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Transcript
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 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 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 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 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 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 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 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, 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.