Transcript HP_20_win
How to Use This Presentation • To View the presentation as a slideshow with effects select “View” on the menu bar and click on “Slide Show.” • To advance through the presentation, click the right-arrow key or the space bar. • From the resources slide, click on any resource to see a presentation for that resource. • From the Chapter menu screen click on any lesson to go directly to that lesson’s presentation. • You may exit the slide show at any time by pressing the Esc key. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Resources Chapter Presentation Transparencies Visual Concepts Sample Problems Standardized Test Prep Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Electromagnetic Induction Table of Contents Section 1 Electricity from Magnetism Section 2 Generators, Motors, and Mutual Inductance Section 3 AC Circuits and Transformers Section 4 Electromagnetic Waves Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Section 1 Electricity from Magnetism Objectives • Recognize that relative motion between a conductor and a magnetic field induces an emf in the conductor. • Describe how the change in the number of magnetic field lines through a circuit loop affects the magnitude and direction of the induced electric current. • Apply Lenz’s law and Faraday’s law of induction to solve problems involving induced emf and current. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Section 1 Electricity from Magnetism Electromagnetic Induction • Electromagnetic induction is the process of creating a current in a circuit by a changing magnetic field. • A change in the magnetic flux through a conductor induces an electric current in the conductor. • The separation of charges by the magnetic force induces an emf. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Section 1 Electricity from Magnetism Electromagnetic Induction in a Circuit Loop Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Section 1 Electricity from Magnetism Electromagnetic Induction, continued • The angle between a magnetic field and a circuit affects induction. • A change in the number of magnetic field lines induces a current. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Section 1 Electricity from Magnetism Ways of Inducing a Current in a Circuit Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Section 1 Electricity from Magnetism Characteristics of Induced Current • Lenz’s Law The magnetic field of the induced current is in a direction to produce a field that opposes the change causing it. • Note: the induced current does not oppose the applied field, but rather the change in the applied field. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Section 1 Electricity from Magnetism Lenz's Law for Determining the Direction of the Induced Current Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Section 1 Electricity from Magnetism Characteristics of Induced Current, continued • The magnitude of the induced emf can be predicted by Faraday’s law of magnetic induction. • Faraday’s Law of Magnetic Induction M emf – N t average induced emf = –the number of loops in the circuit the time rate of change in the magnetic flux • The magnetic flux is given by M = ABcosq. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Section 1 Electricity from Magnetism Sample Problem Induced emf and Current A coil with 25 turns of wire is wrapped around a hollow tube with an area of 1.8 m2. Each turn has the same area as the tube. A uniform magnetic field is applied at a right angle to the plane of the coil. If the field increases uniformly from 0.00 T to 0.55 T in 0.85 s, find the magnitude of the induced emf in the coil. If the resistance in the coil is 2.5 Ω, find the magnitude of the induced current in the coil. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Section 1 Electricity from Magnetism Sample Problem, continued Induced emf and Current 1. Define Given: ∆t = 0.85 s A = 1.8 m2 N = 25 turns R = 2.5 Ω Bi = 0.00 T = 0.00 V•s/m2 Bf = 0.55 T = 0.55 V•s/m2 Unknown: emf = ? I=? q = 0.0º Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Section 1 Electricity from Magnetism Sample Problem, continued Induced emf and Current 1. Define, continued Diagram: Show the coil before and after the change in the magnetic field. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Section 1 Electricity from Magnetism Sample Problem, continued Induced emf and Current 2. Plan Choose an equation or situation. Use Faraday’s law of magnetic induction to find the induced emf in the coil. AB cosq M emf – N –N t t Substitute the induced emf into the definition of resistance to determine the induced current in the coil. emf I R Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Section 1 Electricity from Magnetism Sample Problem, continued Induced emf and Current 2. Plan, continued Rearrange the equation to isolate the unknown. In this example, only the magnetic field strength changes with time. The other components (the coil area and the angle between the magnetic field and the coil) remain constant. B emf – NA cos q t Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Section 1 Electricity from Magnetism Sample Problem, continued Induced emf and Current 3. Calculate Substitute the values into the equation and solve. V•s 0.55 – 0.00 2 m emf –(25)(1.8 m2 )(cos0.0º ) –29 V (0.85 s) –29 V I –12 A 2.5 Ω emf –29 V I –12 A Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Section 1 Electricity from Magnetism Sample Problem, continued Induced emf and Current 4. Evaluate The induced emf, and therefore the induced current, is directed through the coil so that the magnetic field produced by the induced current opposes the change in the applied magnetic field. For the diagram shown on the previous page, the induced magnetic field is directed to the right and the current that produces it is directed from left to right through the resistor. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Section 2 Generators, Motors, and Mutual Inductance Objectives • Describe how generators and motors operate. • Explain the energy conversions that take place in generators and motors. • Describe how mutual induction occurs in circuits. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Section 2 Generators, Motors, and Mutual Inductance Generators and Alternating Current • A generator is a machine that converts mechanical energy into electrical energy. • Generators use induction to convert mechanical energy into electrical energy. • A generator produces a continuously changing emf. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Section 2 Generators, Motors, and Mutual Inductance Induction of an emf in an AC Generator Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Section 2 Generators, Motors, and Mutual Inductance Function of a Generator Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Section 2 Generators, Motors, and Mutual Inductance Generators and Alternating Current, continued • Alternating current is an electric current that changes direction at regular intervals. • Alternating current can be converted to direct current by using a device called a commutator to change the direction of the current. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Section 2 Generators, Motors, and Mutual Inductance Comparing AC and DC Generators Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Section 2 Generators, Motors, and Mutual Inductance Motors • Motors are machines that convert electrical energy to mechanical energy. • Motors use an arrangement similar to that of generators. • Back emf is the emf induced in a motor’s coil that tends to reduce the current in the coil of a motor. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Section 2 Generators, Motors, and Mutual Inductance DC Motors Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Section 2 Generators, Motors, and Mutual Inductance Mutual Inductance • The ability of one circuit to induce an emf in a nearby circuit in the presence of a changing current is called mutual inductance. • In terms of changing primary current, Faraday’s law is given by the following equation, where M is the mutual inductance: M I emf –N –M t t Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Section 2 Generators, Motors, and Mutual Inductance Mutual Inductance Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Section 3 AC Circuits and Transformers Objectives • Distinguish between rms values and maximum values of current and potential difference. • Solve problems involving rms and maximum values of current and emf for ac circuits. • Apply the transformer equation to solve problems involving step-up and step-down transformers. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Section 3 AC Circuits and Transformers Effective Current • The root-mean-square (rms) current of a circuit is the value of alternating current that gives the same heating effect that the corresponding value of direct current does. • rms Current Irms Imax 2 0.707 Imax Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Section 3 AC Circuits and Transformers Effective Current, continued • The rms current and rms emf in an ac circuit are important measures of the characteristics of an ac circuit. • Resistance influences current in an ac circuit. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Section 3 AC Circuits and Transformers rms Current Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Section 3 AC Circuits and Transformers Sample Problem rms Current and emf A generator with a maximum output emf of 205 V is connected to a 115 Ω resistor. Calculate the rms potential difference. Find the rms current through the resistor. Find the maximum ac current in the circuit. 1. Define Given: ∆Vrms = 205 V R = 115 Ω Unknown: ∆Vrms = ? Irms = ? Imax = ? Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Section 3 AC Circuits and Transformers Sample Problem, continued rms Current and emf 2. Plan Choose an equation or situation. Use the equation for the rms potential difference to find ∆Vrms. ∆Vrms = 0.707 ∆Vmax Rearrange the definition for resistance to calculate Irms. Vrms Irms R Use the equation for rms current to find Irms. Irms = 0.707 Imax Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Section 3 AC Circuits and Transformers Sample Problem, continued rms Current and emf 2. Plan, continued Rearrange the equation to isolate the unknown. Rearrange the equation relating rms current to maximum current so that maximum current is calculated. Irms Imax 0.707 Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Section 3 AC Circuits and Transformers Sample Problem, continued rms Current and emf 3. Calculate Substitute the values into the equation and solve. Vrms (0.707)(205 V) 145 V 145 V Irms 1.26 A 115 Ω 1.26 A Imax 1.78 A 0.707 4. Evaluate The rms values for emf and current are a little more than two-thirds the maximum values, as expected. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Section 3 AC Circuits and Transformers Transformers • A transformer is a device that increases or decreases the emf of alternating current. • The relationship between the input and output emf is given by the transformer equation. N V2 2 V1 N1 induced emf in secondary = number of turns in secondary number of turns in primary applied emf in primary Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Section 3 AC Circuits and Transformers Transformers Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Section 3 AC Circuits and Transformers Transformers, continued • The transformer equation assumes that no power is lost between the primary and secondary coils. However, real transformers are not perfectly efficient. • Real transformers typically have efficiencies ranging from 90% to 99%. • The ignition coil in a gasoline engine is a transformer. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Section 3 AC Circuits and Transformers A Step-Up Transformer in an Auto Ignition System Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Section 4 Electromagnetic Waves Objectives • Describe what electromagnetic waves are and how they are produced. • Recognize that electricity and magnetism are two aspects of a single electromagnetic force. • Explain how electromagnetic waves transfer energy. • Describe various applications of electromagnetic waves. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Section 4 Electromagnetic Waves Propagation of Electromagnetic Waves • Electromagnetic waves travel at the speed of light and are associated with oscillating, perpendicular electric and magnetic fields. • Electromagnetic waves are transverse waves; that is, the direction of travel is perpendicular to the the direction of oscillating electric and magnetic fields. • Electric and magnetic forces are aspects of a single force called the electromagnetic force. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Section 4 Electromagnetic Waves Electromagnetic Waves Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Section 4 Electromagnetic Waves Propagation of Electromagnetic Waves, continued • All electromagnetic waves are produced by accelerating charges. • Electromagnetic waves transfer energy. The energy of electromagnetic waves is stored in the waves’ oscillating electric and magnetic fields. • Electromagnetic radiation is the transfer of energy associated with an electric and magnetic field. Electromagnetic radiation varies periodically and travels at the speed of light. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Section 4 Electromagnetic Waves The Sun at Different Wavelengths of Radiation Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Section 4 Electromagnetic Waves Propagation of Electromagnetic Waves, continued • High-energy electromagnetic waves behave like particles. • An electromagnetic wave’s frequency makes the wave behave more like a particle. This notion is called the wave-particle duality. • A photon is a unit or quantum of light. Photons can be thought of as particles of electromagnetic radiation that have zero mass and carry one quantum of energy. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Section 4 Electromagnetic Waves The Electromagnetic Spectrum • The electromagnetic spectrum ranges from very long radio waves to very short-wavelength gamma waves. • The electromagnetic spectrum has a wide variety of applications and characteristics that cover a broad range of wavelengths and frequencies. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Section 4 Electromagnetic Waves The Electromagnetic Spectrum, continued • Radio Waves – longest wavelengths – communications, tv • Microwaves – 30 cm to 1 mm – radar, cell phones • Infrared – 1 mm to 700 nm – heat, photography • Visible light – 700 nm (red) to 400 nm (violet) • Ultraviolet – 400 nm to 60 nm – disinfection, spectroscopy • X rays – 60 nm to 10–4 nm – medicine, astronomy, security screening • Gamma Rays – less than 0.1 nm – cancer treatment, astronomy Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Section 4 Electromagnetic Waves The Electromagnetic Spectrum Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Standardized Test Prep Multiple Choice 1. Which of the following equations correctly describes Faraday’s law of induction? A. emf B. emf C. emf D. emf ( AB tanq ) –N t ( AB cos q ) N t ( AB cos q ) –N t ( AB cos q ) M t Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Standardized Test Prep Multiple Choice, continued 1. Which of the following equations correctly describes Faraday’s law of induction? A. emf B. emf C. emf D. emf ( AB tanq ) –N t ( AB cos q ) N t ( AB cos q ) –N t ( AB cos q ) M t Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Standardized Test Prep Multiple Choice, continued 2. For the coil shown at right, what must be done to induce a clockwise current? F. Either move the north pole of a magnet down into the coil, or move the south pole of the magnet up and out of the coil. G. Either move the south pole of a magnet down into the coil, or move the north pole of the magnet up and out of the coil. H. Move either pole of the magnet down into the coil. J. Move either pole of the magnet up and out of the coil. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Standardized Test Prep Multiple Choice, continued 2. For the coil shown at right, what must be done to induce a clockwise current? F. Either move the north pole of a magnet down into the coil, or move the south pole of the magnet up and out of the coil. G. Either move the south pole of a magnet down into the coil, or move the north pole of the magnet up and out of the coil. H. Move either pole of the magnet down into the coil. J. Move either pole of the magnet up and out of the coil. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Standardized Test Prep Multiple Choice, continued 3. Which of the following would not increase the emf produced by a generator? A. rotating the generator coil faster B. increasing the strength of the generator magnets C. increasing the number of turns of wire in the coil D. reducing the cross-sectional area of the coil Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Standardized Test Prep Multiple Choice, continued 3. Which of the following would not increase the emf produced by a generator? A. rotating the generator coil faster B. increasing the strength of the generator magnets C. increasing the number of turns of wire in the coil D. reducing the cross-sectional area of the coil Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Standardized Test Prep Multiple Choice, continued 4. By what factor do you multiply the maximum emf to calculate the rms emf for an alternating current? F. 2 G. H. 2 1 2 1 J. 2 Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Standardized Test Prep Multiple Choice, continued 4. By what factor do you multiply the maximum emf to calculate the rms emf for an alternating current? F. 2 G. H. 2 1 2 1 J. 2 Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Standardized Test Prep Multiple Choice, continued 5. Which of the following correctly describes the composition of an electromagnetic wave? A. a transverse electric wave and a magnetic transverse wave that are parallel and are moving in the same direction B. a transverse electric wave and a magnetic transverse wave that are perpendicular and are moving in the same direction C. a transverse electric wave and a magnetic transverse wave that are parallel and are moving at right angles to each other D. a transverse electric wave and a magnetic transverse wave that are perpendicular and are moving at right angles to each other Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Standardized Test Prep Multiple Choice, continued 5. Which of the following correctly describes the composition of an electromagnetic wave? A. a transverse electric wave and a magnetic transverse wave that are parallel and are moving in the same direction B. a transverse electric wave and a magnetic transverse wave that are perpendicular and are moving in the same direction C. a transverse electric wave and a magnetic transverse wave that are parallel and are moving at right angles to each other D. a transverse electric wave and a magnetic transverse wave that are perpendicular and are moving at right angles to each other Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Standardized Test Prep Multiple Choice, continued 6. A coil is moved out of a magnetic field in order to induce an emf. The wire of the coil is then rewound so that the area of the coil is increased by 1.5 times. Extra wire is used in the coil so that the number of turns is doubled. If the time in which the coil is removed from the field is reduced by half and the magnetic field strength remains unchanged, how many times greater is the new induced emf than the original induced emf ? F. 1.5 times G. 2 times H. 3 times J. 6 times Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Standardized Test Prep Multiple Choice, continued 6. A coil is moved out of a magnetic field in order to induce an emf. The wire of the coil is then rewound so that the area of the coil is increased by 1.5 times. Extra wire is used in the coil so that the number of turns is doubled. If the time in which the coil is removed from the field is reduced by half and the magnetic field strength remains unchanged, how many times greater is the new induced emf than the original induced emf ? F. 1.5 times G. 2 times H. 3 times J. 6 times Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Standardized Test Prep Multiple Choice, continued Use the passage below to answer questions 7–8. A pair of transformers is connected in series, as shown in the figure below. 7. From left to right, what are the types of the two transformers? A. Both are step-down transformers. B. Both are step-up transformers. C. One is a step-down transformer; and one is a step-up transformer. D. One is a step-up transformer; and one is a step-down transformer. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Standardized Test Prep Multiple Choice, continued Use the passage below to answer questions 7–8. A pair of transformers is connected in series, as shown in the figure below. 7. From left to right, what are the types of the two transformers? A. Both are step-down transformers. B. Both are step-up transformers. C. One is a step-down transformer; and one is a step-up transformer. D. One is a step-up transformer; and one is a step-down transformer. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Standardized Test Prep Multiple Choice, continued Use the passage below to answer questions 7–8. A pair of transformers is connected in series, as shown in the figure below. 8. What is the output potential difference from the secondary coil of the transformer on the right? F. 400 V G. 12 000 V H. 160 000 V J. 360 000 V Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Standardized Test Prep Multiple Choice, continued Use the passage below to answer questions 7–8. A pair of transformers is connected in series, as shown in the figure below. 8. What is the output potential difference from the secondary coil of the transformer on the right? F. 400 V G. 12 000 V H. 160 000 V J. 360 000 V Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Standardized Test Prep Multiple Choice, continued 9. What are the particles that can be used to describe electromagnetic radiation called? A. electrons B. magnetons C. photons D. protons Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Standardized Test Prep Multiple Choice, continued 9. What are the particles that can be used to describe electromagnetic radiation called? A. electrons B. magnetons C. photons D. protons Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Standardized Test Prep Multiple Choice, continued 10. The maximum values for the current and potential difference in an ac circuit are 3.5 A and 340 V, respectively. How much power is dissipated in this circuit? F. 300 W G. 600 W H. 1200 W J. 2400 W Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Standardized Test Prep Multiple Choice, continued 10. The maximum values for the current and potential difference in an ac circuit are 3.5 A and 340 V, respectively. How much power is dissipated in this circuit? F. 300 W G. 600 W H. 1200 W J. 2400 W Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Standardized Test Prep Short Response 11. The alternating current through an electric toaster has a maximum value of 12.0 A. What is the rms value of this current? Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Standardized Test Prep Short Response, continued 11. The alternating current through an electric toaster has a maximum value of 12.0 A. What is the rms value of this current? Answer: 8.48 A Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Standardized Test Prep Short Response, continued 12. What is the purpose of a commutator in an ac generator? Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Standardized Test Prep Short Response, continued 12. What is the purpose of a commutator in an ac generator? Answer: It converts ac to a changing current in one direction only. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Standardized Test Prep Short Response, continued 13. How does the energy of one photon of an electromagnetic wave relate to the wave’s frequency? Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Standardized Test Prep Short Response, continued 13. How does the energy of one photon of an electromagnetic wave relate to the wave’s frequency? Answer: The energy is directly proportional to the wave’s frequency (E = hf ). Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Standardized Test Prep Short Response, continued 14. A transformer has 150 turns of wire on the primary coil and 75 000 turns on the secondary coil. If the input potential difference across the primary is 120 V, what is the output potential difference across the secondary? Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Standardized Test Prep Short Response, continued 14. A transformer has 150 turns of wire on the primary coil and 75 000 turns on the secondary coil. If the input potential difference across the primary is 120 V, what is the output potential difference across the secondary? Answer: 6.0 104 V Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Standardized Test Prep Extended Response 15. Why is alternating current used for power transmission instead of direct current? Be sure to include power dissipation and electrical safety considerations in your answer. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Standardized Test Prep Extended Response, continued 15. Answer: For electric power to be transferred over long distances without a large amount of power dissipation, the electric power must have a high potential difference and low current. However, to be safely used in homes, the potential difference must be lower than that used for long-distance power transmission. Because of induction, the potential difference and current of electricity can be transformed to higher or lower values, but the current must change continuously (alternate) for this to happen. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Standardized Test Prep Extended Response, continued Base your answers to questions 16. Why must the current enter 16–18 on the information below. the coil just as someone comes up to the table? A device at a carnival’s haunted house involves a metal ring that flies upward from a table when a patron passes near the table’s edge. The device consists of a photoelectric switch that activates the circuit when anyone walks in front of the switch and of a coil of wire into which a current is suddenly introduced when the switch is triggered. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Standardized Test Prep Extended Response, continued Base your answers to questions 16. Why must the current enter 16–18 on the information below. the coil just as someone comes up to the table? A device at a carnival’s haunted house involves a metal ring that flies upward from a table when a Answer: The change in current in the coil will produce a patron passes near the table’s changing magnetic field, edge. The device consists of a which will induce a current in photoelectric switch that activates the ring. The induced current the circuit when anyone walks in produces a magnetic field front of the switch and of a coil of that interacts with the wire into which a current is magnetic field from the coil, suddenly introduced when the causing the ring to rise from switch is triggered. the table. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Standardized Test Prep Extended Response, continued Base your answers to questions 17. Using Lenz’s law, explain 16–18 on the information below. why the ring flies upward when there is an increasing A device at a carnival’s haunted current in the coil? house involves a metal ring that flies upward from a table when a patron passes near the table’s edge. The device consists of a photoelectric switch that activates the circuit when anyone walks in front of the switch and of a coil of wire into which a current is suddenly introduced when the switch is triggered. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Standardized Test Prep Extended Response, continued Base your answers to questions 17. Using Lenz’s law, explain 16–18 on the information below. why the ring flies upward when there is an increasing A device at a carnival’s haunted current in the coil? house involves a metal ring that flies upward from a table when a Answer: According to Lenz’s patron passes near the table’s law, the magnetic field edge. The device consists of a induced in the ring must photoelectric switch that activates oppose the magnetic field the circuit when anyone walks in that induces the current in front of the switch and of a coil of the ring. The opposing fields wire into which a current is cause the ring, which can suddenly introduced when the move freely, to rise upward switch is triggered. from the coil under the table’s surface. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Standardized Test Prep Extended Response, continued Base your answers to questions 18. Suppose the change 16–18 on the information below. in the magnetic field is A device at a carnival’s haunted 0.10 T/s. If the radius of house involves a metal ring that the ring is 2.4 cm and flies upward from a table when a the ring is assumed to patron passes near the table’s consist of one turn of edge. The device consists of a wire, what is the emf photoelectric switch that activates induced in the ring? the circuit when anyone walks in front of the switch and of a coil of wire into which a current is suddenly introduced when the switch is triggered. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Standardized Test Prep Extended Response, continued Base your answers to questions 18. Suppose the change 16–18 on the information below. in the magnetic field is A device at a carnival’s haunted 0.10 T/s. If the radius of house involves a metal ring that the ring is 2.4 cm and flies upward from a table when a the ring is assumed to patron passes near the table’s consist of one turn of edge. The device consists of a wire, what is the emf photoelectric switch that activates induced in the ring? the circuit when anyone walks in front of the switch and of a coil of wire into which a current is Answer: 1.8 10–4 V suddenly introduced when the switch is triggered. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 20 Section 1 Electricity from Magnetism Ways of Inducing a Current in a Circuit Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved.