Magnetic Forces and Magnetic Fields

1
Chapter 21

• Magnetic Forces and Magnetic Fields

2
A Little History

• 12th century magnets first fashioned into
  compasses and used for navigation by the Chinese
• 16th century William Gilbert made artificial
  magnets by rubbing pieces of iron against
  lodestone
• 1750 John Michell found that magnetic poles obey
  inverse square law
• 1800s Ampere suggested that electric currents
  are the source of all magnetic phenomena

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Magnetic Fields
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Magnetic Field Lines
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Magnetic Field of the Earth
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The Force that a Magnetic Field Exerts on a
Moving Charge
In order for a charge to experience a magnetic
field
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The Force that a Magnetic Field Exerts on a
Moving Charge
Definition of Magnetic Field The magnitude B of
a magnetic field at any point in space is defined
as where F is the magnitude of the
magnetic force on a positive test charge q0 and v
is the velocity of the charge and makes an angle
? (0? ? ? 180o) with the direction of the
magnetic field. The magnetic field B is a
vector, and its direction can be determined by
using a small compass needle. SI Unit
newtonsecond 1 tesla (T)
coulombmeter
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The Right Hand Rule
Right-Hand Rule No. 1 Extend the right hand so
the fingers point along the direction of the
magnetic field B and the thumb points along the
velocity v of the charge. The palm of the hand
then faces the direction of the magnetic force
that acts on a positive charge.
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The Motion of a Charged Particle in a Magnetic
Field
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Motion of a Charged Particle in a Magnetic Field
The Circular Trajectory
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Motion of a Particle in Magnetic Field
The electric potential of the positive plate is
2100 V greater than that of the negative plate,
so VA-VB 21,000 V. Once outside the capacitor,
the proton travels at a constant velocity until
it enters a region of magnetic field of
magnitude 0.10 T. Find a) the speed of the
proton as it leaves the negative plate of the
capacitor, and b) the radius r, of the circular
path on which the proton moves in the magnetic
field. (p. 636)
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Motion of a Particle in a Bubble Chamber
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The Mass Spectrometer
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The Force on a Current in a Magnetic Field
The magnetic force on a current carrying wire of
length L
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Magnetohydrodynamic