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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?
Slide 24-1
Magnetic Fields Around Us
Slide 24-14
Key Points
• Three types of magnetic interactions
1. no interaction with either pole of a magnet
=> object is non-magnetic
2. attracted to both poles of a magnet
=> object is magnetic
3. Attracted to one pole and repelled by the other pole
=> object is a magnet
• Magnetic field vector from a bar magnet is a super position of
the magnetic field vectors from the N and S poles:
•
•
Vector from N pole points away from N pole
Vector from S pole points towards S pole
• Field lines form complete loops inside and outside of magnet
•
•
•
Field lines outside magnet go from N to S poles
Field lines inside magnet go from S to N poles
Magnetic Field vectors at a point are tangential to Magnetic Field Lines
Magnetic Fields from Two Magnets
Bar Magnets A and B are placed at right angles. Two compasses, X and Y are placed so
that they are equidistant from the two magnets as shown
A.) The arrow in compass X indicates the direction
in which the North pole of the compass is pointing.
Indicate the North and South ends of both magnets
in the diagram
B.) Draw an arrow in compass Y to show the direction
in which the North pole of the compass needle
would point.
Slide 24-2
Electric Currents Also Create Magnetic Fields
A long, straight
wire
A current loop
A solenoid
Slide 24-15
Drawing Field Vectors and Field Lines of a
Current-Carrying Wire
Slide 24-21
The Magnitude of the Field due to a Long, Straight,
Current-Carrying Wire
m0 I
B=
2p r
m0 = permeability constant = 1.257 ´ 10 -6 T× m/A
Slide 24-25
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
The Magnetic Field of a Current Loop
Slide 24-23
The Magnetic Field of a Solenoid
A short solenoid
A long solenoid
Slide 24-24
Slide 24-26
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.
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.
Slide 24-28
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
Slide 24-29
Example
What is the direction and magnitude of the magnetic field at
point P, at the center of the loop?
Slide 24-30
The Magnetic Field Inside a Solenoid
N
B = m0 I
L
Magnetic field inside a solenoid
of length L with N turns.
Slide 24-31
The Force on a Charged Particle Moving in a Magnetic
Field
Slide 24-32
The Right-Hand Rule for Forces
Slide 24-33
Slide 24-34
Paths of Charged Particles in Magnetic Fields
mv
r=
qB
Slide 24-35
Magnetic Fields Exert Forces on Currents
Fwire = ILB
Magnitude of the force on a current segment
of length L perpendicular to a magnetic field
Slide 24-37