Magnetic Forces

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Magnetic Forces
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Magnetism is Goofy
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Cathode Ray Tube
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What direction will the beam be pushed?
What direction is the magnetic field (at the
location of the beam)?
What direction is the force on the beam?
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For a charged particle to experiencea force due
to a magnetic field

• a) the charge must be moving, and
• b) the velocity of the moving charge must have a
  component that is perpendicular to the direction
  of the magnetic field.
• NOTE the direction of the force is always
  perpendicular to both the velocity vector and the
  magnetic field B.

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Right Hand Rule

• Fingers in the direction of v
• Wave into B (might have to turn wrist)
• Thumb in direction of force on positive charge

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Magnetic force

• F qvperpB

What direction?
vperp
vsin?
F qvBsin?
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What is the force on the electron (mag. and dir.)?
What direction?
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velec1x107 m/s
B 5x10-5 T
F qvBsin? F (1.6x10-19 C)(1x107 m/s)(5x10-5
T)sin(70) F 7.5x10-17 N
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What direction is the force?
proton
v
B
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What direction is the force?
charge
B
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Jumping Wire
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The force will point in what direction?
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Another Use for a Pepsi Cup
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Sources of Magnetic Field

• A moving charge reacts to a magnetic field.
• Can a charge create a magnetic field?
• Oersteds Experiment.

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Magnetic Fields

• Due to Wires

Side View
I
End View
I
Rule twice as far, half as strong.
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Do they attract or repel each other?
Parallel Wires, Both Currents Up
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1
Find B at 1 due to 2 Force on 1
B at 2 due to 1 Force on 2
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Do they attract or repel each other?
Parallel Wires, Opposite Currents
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1
Find B at 1 due to 2 Force on 1
B at 2 due to 1 Force on 2
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Parallel Wires