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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?
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 24-1
Checking Understanding
Point P is 5 cm above the wire as you look straight down at
it. In which direction is the magnetic field at P?
Slide 24-19
Answer
Point P is 5 cm above the wire as you look straight down at
it. In which direction is the magnetic field at P?
Slide 24-20
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
0 I
B
2 r
0  permeability constant  1.257  10 T m/A
6
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Slide 24-25
The Magnetic Field of a Current Loop
B
0 I
2R
Magnetic field at the center of
a current loop of radius R
B
0 NI
2R
Magnetic field at the center of
a current loop with N turns
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Slide 24-29
Checking Understanding
The diagram below shows a current loop perpendicular to the page;
the view is a “slice” through the loop. The direction of the current in
the wire at the top and the bottom is shown. What is the direction of
the magnetic field at a point in the center of the loop?
A.
B.
C.
D.
To the left
Up
To the right
Down
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 24-35
Answer
The diagram below shows a current loop perpendicular to the page;
the view is a “slice” through the loop. The direction of the current in
the wire at the top and the bottom is shown. What is the direction of
the magnetic field at a point in the center of the loop?
A.
B.
C.
D.
To the left
Up
To the right
Down
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 24-36
The Magnetic Field Inside a Solenoid
N
B  0 I
L
Magnetic field inside a solenoid
of length L with N turns.
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Slide 24-31
Example
What is the direction and magnitude of the magnetic field at
point P, at the center of the loop?
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Slide 24-30
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
The Force on a Charged Particle Moving in a Magnetic
Field
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Slide 24-32
The Right-Hand Rule for Forces
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Slide 24-33
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Slide 24-34
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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0 LI1 I 2
2d
Slide 24-38
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Slide 24-39
Checking Understanding
The magnetic field at point P is zero. What are the magnitude
and direction of the current in the lower wire?
A.
B.
C.
D.
E.
F.
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10 A to the right.
5 A to the right.
2.5 A to the right.
10 A to the left.
5 A to the left.
2.5 A to the left.
Slide 24-27
Answer
The magnetic field at point P is zero. What are the magnitude
and direction of the current in the lower wire?
E. 5 A to the left.
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Slide 24-28
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
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.
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
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.
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
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.
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.
Slide 24-67