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Chapter 24
Magnetic Fields and Forces
Topics:
•
•
•
•
•
Magnets and the magnetic
field
Electric currents create
magnetic fields
Magnetic fields of wires,
loops, and solenoids
Magnetic forces on charges
and currents
Magnets and magnetic
materials
Sample question:
This image of a patient’s knee was made with magnetic fields, not
x rays. How can we use magnetic fields to visualize the inside of
the body?
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Slide 24-1
Electric Currents Also Create Magnetic Fields
A long, straight
wire
A current loop
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A solenoid
Slide 24-15
The Magnitude of the Field due to a Long, Straight,
Current-Carrying Wire
m0 I
B=
2p r
m0 = permeability constant = 1.257 ´ 10 T× m/A
-6
Also μ0 = 4π x 10-7 T m / A
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Slide 24-25
The Magnetic Field of a Current Loop
B=
m0 I
2R
Magnetic field at the center of
a current loop of radius R
B=
m0 NI
2R
Magnetic field at the center of
a current loop with N turns
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Slide 24-29
The Magnetic Field Inside a Solenoid
N
B = m0 I
L
Magnetic field inside a solenoid
of length L with N turns.
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Slide 24-31
Right Hand Rules for Magnetism
• Right-hand rule 1 (RHR 1) => for finding magnetic force
FB= q*v_vector x B_vector (Cross-Product Rule)
1. Point right hand in the direction the charges are moving (current or velocity)
2. Rotate your right hand until you can point your fingers in the direction of the
magnetic Field
3. Thumb points in direction of force for + charge
Force is in opposite direction for - charges
• Right-hand rule 2 (RHR 2) => Finding direction of B from I
•
•
Point thumb of right hand in direction of current I,
B-field lines curl in direction of fingers
• Right-hand rule 3 (RHR 3) =>
Finding direction of current in a loop from direction of B-field
•
•
Point thumb of right hand in direction of B-field
Fingers of right hand curl in direction of current
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Slide 24-2
Magnetic Field Activities
• Double Trouble
• Solenoid
• Questions from E4 Worksheet 1
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Slide 24-37
Magnetic Force
• Review Magnetic Forces
• Magnetic Force on Moving
Charges
• E4 Worksheet 2 Questions
• Torque on Current Loops
• Double Trouble with Current
Loops
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Slide 24-37
Paths of Charged Particles in Magnetic Fields
mv
r=
qB
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Slide 24-35
The Mass Spectrometer
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Slide 24-36
Velocity Selector
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Slide 24-37
Magnetic Fields Exert Forces on Currents
Fwire = ILB
Magnitude of the force on a current segment
of length L perpendicular to a magnetic field
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Slide 24-37
Forces between Currents
Magnetic force between two
parallel current-carrying wires
Fparallel wires =
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m0 LI1 I 2
2pd
Slide 24-38
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Slide 24-39
Forces between Current Loops
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Slide 24-40
A Current Loop Acts like a Bar Magnet
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Slide 24-41
Magnetic Fields Exert Torques on Current Loops
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Slide 24-42
Applications: Galvanometers, Motors,
Loudspeakers
An electric motor
also takes
advantage of the
torque on a current
loop, to change
electrical energy to
mechanical energy.
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The Torque on a Dipole in a Magnetic Field
t = ( IA ) B sinq
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Slide 24-43
Magnetic Resonance Imaging
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Slide 24-44
Magnetic Resonance Imaging
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Slide 24-45
Electron Magnetic Moments: Ferromagnetism
A nonmagnetic solid (copper)
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A ferromagnetic solid (iron)
Slide 24-47
Inducing a Magnetic Moment in a Piece of Iron
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Slide 24-48
Summary of Chapter 20
• Magnets have north and south poles
• Like poles repel, unlike attract
• Unit of magnetic field: tesla
• Electric currents produce magnetic fields
• A magnetic field exerts a force on an electric
current:
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Summary of Chapter 20
• A magnetic field exerts a force on a moving
charge:
• Magnitude of the field of a long, straight
current-carrying wire:
• Parallel currents attract; antiparallel
currents repel
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Summary of Chapter 20
• Magnetic field inside a solenoid:
• Ampère’s law:
• Torque on a current loop:
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Checking Understanding
The diagram below shows slices through two adjacent current loops.
Think about the force exerted on the loop on the right due to the loop
on the left. The force on the right loop is directed
A.
B.
C.
D.
to the left.
up.
to the right.
down.
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Slide 24-37
Answer
The diagram below shows slices through two adjacent current loops.
Think about the force exerted on the loop on the right due to the loop
on the left. The force on the right loop is directed
A.
B.
C.
D.
to the left.
up.
to the right.
down.
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 24-38
Additional Questions
1. A loop carrying a current as shown rests in a uniform magnetic
field directed to the right. If the loop is free to rotate,
A.
B.
C.
it will rotate clockwise.
it will not rotate.
it will rotate counterclockwise.
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Slide 24-66
Answer
1. A loop carrying a current as shown rests in a uniform magnetic
field directed to the right. If the loop is free to rotate,
A.
B.
C.
it will rotate clockwise.
it will not rotate.
it will rotate counterclockwise.
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Slide 24-67
Motional emf
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Slide 25-12
Induced Current in a Circuit
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Slide 25-13
Magnetic Flux
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Slide 25-14