Download Chapter 18 Magnetism Section 1 Magnets and Magnetic Fields

Chapter 18 Magnetism
CHAPTER OUTLINE
Section 1 Magnets and Magnetic Fields
Key Idea questions
> What happens when the poles of two magnets are brought close
together?
> What causes a magnet to attract or repel another magnet?
> How is Earth’s magnetic field oriented?
Magnets
> What happens when the poles of two magnets are brought close
together?
> Two like poles repel each other. Two unlike poles attract each other.
• magnetic pole: one of two points, such as the ends of a magnet, that
have opposing magnetic qualities
– All magnets have at least one pair of poles, a north pole and a
south pole.
– It is impossible to isolate a magnet’s south pole from the magnet’s
north pole.
• Some materials can be made into permanent magnets.
– A magnetized piece of iron is called a “permanent” magnet, but its
magnetism can be weakened or even removed.
– Iron is a soft magnetic material.
• It is easily magnetized.
• It tends to lose its magnetic properties easily.
– Cobalt is a hard magnetic material.
• It more difficult to magnetize.
• Once magnetized, it doesn’t lose its magnetism easily.
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Super Summary Chapter Outline p. 1
Chapter 18 Magnetism
Magnetic Fields
> What causes a magnet to attract or repel another magnet?
> Magnets repel or attract each other because of the interaction of their
magnetic fields.
• magnetic field: a region where a magnetic force can be detected
• Magnets are sources of magnetic fields.
• Moving charges create magnetic fields.
• magnetic domains: groups of atoms that all line up the same way and
form small, magnetized regions within a material
• Magnetic field lines are used to represent a magnetic field.
– Field lines always form closed loops.
– The magnetic field gets weaker with distance from the magnet.
– Magnetic field lines that are close together indicate a strong
magnetic field.
– Field lines that are farther apart indicate a weaker field.
– Magnetic field lines begin at the north pole of a magnet and end at
the south pole of the magnet.
• Compasses align with Earth’s magnetic field.
– The compass points in the direction that lies along the magnetic
field line at a given point.
– By convention, the pole of a magnet that points north is painted
red.
Earth’s Magnetic Field
> How is Earth’s magnetic field oriented?
> Earth’s magnetic field lines run from geographic south to geographic
north.
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Super Summary Chapter Outline p. 2
Chapter 18 Magnetism
• The pole in Antarctica is a magnetic north pole.
• The pole in northern Canada is a magnetic south pole.
• Earth’s magnetic field has both direction and strength.
• The source of Earth’s magnetism is not yet fully understood.
– The iron in the core is too hot to retain any magnetic properties.
– The circulation of ions or electrons in the liquid layer of Earth’s
core may be the source of the magnetism.
• The sun also has a magnetic field and ejects charged particles into
space.
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Super Summary Chapter Outline p. 3
Chapter 18 Magnetism
Section 2 Magnetism from Electric Currents
Key Idea questions
> What happens to a compass near a wire that is carrying a current?
> Why are electric motors useful?
Electromagnetism
> What happens to a compass near a wire that is carrying a current?
> When the wire carries a strong, steady current, all of the compass
needles move to align with the magnetic field created by the electric
current.
• Hans Christian Oersted found that magnetism is produced by moving
electric charges.
– Electric currents produce magnetic fields.
• Use the right-hand rule to find the direction of the magnetic field
produced by a current.
– right-hand rule: If you hold a wire in your right hand and point your
thumb in the direction of the positive current, the direction that
your fingers curl is the direction of the magnetic field.
• Solenoids and bar magnets have similar magnetic fields.
• solenoid: a coil of wire with an electric current in it
– In a solenoid, the magnetic field of each loop of wire adds to the
strength of the magnetic field of any neighboring loops.
• The strength of a solenoid can be increased.
– More loops or more current can create a stronger magnetic field.
• electromagnet: a coil that has a soft iron core and that acts as a
magnet when an electric current is in the coil
– The magnetic field of the rod adds to the coil’s field.
• Moving charges cause magnetism.
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Super Summary Chapter Outline p. 4
Chapter 18 Magnetism
– Negatively charged electrons moving around the nuclei of atoms
make magnetic fields.
– Atomic nuclei also have magnetic fields because protons move
within the nucleus.
– Each electron has a property called electron spin, which also
produces a tiny magnetic field.
• The magnetism of the uncanceled fields in certain materials
combines to make the materials magnetic overall.
Electromagnetic Devices
> Why are electric motors useful?
> A motor can perform mechanical work when it is attached to an external
device.
• electric motor: a device that converts electrical energy to mechanical
energy
• Galvanometers detect current.
– galvanometer: an instrument that detects, measures, and
determines the direction of a small electric current
• ammeter: measures current
• voltmeter: measures voltage
• Motors use a commutator to spin in one direction.
– commutator: a device used to make the current change direction
every time the flat coil makes a half revolution.
– brushes: devices that connect the wires to the commutator
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Super Summary Chapter Outline p. 5
Chapter 18 Magnetism
Section 3 Electric Currents from Magnetism
Key Idea questions
> 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?
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: An electric current can be produced in a circuit by a
changing magnetic field crossing the circuit.
• 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.
• 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.
• Generators convert mechanical energy into electrical energy.
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Super Summary Chapter Outline p. 6
Chapter 18 Magnetism
• 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.
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.
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
• Current in the primary coil, this current creates a changing magnetic
field that magnetizes the iron core.
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Super Summary Chapter Outline p. 7
Chapter 18 Magnetism
• 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.
• 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.
• 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.
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Super Summary Chapter Outline p. 8