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
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
3-D Arrows, Cross Products, and Right Hand Rule 1
• Showing vectors in 3D
• Cross ProductC = A ´ B
C = A B sin a
For direction use Right-hand rule 1
• Right-hand rule 1 (RHR 1)
=> for finding direction of cross-product vector
(Cross-Product Rule)
C = A´ B
1. Point right hand in the direction of the first vector (vector A)
2. Rotate your right hand until you can point your fingers in the
direction of the second vector (vector B)
3. Thumb points in direction the cross-product vector (vector C)
Slide 24-2
Electric vs. Magnetic Interactions
1. Nature of Magnetic Interactions
Slide 24-2
Discovering Magnetism
Slide 24-6
The Magnetic Field
Slide 24-7
Mapping Out the Field of a Bar Magnet
Slide 24-8
Mapping Out the Magnetic Field Using Iron Filings
Slide 24-9
Drawing Field Lines of a Bar Magnet
Slide 24-10
Magnetic Fields Produced by Bar Magnets
A single bar magnet
A single bar magnet
(closeup)
Slide 24-11
Magnetic Fields Produced by Bar Magnets
Two bar magnets,
unlike poles facing
Two bar magnets,
like poles facing
Slide 24-12
Checking Understanding
Slide 24-13
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
Magnetic Fields Around Us
Slide 24-14
Electric Currents Also Create Magnetic Fields
A long, straight
wire
A current loop
A solenoid
Slide 24-15
The Magnetic Field of a Straight Current-Carrying Wire
Slide 24-16
Slide 24-17
Representing Vectors and Currents That Are
Perpendicular to the Page
Slide 24-18
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
Drawing Field Vectors and Field Lines of a
Current-Carrying Wire
Slide 24-21
Drawing a Current Loop
Slide 24-22
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
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
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