Induction and Alternating Current with teacher's notes

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$Holt Math Minnesota Test Prep Workbook for Grade 9$

$Math Skills: More Practice$

A person can receive an electric shock by touching a conducting or “live” wire while in contact with a lower electric potential, or ground. The ground contact might be made by touching a water pipe (which is normally at zero potential) or by standing on the floor with wet feet (because impure water is a good conductor). Electric shock can result in fatal burns or can cause the muscles of vital organs, such as the heart, to malfunction. The degree of damage to the body depends on the magnitude of the current, the length of time it acts, and the part of the body through which it passes. A current of 100 milliamps (mA) can be fatal. If the current is larger than about 10 mA, the hand muscles contract and the person may be unable to let go of the wire. Section Review To prevent electrocution, any wires designed to have such currents in them are wrapped in insulation, usually plastic or rubber. However, with frequent use, electric Copyright © by Holt, Rinehart and Winston. All rights reserved. Avoiding Electrocution cords can fray, exposing some of the conductors. To further prevent electrocution in these and other situations in which electrical contact can be made, devices called a Ground Fault Circuit Interrupter (GFCI) and a Ground Fault Interrupter (GFI) are mounted in electrical outlets and individual appliances. GFCIs and GFIs provide protection by comparing the current in one side of the electrical outlet socket to the current in the other socket. If there is even a 5 mA difference, the interrupter opens the circuit in a few milliseconds (thousandths of a second). If you accidentally touch a bare wire and create an alternate conducting path through you to ground, the device detects this redirection of current and you get only a small shock or tingle. Despite these safety devices, you can still be electrocuted. Never use electrical appliances near water or with wet hands. Use a battery-powered radio near water because batteries cannot supply enough current to harm you. It is also a good idea to replace old outlets with GFCI-equipped units or to install GFI-equipped circuit breakers. 1. A loop with an area of 0.33 m 2 is rotating at 281 rad/s with its axis of rotation perpendicular to a uniform magnetic field. The magnetic field has a strength of 0.035 T. If the loop contains 37 turns of wire, what is the maximum potential difference induced in the loop? 2. A generator develops a maximum emf of 2.8 V. If the generator coil has 25 turns of wire, a cross-sectional area of 36 cm 2 , and rotates with a constant frequency of 60.0 Hz, what is the strength of the magnetic field in which the coil rotates? 3. What is the purpose of a commutator in a motor? Explain what would happen if a commutator were not used in a motor. 4. Physics in Action The rms current produced by a single coil of an electric guitar is 0.025 mA. How large is the maximum instantaneous current? If the coil’s resistance is 4.3 kΩ, what are the rms and maximum potential differences produced by the coil? Induction and Alternating Current 813 SECTION 22-2 BACKGROUND Ground Fault Interrupters make use of Faraday’s law. Both the wire that leads from the wall outlet to the appliance and the wire that leads from the appliance back to the wall pass through an iron ring. Part of the iron ring is wrapped in a coil called a sensing coil. When the current to the appliance equals the current from the appliance, the net magnetic field through the sensing coil is zero. If a short circuit occurs, the net magnetic field through the coil is no longer zero. Because the current in the wire is alternating, an ac potential difference is induced in the sensing coil. This induced potential difference in the coil is used to trigger a circuit breaker, stopping the current before it reaches a level that might be harmful to the person using the appliance. Section Review ANSWERS 1. 120 V 2. 8.3 × 10 −2 T 3. The commutator reverses the direction of the current in the coil so that the magnetic field produced by the current always opposes the external field and the coil turns continuously. Without the commutator, the coil would turn until the coil’s magnetic field was aligned with the external field. The coil would then cease to turn. 4. 0.035 mA; 0.11 V, 0.15 V 813
Section 22-3 Demonstration 8 Transformers Purpose Show students how a transformer works, and reinforce the idea that a changing current is required. Materials iron bar, 9 V battery, knife switch, flashlight bulb in holder, two wires (each about 1 m long) Procedure Set up the primary side of the transformer before class by connecting the battery and switch in series with one of the wires coiled around the iron bar. The primary coil should have 50 turns. Set up the secondary coil with 25 turns, and connect the flashlight bulb to the secondary coil. In class, demonstrate this stepdown transformer by momentarily closing the switch and then opening it again. Have students discuss the transfer of energy that occurs in this situation. Close the switch and keep it closed. Have students note the behavior of the bulb. Discuss the brief illumination and fading of the bulb with students. Lead students to consider the concept of changing current. Open the switch, and note the illumination. Discuss this effect. 814 22-3 SECTION OBJECTIVES • Describe how mutual induction occurs in circuits. • Calculate the potential difference from a step-up or step-down transformer. • Describe how self-induction occurs in an electric circuit. mutual inductance a measure of the ability of one circuit carrying a changing current to induce an emf in a nearby circuit Figure 22-18 Faraday’s electromagnetic-induction experiment used a changing current in one circuit to induce a current in another circuit. 814 Chapter 22 22-3 Inductance MUTUAL INDUCTANCE The basic principle of electromagnetic induction was first demonstrated by Michael Faraday. His experimental apparatus, which resembled the arrangement shown in Figure 22-18, used a coil connected to a switch and a battery instead of a magnet to produce a magnetic field. This coil is called the primary coil, and its circuit is called the primary circuit. The magnetic field is strengthened by the magnetic properties of the iron ring around which the primary coil is wrapped. A second coil is wrapped around another part of the iron ring and is connected to a galvanometer. An emf is induced in this coil, called the secondary coil, when the magnetic field of the primary coil is changed. When the switch in the primary circuit is closed, the galvanometer in the secondary circuit deflects in one direction and then returns to zero. When the switch is opened, the galvanometer deflects in the opposite direction and again returns to zero. When there is a steady current in the primary circuit, the galvanometer reads zero. The magnitude of this emf is predicted by Faraday’s law of induction. However, Faraday’s law can be rewritten so that the induced emf is proportional to the changing current in the primary coil. This can be done because of the direct proportionality between the magnetic field produced by a current in a coil, or solenoid, and the current itself. The form of Faraday’s law in terms of changing primary current is as follows: emf =−N⎯ ∆[AB (cos q)] ⎯ =−M ⎯ ∆t ∆I ⎯ ∆t The constant, M, is called the mutual inductance of the two-coil system. The mutual inductance depends on the geometrical properties of the coils and + Primary coil Iron ring Secondary coil Galvanometer 100 200 300 400 500 600 700 800 900 0 + 100 200 300 400 500 600 700 800 900 1000 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 and 20: SECTION 22-2 PRACTICE GUIDE 22C Sol
Page 21: SECTION 22-2 Teaching Tip Point out
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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