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Copyright © by Holt, Rinehart and Winston. All rights reserved.

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Section 1 Electricity from MagnetismChapter 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.



Chapter 20

Electromagnetic Induction in a Circuit Loop

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.




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.




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

average induced emf = –the number of loops in the circuit

the time rate of change in the magnetic flux

– Memf Nt

• The magnetic flux is given by M = ABcos



Section 2 Generators, Motors, and Mutual InductanceChapter 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.



Chapter 20

Induction of an emf in an AC Generator

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.




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.



Section 3 AC Circuits and TransformersChapter 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

maxmax0.707

2rms

II I




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.




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. DefineGiven:

∆Vrms = 205 VR = 115 ΩUnknown:

∆Vrms = ? Irms = ? Imax = ?




Sample Problem, continued

rms Current and emf2. 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.

rmsrms

VI

R

Use the equation for rms current to find Irms.

Irms = 0.707 Imax





rms Current and emf2. Plan, continued

Rearrange the equation to isolate the unknown. Rearrange the equation relating rms current to maximum current so that maximum current is calculated.

max 0.707rmsI

I





rms Current and emf3. Calculate

Substitute the values into the equation and solve.

max

(0.707)(205 V) 145 V

145 V1.26 A

115 Ω1.26 A

1.78 A0.707

rms

rms

V

I

I

4. Evaluate The rms values for emf and current are a little more than two-thirds the maximum values, as expected.




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.

22 1

1

induced emf in secondary =

number of turns in secondaryapplied emf in primary

number of turns in primary

NV V

N




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 20

A Step-Up Transformer in an Auto Ignition System

Section 3 AC Circuits and Transformers

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