1. 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.

2. Electromagnetic Induction in a Circuit Loop
Section 1 Electricity from Magnetism
Chapter 20
Electromagnetic Induction in a Circuit Loop
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3. 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.
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4. 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.

5. 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.

6. 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.

7. 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
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8. 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.

9. 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.

10. 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

11. 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.

12. 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 = ?

13. 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

14. 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.

15. 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.

16. 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.

17. 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.

18. 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
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