic force on the current loop causes it to rotate (see Figure 22-17). A motor is almost identical in construction to a dc generator. The coil of wire is mounted on a rotating shaft and is positioned between the poles of a magnet. Brushes make contact with a commutator, which alternates the current in the coil. This alternation of the current causes the magnetic field produced by the current to regularly reverse and thus always be repelled by the fixed magnetic field. Thus, the coil and the shaft are kept in continuous rotational motion. A motor can perform useful mechanical work when a shaft connected to its rotating coil is attached to some external device. As the coil in the motor rotates, however, the changing normal component of the magnetic field through it induces an emf that acts to reduce the current in the coil. If this were not the case, Lenz’s law would be violated. This induced emf is called the back emf. The back emf increases in magnitude as the magnetic field changes at a higher rate. In other words, the faster the coil rotates, the greater the back emf becomes. The potential difference available to supply current to the motor equals the difference between the applied potential difference and the back emf. Consequently, the current in the coil is also reduced because of the presence of back emf. The faster the motor turns, the smaller the net potential difference across the motor, and the smaller the net current in the coil, becomes. Commutator Brush N DC Motor emf Brush S + Copyright © by Holt, Rinehart and Winston. All rights reserved.
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 and 12: SECTION 22-1 Section Review ANSWERS
Page 13 and 14: SECTION 22-2 STOP 804 Misconception
Page 15 and 16: SECTION 22-2 Teaching Tip Point out
Page 17 and 18: SECTION 22-2 Demonstration 6 Effect
Page 19: 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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