Chapter 21 Magnetism 21.1 Magnets and Magnetic Fields

Chapter 21Chapter 21MagnetismMagnetism

21.1 Magnets and 21.1 Magnets and Magnetic FieldsMagnetic Fields

Magnetic forceMagnetic force is the force a magnet is the force a magnet exerts on another magnet, on iron or a exerts on another magnet, on iron or a similar metal, or on moving charges. similar metal, or on moving charges.

• Magnetic forces, like electric forces, act Magnetic forces, like electric forces, act over a distance.over a distance.

• Magnetic force, like electric force, Magnetic force, like electric force, varies with distance.varies with distance.

Magnetic Forces

All magnets have two All magnets have two magnetic polesmagnetic poles, , regions where the magnet’s force is regions where the magnet’s force is strongest. strongest.

• One end of a magnet is its north pole.One end of a magnet is its north pole.• The other end is its south pole. The other end is its south pole.

Magnetic Forces

A A magnetic fieldmagnetic field surrounds a magnet and surrounds a magnet and can exert magnetic forces. Magnetic field can exert magnetic forces. Magnetic field lines begin near the north pole and extend lines begin near the north pole and extend toward the south pole. toward the south pole.

• The arrows on the field lines indicate The arrows on the field lines indicate what direction a compass needle would what direction a compass needle would point at each point in space. point at each point in space.

• Where lines are close together, the Where lines are close together, the field is strong. field is strong.

• Where lines are more spread out, the Where lines are more spread out, the field is weak.field is weak.

Magnetic Field Around EarthMagnetic Field Around EarthEarth is like a giant magnet Earth is like a giant magnet surrounded by a magnetic field. The surrounded by a magnetic field. The area surrounding Earth that is area surrounding Earth that is influenced by this field is the influenced by this field is the magnetospheremagnetosphere..

A compass points north because it A compass points north because it aligns with Earth’s magnetic field. aligns with Earth’s magnetic field.

Magnetic Fields

A property of electrons called “spin” A property of electrons called “spin” causes electrons to act like tiny magnets. causes electrons to act like tiny magnets.

• In many materials, each electron is In many materials, each electron is paired with another having an opposite paired with another having an opposite spin so magnetic effects mostly cancel spin so magnetic effects mostly cancel each other.each other.

• Unpaired electrons in some materials Unpaired electrons in some materials produce magnetic fields that don’t produce magnetic fields that don’t combine because of the arrangement combine because of the arrangement of the atoms.of the atoms.

Magnetic Materials

In a few materials, such as iron, nickel, In a few materials, such as iron, nickel, and cobalt, the unpaired electrons and cobalt, the unpaired electrons make a strong magnetic field. make a strong magnetic field.

• The fields combine to form The fields combine to form magnetic domains. magnetic domains.

• A A ferromagnetic materialferromagnetic material, such , such as iron, as iron, can be magnetized can be magnetized because it contains magnetic because it contains magnetic domains.domains.

Magnetic Materials

Magnetized MaterialsMagnetized MaterialsIf you place a nonmagnetized ferromagnetic If you place a nonmagnetized ferromagnetic material in a magnetic field, it will become a material in a magnetic field, it will become a magnet when the domains are aligned. magnet when the domains are aligned.

• Magnetization can be temporary. If the Magnetization can be temporary. If the material is moved away from the magnet, material is moved away from the magnet, the magnetic domains become random.the magnetic domains become random.

• In some ferromagnetic materials, the In some ferromagnetic materials, the domains stay aligned for a long time. These domains stay aligned for a long time. These materials are called materials are called permanent magnetspermanent magnets. .

Magnetic Materials

If you cut a magnet in half, If you cut a magnet in half, each half will have its own each half will have its own north pole and south pole north pole and south pole because the domains will still because the domains will still be aligned. be aligned.

A magnet can never have just A magnet can never have just a north pole or just a south a north pole or just a south pole.pole.

Magnetic Materials

Assessment Questions

1.1. Where does the magnetic field of a Where does the magnetic field of a magnet have the strongest effect magnet have the strongest effect on another magnet? on another magnet?

a.a. the north polethe north pole

b.b. the south polethe south pole

c.c. both poles equallyboth poles equally

d.d. midway between the two polesmidway between the two poles

2.2. How are the magnetic field lines drawn to How are the magnetic field lines drawn to show the interaction of two bar magnets that show the interaction of two bar magnets that are lined up with their north poles near one are lined up with their north poles near one another? another?

a.a. Field lines begin at the north pole of each Field lines begin at the north pole of each magnet and extend to the south pole of magnet and extend to the south pole of the other magnet.the other magnet.

b.b. Field lines begin at each magnet’s north Field lines begin at each magnet’s north pole and extend toward its south pole.pole and extend toward its south pole.

c.c. Field lines extend from the north pole of Field lines extend from the north pole of one magnet to the north pole of the other one magnet to the north pole of the other magnet.magnet.

d.d. Field lines cannot be drawn because the Field lines cannot be drawn because the magnetic forces cancel one another.magnetic forces cancel one another.

3.3. Why does a compass not point exactly Why does a compass not point exactly toward the geographic north pole? toward the geographic north pole?

a.a. Earth’s magnetic field is constantly Earth’s magnetic field is constantly changing due to effects of the solar wind.changing due to effects of the solar wind.

b.b. The magnetic pole is near but not exactly The magnetic pole is near but not exactly at the geographic pole.at the geographic pole.

c.c. Earth’s magnetic field lines are too broad Earth’s magnetic field lines are too broad for a compass point exactly toward the for a compass point exactly toward the pole.pole.

d.d. Daily variations in the magnetic field Daily variations in the magnetic field mean that compasses are not very mean that compasses are not very accurate.accurate.

4.4. What happens to a permanent magnet What happens to a permanent magnet if its magnetic domains lose their if its magnetic domains lose their alignment? alignment?

a.a. The magnetic field reverses The magnetic field reverses direction.direction.

b.b. It loses its magnetic field.It loses its magnetic field.

c.c. It has several north poles and It has several north poles and several south poles.several south poles.

d.d. It is no longer a ferromagnetic It is no longer a ferromagnetic material.material.

Chapter 21 Chapter 21 MagnetismMagnetism

21.2 21.2 ElectromagnetismElectromagnetism

In 1820 Hans In 1820 Hans Oersted Oersted discovered how discovered how magnetism and magnetism and electricity are electricity are connected. A unit connected. A unit of measure of of measure of magnetic field magnetic field strength, the strength, the oersted, is oersted, is named after him.named after him.

Electricity and magnetism are different Electricity and magnetism are different aspects of a single force known as the aspects of a single force known as the electromagnetic forceelectromagnetic force. .

• The electric force results from The electric force results from charged particles. charged particles.

• The magnetic force usually results The magnetic force usually results from the movement of electrons in from the movement of electrons in an atom. an atom.

Electricity and Magnetism

Magnetic Fields Around Moving Magnetic Fields Around Moving ChargesCharges

Moving charges create a magnetic Moving charges create a magnetic field.field.

• Magnetic field lines form circles Magnetic field lines form circles around a straight wire carrying a around a straight wire carrying a current.current.


If you point the thumb of your right If you point the thumb of your right hand in the direction of the current, hand in the direction of the current, your fingers curve in the direction of your fingers curve in the direction of the magnetic field.the magnetic field.

Direction of electron flow

Direction of current

Current-carrying wire

Direction of magnetic field

Forces Acting on Moving ChargesForces Acting on Moving Charges

A magnetic field exerts a force on a A magnetic field exerts a force on a moving charge.moving charge.

• A charge moving in a magnetic field is A charge moving in a magnetic field is deflected in a direction perpendicular to deflected in a direction perpendicular to both the field and to the velocity of the both the field and to the velocity of the charge.charge.

• A current-carrying wire in a magnetic A current-carrying wire in a magnetic field will be pushed in a direction field will be pushed in a direction perpendicular to both the field and the perpendicular to both the field and the direction of the current. direction of the current.

• Reversing the direction of Reversing the direction of the current will still cause the current will still cause the wire to be deflected, but the wire to be deflected, but in the opposite direction.in the opposite direction.

• If the current is parallel to If the current is parallel to the magnetic field, the force the magnetic field, the force is zero and there is no is zero and there is no deflection.deflection.


If a current-carrying wire has a If a current-carrying wire has a loop in it, the magnetic field in loop in it, the magnetic field in the center of the loop points the center of the loop points right to left through the loop. right to left through the loop.

Multiple loops in the wire make Multiple loops in the wire make a coil. The magnetic fields of a coil. The magnetic fields of the loops combine so that the the loops combine so that the coiled wire acts like a bar coiled wire acts like a bar magnet. magnet.

Solenoids and Electromagnets

The field through the center The field through the center of the coil is the sum of the of the coil is the sum of the fields from all the turns of the fields from all the turns of the wire. wire.

A coil of current-carrying A coil of current-carrying wire that produces a wire that produces a magnetic field is called a magnetic field is called a solenoid.solenoid.


If a ferromagnetic material, such as an iron If a ferromagnetic material, such as an iron rod is placed inside the coil of a solenoid, rod is placed inside the coil of a solenoid, the strength of the magnetic field the strength of the magnetic field increases.increases.

• The magnetic field produced by the The magnetic field produced by the current causes the iron rod to become current causes the iron rod to become a magnet.a magnet.

• An An electromagneelectromagnet t is a solenoid with a is a solenoid with a ferromagnetic core. ferromagnetic core.

• The current can be used to turn the The current can be used to turn the magnetic field on and off.magnetic field on and off.

The strength of an electromagnet depends The strength of an electromagnet depends on the current in the solenoid, the number on the current in the solenoid, the number of loops in the coil, and the type of core. of loops in the coil, and the type of core. The strength of an electromagnet can be The strength of an electromagnet can be increased using the following methods.increased using the following methods.

• Increase the current flowing through Increase the current flowing through the solenoid. the solenoid.

• Increase the number of turns.Increase the number of turns.• Use cores that are easily magnetized.Use cores that are easily magnetized.


Electromagnets can convert electrical Electromagnets can convert electrical energy into motion that can do work. energy into motion that can do work.

• A A galvanometergalvanometer measures current in a measures current in a wire through the deflection of a wire through the deflection of a solenoid in an external magnetic field. solenoid in an external magnetic field.

• An An electric motorelectric motor uses a rotating uses a rotating electromagnet to turn an axle.electromagnet to turn an axle.

• A loudspeaker uses a solenoid to A loudspeaker uses a solenoid to convert electrical signals into sound convert electrical signals into sound waves.waves.

Electromagnetic Devices

GalvanometersGalvanometersA A galvanometergalvanometer is a device that is a device that uses a solenoid to measure uses a solenoid to measure small amounts of current. small amounts of current.


Electric MotorsElectric MotorsAn An electric motorelectric motor is a device that is a device that uses an electromagnet to turn an axle. uses an electromagnet to turn an axle.

• A motor has many loops of wire A motor has many loops of wire around a central iron core. around a central iron core.

• In the motor of an electric In the motor of an electric appliance, the wire is connected to appliance, the wire is connected to an electrical circuit in a building.an electrical circuit in a building.


LoudspeakersLoudspeakers

A loudspeaker contains a A loudspeaker contains a solenoid placed around one solenoid placed around one pole of a permanent magnet. pole of a permanent magnet.

The current in the wires The current in the wires entering the loudspeaker entering the loudspeaker changes direction and changes direction and increases or decreases.increases or decreases.


Assessment Questions1.1. A charged particle is moving across a plane A charged particle is moving across a plane

from left to right as it enters a magnetic field from left to right as it enters a magnetic field that runs from top to bottom. How will the that runs from top to bottom. How will the motion of the particle be changed as it motion of the particle be changed as it enters the magnetic field? enters the magnetic field?

a.a. It will accelerate.It will accelerate.

b.b. It will deflect either up or down on the It will deflect either up or down on the plane.plane.

c.c. It will deflect perpendicular to the plane.It will deflect perpendicular to the plane.

d.d. Its motion will not be affected.Its motion will not be affected.

Assessment Questions2.2. Which change will increase the Which change will increase the

strength of an electromagnet made by strength of an electromagnet made by wrapping a conductive wire around an wrapping a conductive wire around an iron nail? iron nail? a.a. reversing the direction of current flowreversing the direction of current flowb.b. replacing the nail with a wooden replacing the nail with a wooden

doweldowelc.c. increasing the number of coils of wire increasing the number of coils of wire

around the nailaround the naild.d. using a longer nailusing a longer nail


3.3. A loudspeaker uses a magnet to cause A loudspeaker uses a magnet to cause which energy conversion?which energy conversion?

a.a. mechanical energy to magnetic mechanical energy to magnetic energyenergy

b.b. electrical energy to mechanical electrical energy to mechanical energyenergy

c.c. electrical energy to magnetic energyelectrical energy to magnetic energy

d.d. mechanical energy to electrical mechanical energy to electrical energyenergy


4.4. The motion of an electric The motion of an electric charge creates an electrical charge creates an electrical field. field.

TrueTrueFalseFalse

Chapter 21 Chapter 21 MagnetismMagnetism

21.3 Electrical Energy 21.3 Electrical Energy Generation and Generation and TransmissionTransmission

A magnetic field can be used to produce A magnetic field can be used to produce an electric current. an electric current.

• Electromagnetic inductionElectromagnetic induction is the is the process of generating a current by process of generating a current by moving an electrical conductor relative moving an electrical conductor relative to a magnetic field. to a magnetic field.

• Changing the magnetic field through a Changing the magnetic field through a coil of wire induces a voltage in the coil of wire induces a voltage in the coil. coil.

• A current results if the coil is part of a A current results if the coil is part of a complete circuit.complete circuit.

Most of the electrical energy used in homes Most of the electrical energy used in homes and businesses is produced at large power and businesses is produced at large power plants using generators. plants using generators.

• A A generatorgenerator is a device that converts is a device that converts mechanical energy into electrical energy mechanical energy into electrical energy by rotating a coil of wire in a magnetic by rotating a coil of wire in a magnetic field. field.

• Electric current is generated by the Electric current is generated by the relative motion of a conducting coil in a relative motion of a conducting coil in a magnetic field. magnetic field.

Generators

AC GeneratorsAC GeneratorsAn An AC generatorAC generator produces alternating produces alternating current, in which charges flow first in current, in which charges flow first in one direction and then in the other one direction and then in the other direction. direction.

The generator looks very similar to an The generator looks very similar to an electric motor. While a motor converts electric motor. While a motor converts electrical energy into mechanical electrical energy into mechanical energy, a generator does the opposite.energy, a generator does the opposite.

Generators

DC GeneratorsDC GeneratorsA A DC generatorDC generator produces a direct current. produces a direct current.

• Its design is very much like the design Its design is very much like the design of an AC generator except that a of an AC generator except that a commutator replaces the slip rings. commutator replaces the slip rings.

• As opposite sides of the commutator As opposite sides of the commutator touch the brush, the current that leaves touch the brush, the current that leaves the generator flows in only one the generator flows in only one direction.direction.

Generators

The number of turns in the primary The number of turns in the primary and secondary coils determines the and secondary coils determines the voltage and current. voltage and current.

• To calculate the voltage, divide To calculate the voltage, divide the number of turns in the the number of turns in the secondary coil by the number of secondary coil by the number of turns in the primary coil. turns in the primary coil.

• The result is the ratio of the The result is the ratio of the output voltage to the input output voltage to the input voltage.voltage.

Transformers

Types of TransformersTypes of TransformersA A step-down transformerstep-down transformer decreases voltage and decreases voltage and increases current. increases current.

Transformers

A A step-up transformerstep-up transformer increases voltage and increases voltage and decreases current. decreases current.

Transformers

A A turbineturbine is a device with fanlike blades that is a device with fanlike blades that turn when pushed, for example, by water or turn when pushed, for example, by water or steam.steam.

• Burning fossil fuels or nuclear reactions can Burning fossil fuels or nuclear reactions can heat water to produce steam that spins a heat water to produce steam that spins a turbine. turbine.

• Water from a reservoir behind a dam can Water from a reservoir behind a dam can also turn a turbine.also turn a turbine.

• To produce electrical energy, the turbine To produce electrical energy, the turbine may turn the coils of a generator, or it may may turn the coils of a generator, or it may spin magnets around the coils of wire.spin magnets around the coils of wire.

Electrical Energy for Your Home

A A power plantpower plant transmits electrical energy at transmits electrical energy at hundreds of thousands of volts. hundreds of thousands of volts.

• After the current passes travels through After the current passes travels through high-voltage transmission lines, the voltage high-voltage transmission lines, the voltage is stepped down at a substation, to a few is stepped down at a substation, to a few thousand volts.thousand volts.

• The electrical energy is then distributed and The electrical energy is then distributed and stepped down to between 220 and 240 stepped down to between 220 and 240 volts. volts.

• Appliances like an electric stove use 240-Appliances like an electric stove use 240-volt circuits. Most other appliances in the volt circuits. Most other appliances in the home use 120 volts.home use 120 volts.


1.1. In a DC generator, the commutator In a DC generator, the commutator

a.a. generates an electric current.generates an electric current.

b.b. converts an alternating current to converts an alternating current to a direct current.a direct current.

c.c. reduces the voltage.reduces the voltage.

d.d. reverses the direction of the reverses the direction of the direct current.direct current.


2.2. A transformer has 400 turns on the A transformer has 400 turns on the primary coil and 1600 turns on the primary coil and 1600 turns on the secondary coil. What is the output secondary coil. What is the output voltage if the input is 1,000 volts? voltage if the input is 1,000 volts? a.a. 250 V250 Vb.b. 500 V500 Vc.c. 2,000 V2,000 Vd.d. 4,000 V4,000 V


3.3. Which property would you want to Which property would you want to increase in transmitting electrical increase in transmitting electrical energy as efficiently as possible energy as efficiently as possible over long distances? over long distances?

a.a. currentcurrent

b.b. voltagevoltage

c.c. resistanceresistance

d.d. insulationinsulation

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Chapter 21 Magnetism 21.1 Magnets and Magnetic Fields