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**Electromagnetic Induction**

Section 1 Electricity from Magnetism Chapter 20 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.

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**Electromagnetic Induction in a Circuit Loop**

Section 1 Electricity from Magnetism Chapter 20 Electromagnetic Induction in a Circuit Loop Insert High-Res image from Figure 1, page 708

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**Electromagnetic Induction, continued**

Section 1 Electricity from Magnetism Chapter 20 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. Insert High-Res art from Figure 3 page 709

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**Characteristics of Induced Current**

Section 1 Electricity from Magnetism Chapter 20 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.

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**Characteristics of Induced Current, continued**

Section 1 Electricity from Magnetism Chapter 20 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 The magnetic flux is given by FM = ABcosq.

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**Generators and Alternating Current**

Section 2 Generators, Motors, and Mutual Inductance Chapter 20 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.

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**Induction of an emf in an AC Generator**

Section 2 Generators, Motors, and Mutual Inductance Chapter 20 Induction of an emf in an AC Generator Insert High-Res image from TR109

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**Generators and Alternating Current, continued**

Section 2 Generators, Motors, and Mutual Inductance Chapter 20 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.

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**Section 2 Generators, Motors, and Mutual Inductance**

Chapter 20 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.

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**Chapter 20 Effective Current**

Section 3 AC Circuits and Transformers Chapter 20 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

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**Effective Current, continued**

Section 3 AC Circuits and Transformers Chapter 20 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.

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**Chapter 20 Sample Problem rms Current and emf**

Section 3 AC Circuits and Transformers Chapter 20 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 = ?

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**Sample Problem, continued**

Section 3 AC Circuits and Transformers Chapter 20 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 = ∆Vmax Rearrange the definition for resistance to calculate Irms. Use the equation for rms current to find Irms. Irms = Imax

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**Sample Problem, continued**

Section 3 AC Circuits and Transformers Chapter 20 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.

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**Sample Problem, continued**

Section 3 AC Circuits and Transformers Chapter 20 Sample Problem, continued rms Current and emf 3. Calculate Substitute the values into the equation and solve. 4. Evaluate The rms values for emf and current are a little more than two-thirds the maximum values, as expected.

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**Section 3 AC Circuits and Transformers**

Chapter 20 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.

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**Transformers, continued**

Section 3 AC Circuits and Transformers Chapter 20 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.

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**A Step-Up Transformer in an Auto Ignition System**

Section 3 AC Circuits and Transformers Chapter 20 A Step-Up Transformer in an Auto Ignition System Insert High-Res image from TR112

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