Post on 01-Apr-2015
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
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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.
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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)
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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
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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