Download Electricity Ch. 18 Sect. 3

Magnetism
Section 3
Section 3: Electric Currents from Magnetism
Preview
• Key Ideas
• Bellringer
• Electromagnetic Induction
• The Electromagnetic Force
• Transformers
Magnetism
Section 3
Key Ideas
〉What happens when a magnet is moved into or
out of a coil of wire?
〉How are electricity and magnetism related?
〉What are the basic components of a
transformer?
Magnetism
Section 3
Bellringer
1. List as many items as you can that use DC current.
(Hint: Batteries supply DC current.)
2. List as many items as you can that use AC current.
(Hint: Standard wall outlets supply AC current.)
3. If you want to plug a CD player that normally uses
batteries into a wall socket, an AC adapter is
required. What is the function of the AC adapter?
4. Electric power that goes into a neighborhood must be
stepped down, or decreased, in voltage before it
goes into a home. Explain why this is necessary.
Magnetism
Section 3
Electromagnetic Induction
〉What happens when a magnet is moved into
or out of a coil of wire?
〉Moving a magnet into and out of a coil of wire
causes charges in the wire to move.
• electromagnetic induction: the process of creating
a current in a circuit by changing a magnetic field
• Faraday’s law states:
An electric current can be produced in a circuit
by a changing magnetic field crossing the circuit.
Magnetism
Section 3
Electromagnetic Induction, continued
• As the loop moves in and out of the magnetic field of
the magnet, a current is induced in the circuit.
• Rotating the circuit or changing the strength of the
magnetic field will also induce a current in the circuit.
• Electromagnetic induction obeys conservation of
energy.
– Pushing a loop through a magnetic field requires
work.
Magnetism
Section 3
Electromagnetic Induction, continued
Magnetism
Section 3
Visual Concept: Ways of Inducing a Current in
a Circuit
Magnetism
Section 3
Electromagnetic Induction, continued
• The magnetic force acts on moving electric charges.
– The force is at its maximum value when the
charge moves perpendicularly to the field.
– As the angle between the charge’s direction and
the direction of the magnetic field decreases, the
force on the charge decreases.
• The magnetic force acts on wires carrying a current.
Magnetism
Section 3
Electromagnetic Induction, continued
Magnetism
Section 3
Electromagnetic Induction, continued
• Generators convert mechanical energy into electrical
energy.
• generator: a machine that converts mechanical
energy to electrical energy
• alternating current (AC): an electric current that
changes direction at regular intervals
– For each half rotation of the loop, the current
produced by the generator reverses direction.
• AC generators produce the electrical energy you use
in your home.
Magnetism
AC Generator
Section 3
Magnetism
Induced Current
Section 3
Magnetism
Section 3
Visual Concept: Function of a Generator
Magnetism
Section 3
The Electromagnetic Force
〉How are electricity and magnetism related?
〉Electricity and magnetism are two aspects of a
single force, the electromagnetic force.
• The energy that results from these two forces is called
electromagnetic (EM) energy.
• Light is a form of electromagnetic energy.
• EM waves are made up of oscillating electric and
magnetic fields that are perpendicular to each other.
Magnetism
Section 3
Visual Concept: Electromagnetic Waves
Magnetism
Section 3
Transformers
〉 What are the basic components of a transformer?
〉In its simplest form, a transformer consists of
two coils of wire wrapped around opposite
sides of a closed iron loop.
• transformer: a device that increases or decreases
the voltage of alternating current
• primary coil: wire attached to a source of alternating
current
• secondary circuit: wire attached to an appliance
Magnetism
Section 3
Transformers, continued
• Current in the primary coil creates a changing
magnetic field that magnetizes the iron core.
• The changing magnetic field of the iron core then
induces a current in the secondary coil.
• Transformers can increase or decrease voltage.
– The voltage induced in the secondary coil of a
transformer depends on the number of loops, or
turns, in the coil.
Magnetism
Section 3
Transformers, continued
• In a step-up transformer, the primary coil has fewer
turns than the secondary coil does.
– The voltage across the secondary coil is greater
than the voltage across the primary coil.
• In a step-down transformer, the secondary coil has
fewer loops than the primary coil does.
– The voltage across the secondary coil is lower than
the voltage across the primary coil.
Magnetism
Section 3
Transformers, continued
• Transformers must obey the law of conservation of
energy.
– The current in the secondary coil of a step-up
transformer is always less than the current in the
primary coil.
• Transformers are used in the transfer of electrical
energy.
Magnetism
Transformers, continued
Section 3
Magnetism
Section 3
Visual Concept: Transformer