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Magnetic Field & Forces Ch. 27
Magnetism
(sec. 27.1)
Magnetic field
(sec. 27.2)
Magnetic field lines and magnetic flux (sec. 27.3)
Motion of charges in a B field
(sec. 27.4)
Applications - moving charged particles (sec. 27.5)
Magnetic force on
conductor with current
(sec. 27.6)
Force and torque on a current loop
(sec. 27.7)
Direct current motor
(sec. 27.8)
The Hall effect
(sec. 27.9)
C 2012 J. F. Becker
Learning Goals - we will learn: ch 27
• The nature of the force that a moving
charged particle experiences in a
magnetic field.
• How to analyze the motion of a charged
particle in a magnetic field.
• How to analyze and calculate the magnetic
forces on current-carrying conductors
and loops.
Forces between bar
magnets
(or permanent
magnets )
Earth’s magnetic field
(Note the N-S poles of the bar magnet!)
The Van Allen radiation belts
around the Earth
Magnetic force acting on
a moving (+) charge
Compass over a horizontal
current-carrying wire
Magnetic field lines associated with a permanent
magnet, coil, iron-core electromagnet, current in wire,
current loop
MAGNETIC FLUX through an
area element dA
F = q v x B
Orbit of a charged
particle in a uniform
magnetic field is a
circle, so
F = qvB =
2
m(v /R)
and
R=mv/qB
qvB=qE
v=E/B
Velocity selector for charged particles uses
perpendicular E and B fields
Mass spectrometer uses a velocity
selector to produce particles with
uniform speed. And from
R = m v / q B we get
q/m=v/BR
Q27.10
A charged particle moves through a region of space that has both
a uniform electric field and a uniform magnetic field. In order for
the particle to move through this region at a constant velocity,
A. the electric and magnetic fields must point in the same
direction.
B. the electric and magnetic fields must point in opposite
directions.
C. the electric and magnetic fields must point in perpendicular
directions.
D. The answer depends on the sign of the particle’s electric
charge.
I
Force on a moving
positive charge in a
current-carrying
conductor:
L
F=ILxB
For vector direction
use
“RIGHT HAND RULE”
I
Right
hand
rule
F=ILxB
Magnetic force on a straight wire carrying current
I in a magnetic field B
Magnetic field B, length L, and force F vectors for a
straight wire carrying a current I
Components of a
loudspeaker
F = I L x B
Forces on the sides of a current-carrying loop in
a uniform magnetic field.
This is how a motor works!
Right hand rule determines the direction of the
magnetic moment (m) of a current-carrying loop
Torque (m x B) on this solenoid in a uniform magnetic
field is into the screen
thus rotating the solenoid clockwise
Current loops in a non-uniform B field
Atomic magnetic
moments in an iron bar
(a) unmagnetized
(b) magnetized
(c) torgue on a bar
magnet in a B field
Bar magnet attracts an unmagnetized piece of iron; the
B field gives rise to
a net magnetic moment in the object
A simple DC motor
The Hall effect – forces on charge carriers in a
conductor in a B field.
With a simple voltage measurement we can
determine whether the “charge carriers” are
positive or negative.
A linear motor
Electromagnetic pump
Review
See
www.physics.sjsu.edu/becker/physics51
C 2012 J. F. Becker