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.
Electromagnetic Induction in a Circuit Loop
•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.
•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
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.
Induction of an emf in an AC Generator
•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 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
•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.
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.
•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.
A Step-Up Transformer in an Auto Ignition System