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Chapter 19, part 2
PHY213
1
Ch. 19: Magnetism (Cont.)
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Skip 9.5
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Motion of a charged particle in a
magnetic field
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Magnetic field around a wire
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Magnetic field of a current loop
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Magnetic force between two wires
(Ampere defined)
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Solenoids
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Magnetic Domains
Force on a Charged
Particle in a Magnetic Field
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Consider a particle
moving in an external
magnetic field so that its
velocity is perpendicular
to the field
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The force is always
directed toward the
center of the circular path
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The magnetic force
causes a centripetal
acceleration, changing
the direction of the
velocity of the particle
Force on a Charged
Particle
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Equating the magnetic and centripetal
forces:
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Solving for r:
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r is proportional to the momentum of the
particle and inversely proportional to the
magnetic field
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Sometimes called the
cyclotron equation
r
mv
qvB
F
2
=
=
qB
mv
r
=
Particle Moving in an
External Magnetic Field
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If the particle’s
velocity is not
perpendicular to
the field, the path
followed by the
particle is a spiral
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The spiral path is
called a helix
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View Full DocumentChapter 19, part 2
PHY213
2
Lecture Quiz #8
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The find the period,
T
, of the cyclotron
orbit in terms of
q,m,v,B,
and
r
. the period
is The time for one complete orbit.
(Hint: Start with
v=d/T
, where d is the
circumference of the orbit)
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What variable(s) does the period depend
on? What does it not depend on?
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Cyclotron frequency = 1/
T
. What is the
cyclotron frequency for a proton in a
magnetic field of B=2.0 Tesla.
Hans Christian Oersted
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1777 – 1851
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Best known for
observing that a
compass needle
deflects when placed
near a wire carrying
a current
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First evidence of a
connection between
electric and magnetic
phenomena
Magnetic Fields –
Long Straight Wire
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A currentcarrying
wire produces a
magnetic field
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The compass needle
deflects in directions
tangent to the circle
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The compass needle
points in the direction
of the magnetic field
produced by the
current
Direction of the Field of a
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 Summer '08
 Cao
 Physics, Charge, Current, Magnetism, Force

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