THE MAGNETIC FIELD
•B is the symbol for the magnetic field.
•Magnetic field lines run from north poles to
south poles.
•Like poles repel, and unlike poles attract.
•Poles cannot be isolated. They occur only in
pairs, as dipoles.
Physical Modeling, Fall 2006
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Charges in a Magnetic Field
Moving charges experience a force due to a
magnetic field.
FB = qv × B
where f is the angle between v and B.
Direction is from the right hand rule.
Magnitude of FB is: FB = qvB sin f
Physical Modeling, Fall 2006
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Direction of the Magnetic Force
Physical Modeling, Fall 2006
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Charges in a Magnetic Field
•The magnetic force on a moving charge is
perpendicular to the direction of the
magnetic field, and perpendicular to the
direction of the velocity of the charge.
•If a charge moves parallel to the direction of
a magnetic field, it experiences no magnetic
force.
Physical Modeling, Fall 2006
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Charges in a Magnetic Field
If a charge moves perpendicular to a uniform
magnetic field, it will travel in a circular path.
F  ma
2
v
qvB sin f  qvB  m
r
mv
r
qB
Physical Modeling, Fall 2006
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Application
Earth’s magnetic field shields us from
incoming charged particles. However, since
Earth’s magnetic field goes from the south
pole to the north, particles can travel parallel
to the field and enter the atmosphere near the
poles. The “aurora” is the result.
Physical Modeling, Fall 2006
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Aurora
Physical Modeling, Fall 2006
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Frequency or Period
Frequency or period of a circular orbit in a
magnetic field does not depend on radius.
F  ma
qvB  q rB  m r
qB  m
qB

m
2
Physical Modeling, Fall 2006
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Frequency or Period

qB
f 

2 2 m
1 2 m
T
f

qB
Physical Modeling, Fall 2006
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