Magnetism

Chapter 19Chapter 19

Introduction

Applications involving magnetismApplications involving magnetism LoudspeakersLoudspeakers MetersMeters ElectromagnetsElectromagnets Recording devicesRecording devices

TapesTapes Computer disksComputer disks

MRIMRI

Important Concepts

All magnetic fields are produced by All magnetic fields are produced by charges in motion.charges in motion.

Magnetic fields will affect charges Magnetic fields will affect charges moving through them.moving through them.

Magnetic Poles

Magnets have poles.Magnets have poles. NorthNorth SouthSouth

191191

The Law of Poles

Like poles repel, unlike poles attract.Like poles repel, unlike poles attract.

Can a magnet have only one pole?Can a magnet have only one pole?NoNo

Can a magnet have more than two Can a magnet have more than two poles?poles?YesYes

Effects of Cutting?

If a magnet is cut in half, lengthwise, will If a magnet is cut in half, lengthwise, will the two halves attract or repel?the two halves attract or repel?

RepelRepel If a magnet is cut in half, across the width, If a magnet is cut in half, across the width,

will the two halves attract or repel?will the two halves attract or repel?

AttractAttract

36-236-2

Magnetic Induction

Magnetism may be induced in iron and Magnetism may be induced in iron and other materials.other materials. Stroking with a permanent magnetStroking with a permanent magnet Heating and then cooling in a strong Heating and then cooling in a strong

magnetic fieldmagnetic field

Natural Magnets

Magnetite (lodestone)Magnetite (lodestone) A naturally occurring magnetic materialA naturally occurring magnetic material

Produced by heating and then cooling in the Produced by heating and then cooling in the earth’s magnetic fieldearth’s magnetic field

Soft Magnetic Materials IronIron

Easily magnetizedEasily magnetized Weakens quicklyWeakens quickly

Hard Magnetic Materials Cobalt and nickelCobalt and nickel

Difficult to magnetizeDifficult to magnetize Stays strong longerStays strong longer

Magnetic Fields

All charges are surrounded by an All charges are surrounded by an electric field.electric field.

Charges in motion are surrounded Charges in motion are surrounded by a magnetic field.by a magnetic field.

Magnet Fields

Field descriptions must include:Field descriptions must include:Magnitude (strength)Magnitude (strength)DirectionDirection

The direction that an isolated north The direction that an isolated north pole would move when placed in pole would move when placed in the fieldthe field

19.2, 192 19.2, 192

Magnetic Field Lines

Magnetic field lines can be shown by Magnetic field lines can be shown by using small iron filingsusing small iron filings

7979

Magnetic Field Of The Earth

North-seeking and south seeking polesNorth-seeking and south seeking poles CompassesCompasses Geographic poles vs. magnetic polesGeographic poles vs. magnetic poles

Location (north magnetic is in Canada)Location (north magnetic is in Canada) Dip angleDip angle Magnetic declinationMagnetic declination

The angle between true north and magnetic The angle between true north and magnetic northnorth

19.419.4

What is believed to be the source of the What is believed to be the source of the earth’s magnetic field?earth’s magnetic field?

The earth’s magnetic field reverses every The earth’s magnetic field reverses every few million yearsfew million years Evidence?Evidence?

Animals and Magnetism

Some animals use the earth’s magnetic field Some animals use the earth’s magnetic field for navigation.for navigation. Anaerobic bacteriaAnaerobic bacteria LobstersLobsters BirdsBirds

Magnetic Forces

A charge experiences a force when A charge experiences a force when moving through a magnetic field.moving through a magnetic field. Maximum force occurs when the motion Maximum force occurs when the motion

is perpendicular to the field.is perpendicular to the field. Zero force occurs when the motion is Zero force occurs when the motion is

parallel or antiparallel to the field.parallel or antiparallel to the field.

19.519.5

Factors Influencing Magnetic Force

Which factors will determine the Which factors will determine the magnitude of the force on a charge that magnitude of the force on a charge that is moving through a magnetic field?is moving through a magnetic field? Magnetic field strengthMagnetic field strength Quantity of charge Quantity of charge Velocity of the chargeVelocity of the charge Direction of the velocityDirection of the velocity

Magnetic Symbols

New SymbolsNew Symbols Magnetic force (F)Magnetic force (F) Magnetic field strength (B)Magnetic field strength (B) Angle Angle (measured between v and B)(measured between v and B)

FormulaFormula

€

F = qvBsinθ

Magnetic Field Strength

Magnetic field strength Magnetic field strength unitsunits SISI

Tesla (T) or (1 Wb/mTesla (T) or (1 Wb/m22))

Magnetic Force

FormulaFormula

Right Hand RuleRight Hand Rule

19.6/19.7, 19819.6/19.7, 198

€

Fmax = qvB

Magnetic Force On A Current Carrying-Conductor

A current-carrying wire will A current-carrying wire will experience a force when placed in experience a force when placed in a magnetic field.a magnetic field.

Symbols for current directionSymbols for current directionHeadHeadTailTail

2626

Magnetic Field Around a Conductor

Magnetic Force On A Straight Wire

FormulaFormula

There is no force when the current in the There is no force when the current in the wire is in the same or opposite direction of wire is in the same or opposite direction of the field.the field.

€

F = BIl sinθ

Magnetic Force Applications

Stereo speakerStereo speaker Voice coilVoice coil

Electromagnetic pumpElectromagnetic pump No moving partsNo moving parts

Possible heart replacementPossible heart replacementMagnetohydrodynamic propulsion (MHD)Magnetohydrodynamic propulsion (MHD)

199199

Torque On A Current Loop

A torque is exerted on a current-carrying A torque is exerted on a current-carrying loop when it is placed in a magnetic fieldloop when it is placed in a magnetic field ApplicationsApplications

GalvanometersGalvanometersMotorsMotorsGeneratorsGenerators

19.15, 19.17, 200, 20119.15, 19.17, 200, 201

Torque formulaTorque formula

Shape of coil is not importantShape of coil is not important Angle Angle is measured is measured betweenbetween B and the axis B and the axis

of the coilof the coil Number of Turns (N)Number of Turns (N)

€

τ =NBIAsinθ

Motion Of A Charged Particle In A Magnetic Field

If the velocity is perpendicular to the If the velocity is perpendicular to the field, the magnetic force is always field, the magnetic force is always toward the center of the circular path.toward the center of the circular path. Centripetal accelerationCentripetal acceleration

No change in speedNo change in speedDirection changeDirection change

195, 80, 19.19195, 80, 19.19

Curved Motion in a Magnetic Field

Radius of curvature formula:Radius of curvature formula:

197197

€

r =mv

qB

QUESTIONS

1 – 81 – 8

Pg. 654Pg. 654

Electricity and Magnetism

Oersted lectureOersted lecture An electric field deflected a nearby An electric field deflected a nearby

compass needlecompass needleThis was the first realization of a link This was the first realization of a link

between electricity and magnetismbetween electricity and magnetism

Magnetic Field Of A Long Straight Wire

Oersted demonstrationOersted demonstrationCompasses surrounding a current-Compasses surrounding a current-

carrying wirecarrying wire The magnetic field lines form circles The magnetic field lines form circles

around the wire.around the wire.

19.23a,b19.23a,b

Another Right-Hand Rule

The Right-Hand-Rule for a currentThe Right-Hand-Rule for a current Indicates the directions for current, Indicates the directions for current,

magnetic field , and forcemagnetic field , and force

19.23a19.23a

Magnetic Field Strength

Formula for the magnetic field strength due Formula for the magnetic field strength due to a current in a long straight wireto a current in a long straight wire

o o is the permeability of free spaceis the permeability of free space

o = o = 44 x 10 x 10-7-7 T T..m/Am/A

€

B =μoI

2πr

Magnetic Force Between Two Parallel Conductors

Two parallel current-carrying conductors will Two parallel current-carrying conductors will exert a force on each other. (very important)exert a force on each other. (very important)

19.2819.28

€

B =μoI22πr

€

F =lμoI1I2

2πd

Defining The Ampere

Definition of the Ampere (1 A)Definition of the Ampere (1 A) Two long parallel wiresTwo long parallel wires One meter apartOne meter apart Same current in eachSame current in each Force/length = 2 x 10Force/length = 2 x 10-7-7 N/m N/m Remember:Remember:

1 Ampere = 1 Coulomb/second1 Ampere = 1 Coulomb/second

Magnetic Field Of A Current Loop

A current loop can increase the A current loop can increase the strength of the magnetic field.strength of the magnetic field.

Current loops have poles.Current loops have poles.

203, 19.28, 19.31203, 19.28, 19.31

Magnetic Field Of A Solenoid

A solenoid is made up of several A solenoid is made up of several closely spaced loops.closely spaced loops. Electromagnets are solenoidsElectromagnets are solenoids

ApplicationsApplications Solenoids have polesSolenoids have poles Field lines are nearly parallel withinField lines are nearly parallel within

204, 19.33, 19.32, 158, 159, 207204, 19.33, 19.32, 158, 159, 207

Formula for the magnetic field of a solenoid: Formula for the magnetic field of a solenoid:

n = N/n = N/LL

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B = μonI

Causes of Magnetism Orbital motion produces magnetic Orbital motion produces magnetic

fields.fields. These fields tend to cancel out.These fields tend to cancel out.

Electron spin produces magnetic fieldsElectron spin produces magnetic fields In most substances, electron pairs have In most substances, electron pairs have

opposite spins which cancel.opposite spins which cancel.Not in iron, nickel and cobaltNot in iron, nickel and cobalt

Magnetic Domains

OrientationOrientation AlignmentAlignment GrowthGrowth

Ferromagnetic Materials

Ferromagnetic materials are able to be Ferromagnetic materials are able to be magnetized. (permanent magnets)magnetized. (permanent magnets)

Magnetic cores are used in Magnetic cores are used in electromagnets.electromagnets.

206206

Questions

10 - 12, 14 - 1710 - 12, 14 - 17

Pg. 655Pg. 655