Physics 7C SS1 Lecture 8: Electricity

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Physics 7C SS1Lecture 8 Electricity Magnetism

• Magnetism RHR 1 2
• Light as EM wave
• Polarizers

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Whats left?

• 2 Lectures
• 1 Quiz
• 4-5 DL
• Optional review lecture
• A 1hr15min final exam

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Field Model of Magnetism

• A source moving charge creates a magnetic fields
  in a direction given by RHR1.
• Another moving charge, placed in a magnetic
  field, experiences a magnetic force
• Magnitude given by FqvBsin?
• Direction of force given by RHR2

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Experiments with magnets and compasses

• Magnets have poles that repel and attract

N
N
S
S
N
S
S
N
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Experiments with magnets and compasses

• Magnets attract one end of a compass needle and
  repel the other.

S
N
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Iron fillings align with the magnetic field.
Dipole Field
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Cutting a magnet

• Slicing a magnet in half results in two weaker
  magnets, each with a north and south pole.

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What happens with a compass is brought near a
charged rod?

• South pole is attracted to rod
• North pole is attracted to rod
• Neither pole is attracted to rod

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What happens with a compass is brought near a
charged rod?

• South pole is attracted to rod
• North pole is attracted to rod
• Neither pole is attracted to rod

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Which rule is used to find the magnetic field at
point A?

• RHR1
• RHR2
• LHR1
• You can use either hand--just be sure to point
  your thumb in the direction of the current and
  curl your fingers to find B!

A
I
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Which direction is the magnetic field at point A?

• Left
• Right
• Up
• Down
• Into screen
• Out of screen
• Away from wire
• Toward wire
• Something else

A
I
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Which direction is the magnetic field at point A?

• Left
• Right
• Up
• Down
• Into screen
• Out of screen
• Away from wire
• Toward wire
• Something else

A
I
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Which direction is the magnetic field at point B?

• Left
• Right
• Up
• Down
• Into screen
• Out of screen
• Away from wire
• Toward wire
• Something else

B
I
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Which direction is the magnetic field at point C?

• Left
• Right
• Up
• Down
• Into screen
• Out of screen
• Away from wire
• Toward wire
• Something else

C
I1
I2
You may assume I1I2
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Which direction is the magnetic field at point C?
Btot

• Left
• Right
• Up
• Down
• Into screen
• Out of screen
• Away from wire
• Toward wire
• Something else

C
I1
I2
You may assume I1I2
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Magnetic Force

• Suppose a large magnetic field points downward at
  every point in the room. What direction is the
  force on a positive particle traveling along the
  chalkboards, to your left?

• Into the board
• Out of the board
• Left (along particle path)
• Right (opposite path)
• Down
• Up
• No Force

v
B
q
F qvBsinq, where q is the angle between B and v
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Magnetic Force

• Suppose a large magnetic field points downward at
  every point in the room. What direction is the
  force on a positive particle traveling out of the
  board, to the back of the room?

• Into the board
• Out of the board
• Left
• Right
• Down
• Up
• No Force

v
B
q
F qvBsinq, where q is the angle between B and v
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Magnetic Force

• Suppose a large magnetic field points downward at
  every point in the room. What direction is the
  force on a positive particle traveling upward,
  toward the ceiling?

• Into the board
• Out of the board
• Left
• Right
• Down
• Up
• No Force

v
B
q
F qvBsinq, where q is the angle between B and v
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Magnetic Force

• Suppose a large magnetic field points downward at
  every point in the room. What direction is the
  force on a positive particle traveling upward,
  toward the ceiling?

• Into the board
• Out of the board
• Left
• Right
• Down
• Up
• No Force

v
B
q
F qvBsinq, where q is the angle between B and v
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Using both RHRs

• Look at the demonstration. Notice how the wires
  are bending. If you know the current in the left
  wire flows upward, which way is the current in
  the right wire?
• Up
• Down
• Either
• No current

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Switching Gears Rethinking Light

• What waves in light?
• What propagates?

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Image from http//www.monos.leidenuniv.nl/smo/inde
x.html?basics/light.htm
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A vertical wave traveling through a vertical
fence passes unimpeded. The second fence also
lets the wave pass.
If we place the second fence with horizontal
slats, the vertical vibrations cannot pass
through the fence.
Image from http//www.glenbrook.k12.il.us/GBSSCI/P
HYS/CLASS/light/u12l1e.html
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Image from http//www.lbl.gov/MicroWorlds/teachers
/polarization.pdf
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