Physics 7C SS1, Lecture 9: Field Model

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Physics 7C SS1, Lecture 9Field Model EM Waves

• Magnetic Field
• Electromagnetic Waves
• Polarization

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Final Exam Review Sessions

• Definitely some on Monday. Which other day would
  you like most?
• Thursday
• Friday
• Saturday
• Sunday

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Magnetic Force
F out of the screen
v
RHR2 (for positive charge) your thumb points in
the direction of the moving charge, B is along
your index finger, and F is the middle finger.
Very Bad Finger
B
v
B
q
F
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 along the
  chalkboards, to your left?

1. Into the board
2. Out of the board
3. Left (along particle path)
4. Right (opposite path)
5. Down
6. Up
7. No Force

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Magnetic Force

• Suppose a large magnetic field points downward at
  every point in the room. Which direction should
  the particle be traveling in to experience a
  force to the left?

1. Into the board
2. Out of the board
3. Left
4. Right
5. Down
6. Up
7. No Force

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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 upwards,
  toward the ceiling?

1. Into the board
2. Out of the board
3. Left (along particle path)
4. Right (opposite path)
5. Down
6. Up
7. No Force

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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
• Reverse direction for negative test charge

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Inducing current

• Imagine a region with a magnetic field away from
  you in some regions (into the screen) and zero in
  other regions, as shown below.

• Right wire is blue wire. Left wire is red wire.
  At t0, loop is outside the field. Our goal
• What happens as the loop enters the magnetic
  field?
• What happens while the loops moves within B.
• What happens as the loop exits the magnetic
  field?
• Connecting to chaning fields.

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Applying RHR2t0, before entering the field

• Describe the force at the instant shown on
  positive charges in the blue wire
• Left
• Right
• Up
• Down
• Into Screen
• Out of screen
• Zero
• Other

Why?
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Applying RHR2As entering the field

• Describe the force at the instant shown on
  positive charges in the blue wire
• Left
• Right
• Up
• Down
• Into Screen
• Out of screen
• Zero
• Other

Why?
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Applying RHR2As entering the field

• Repeat for charges in the top, bottom, and red
  wire
• Left
• Right
• Up
• Down
• Into Screen
• Out of screen
• Zero
• Other

Why?
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Applying RHR2As entering the field

• Draw the current that results from the forces we
  just describes as loop enters field.
• Draw the magnetic field from the induced current.
  (focus on the inside the loop)
• Would this analysis change if I had asked for the
  forces on the electrons?

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Applying RHR2Within the field

• Describe the force at the instant shown on
  positive charges in the blue wire
• Left
• Right
• Up
• Down
• Into Screen
• Out of screen
• Zero
• Other

Why?
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Applying RHR2Leaving the field

• Describe the force at the instant shown on
  positive charges in the blue wire
• Left
• Right
• Up
• Down
• Into Screen
• Out of screen
• Zero
• Other

Why?
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Applying RHR2As leaving the field

• Draw the current that results from the forces we
  just describes as loop leaves field.
• Draw the magnetic field from the induced current.
  (focus on the inside the loop)
• Would this analysis change if I had asked for the
  forces on the electrons?

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A new way to analyze situations with changing B

• Magnetic Flux the amount of B-field through an
  area
• Depends on
• B strength of B-field
• A area bound by conductor
• ? orientation of loop with respect to B-field.

?
How should B-field be oriented for maximum
magnetic field to pass through the loop?
B
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Applying Magnetic Flux

• In which of the previous times was the amount of
  field passing through the loop changing?
• Before entering field
• While entering field
• Within field
• Leaving field

When was there an induced current?
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A changing B-field induces a current that creates
another B-field

• Induced current makes a field opposite to the
  change in amount of field through loop

1) Entering field
3) Leaving field
ti
tf
ti
tf
Iind
Iind
No field
Ext field into page
No field
Ext field into page
Induced field out of page
Induced field out of page
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Consequences of changing magnetic fields

• Anything that changes the flux through a
  conductor causes a current to flow in the
  conductor.
• Before cause of current flow is a voltage
  difference (like from a battery).
• New model changing magnetic flux induces voltage
  differences (which cause induced currents and
  induced magnetic fields)

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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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What happens in each of the following cases? (use
RHR2)
(a) Wire carrying current is placed in a B-field
N
S
X
(b) A wire (without current) is moved up and down
as shown
N
S
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What happens in each of the following cases? (use
RHR2)
(c) A loop of wire (no current) is turned within
a magnetic field
N
S
(b) A loop of wire carrying current is placed in
a B-field
N
S
I
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What happens in each of the following cases? (use
?B)
(c) A loop of wire (no current) is turned within
a magnetic field
N
S