Transcript Welcome to Physics 7C

Physics 7C Fa 2008
Lecture 8: Electricity &
Magnetism
Magnetism: RHR 1 & 2
Light as EM wave
Polarizers
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 F=qvBsin
Direction of force given by
RHR2
B

v
F
2
Which direction is the magnetic
field at the following points?
Where is it biggest?
1)
2)
3)
4)
5)
6)
7)
8)
9)
Left
Right
Up
Down
Into screen
Out of screen
Away from wire
Toward wire
Something else
A
C
B
D
I
Answers added as slides at end of lecture.
3
Which direction is the magnetic
field at the following points?
Where is it biggest?
1)
2)
3)
4)
5)
6)
7)
8)
9)
Left
Right
Up
Down
Into screen
Out of screen
Away from wire
Toward wire
Something else
W
I
X
Y
Z
Answers added as slides at end of lecture.
4
Which direction is the
magnetic field at point C?
1)
2)
3)
4)
5)
6)
7)
8)
9)
Left
Right
Up
Down
Into screen
Out of screen
Away from wire
Toward wire
Something else
C
I1
I2
You may assume I1=I2
5
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
B

v
3) Left (along particle path)
4) Right (opposite path)
5) Down
6) Up
7) No Force
F = qvBsin,
where  is the
angle between B
and v
6
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? 1) Into the board
2) Out of the board
B

v
3) Left
4) Right
5) Down
6) Up
7) No Force
F = qvBsin,
where  is the
angle between B
and v
7
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?
1) Into the board
2) Out of the board
B

v
3) Left
4) Right
5) Down
6) Up
7) No Force
F = qvBsin,
where  is the
angle between B
and v
8
Magnetic Induction

Magnetic Flux: the “amount of B-field through
an area”



Stronger B-field means more field passes through
Larger area means more field passes through
Different orientations permit more field to pass
through
How should B-field be
oriented for maximum
magnetic field to pass
through the loop?
9
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 t=0, loop is
outside the field. Our goal:
1) What happens as the loop
enters the magnetic field?
2) What happens while the
loops moves within B.
3) What happens as the loop
exits the magnetic field?
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4) Connecting to chaning fields.
Applying RHR2:

0) Before we enter the field:
Describe the force at the instant
shown on positive charges in
the blue wire:
1) Left
2) Right
3) Up
Why?
4) Down
5) Into Screen
6) Out of screen
7) Zero
8) Other
11
Applying RHR2:

1) Entering the field:
Describe the force at the instant
shown on positive charges in
the blue wire:
1) Left
2) Right
3) Up
Why?
4) Down
5) Into Screen
6) Out of screen
7) Zero
8) Other
12
Applying RHR2:

1) Entering the field:
Repeat for red, top and
bottom wires:
1) Left
2) Right
3) Up
Why?
4) Down
5) Into Screen
6) Out of screen
7) Zero
8) Other
13
Applying RHR2:

1) Entering the field:
Draw the current the results
from the forces we just
describes as loop enters
field.
Draw the magnetic field from
the induced current.
Would this analysis change if I
had asked for the forces on
the electrons?
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Applying RHR2:

2) Within the field:
Describe the force at the instant
shown on positive charges in
the blue wire:
1) Left
2) Right
3) Up
Why?
4) Down
5) Into Screen
6) Out of screen
7) Zero
8) Other
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Applying RHR2:

2) Within the field:
Repeat for red, top and bottom
wires:
1) Left
2) Right
3) Up
Why?
4) Down
5) Into Screen
6) Out of screen
7) Zero
8) Other
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Applying RHR2:

2) Within the field:
Draw the current the results
from the forces we just
describes as loop moves
within field.
Draw the magnetic field from
the induced current.
Would this analysis change if I
had asked for the forces on
the electrons?
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Applying RHR2:

3) Leaving the field:
Describe the force at the instant
shown on positive charges in
the blue wire:
1) Left
2) Right
3) Up
4) Down
5) Into Screen
6) Out of screen
7) Zero
8) Other
18
Applying RHR2:

3) Leaving the field:
Repeat for red, top and bottom
wires:
1) Left
2) Right
3) Up
4) Down
5) Into Screen
6) Out of screen
7) Zero
8) Other
19
Applying RHR2:

3) Leaving the field:
Draw the current the results
from the forces we just
describes as loop leaves
field.
Draw the magnetic field from
the induced current.
Would this analysis change if I
had asked for the forces on
the electrons?
20
Connecting to changes in
amount of field through loop

In which of the previous times was the
amount of field passing through the loop
changing?
a)
b)
c)
d)
Before entering field
While entering field
Within field
Leaving field
In which cases was a current induced?
21
Changing fields induces a
current that creates a field

Induced current makes a field opposite to
the change in amount of field through loop:
1) Entering field
ti
tf
3) Leaving field
ti
Iind
No field
Ext field into page
Induced field out of page
tf
Iind
Ext field into page
No field
Induced field out of page 22
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)
23
Switching Gears: Rethinking
Light

What “waves” in light?

What propagates?
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25
Image from http://www.monos.leidenuniv.nl/smo/index.html?basics/light.htm
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.
26
Image from http://www.glenbrook.k12.il.us/GBSSCI/PHYS/CLASS/light/u12l1e.html
27
Image from http://www.lbl.gov/MicroWorlds/teachers/polarization.pdf
28
Which direction is the magnetic
field at the following points?
Where is it biggest?
1)
2)
3)
4)
5)
6)
7)
8)
9)
Left
Right
BC>BD>BA>BB
Up
Down
Into screen
Out of screen
Away from wire
Toward wire
Something else
A
C
B
D
I
29
Which direction is the magnetic
field at the following points?
Where is it biggest?
1)
2)
3)
4)
5)
6)
7)
8)
9)
Left
Right
Up
BW=BX>BY>BZ
Down
Into screen
Out of screen
Away from wire
Toward wire
Something else
W
I
X
Y
Z
30
Which direction is the
magnetic field at point C?
1)
2)
3)
4)
5)
6)
7)
8)
9)
Left
Right
Up
Down
Into screen
Out of screen
Away from wire
Toward wire
Something else
Bnet
I1
B1
C
B2
I2
You may assume I1=I2
31
Force on particles in uniform B
v
v
F
F
B
B
Movement out of page,
Force right
Movement left,
Force out of page
v
v up, B down, so angle is 180º,
and there is no force.
B
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