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8/9/2019 AP Physics B Ch 21 Review PPT
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8/9/2019 AP Physics B Ch 21 Review PPT
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When the magnetic field passing through a conducting loop ischanging with respect to time an emf (E= potentialdifference), is induced in the loop causing induced current toflow through the loop.
Electromagnetic Induction
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2 ways to change B field through loop
1) change the size of the area that the B field is penetrating
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2 ways to change B field through loop
2) change the number of field lines or the intensity of the B
field
y yB
y y
y y
t = 0
t = 2 sy y y yBy y y y
y y y y
B field is increasing out of the page
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Magnetic Flux *B
product of B field and area measure of how much field is passing through the loop
perpendicular to surface area
( cos )B
B AJ* ! y
B(cosJ) is component of field that
is perpendicular to area A
Units are Webers (Wb)
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Magnetic Flux *B
Flux is proportional to how many field
lines pass in a perpendicular direction
through an area
max flux reduced flux zero flux
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Flux Change Induces emfE
constant flux
E = 0
flux increasing
induced current flows one
way
flux decreasing
induced current flows
opposite direction
solenoid, magnet demo
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8demo
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Faradays Law of Electromagnetic Induction
BN
t
I(*
!
(
emf induced
potential differencein Volts
number of
loops
rate of change of
magnetic flux with
time (Wb/s)
( )sign indicates that induced current opposes the change in
flux = Lenzs Law
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Many ways to change flux through a loop
change B intensity
change current in
solenoidpull loop into/out of B
slide a bar across a rail system
causing loop to increase in sizegenerator: rotating
coil in fixed B field
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21.3 EMF Induced in a Moving Conductor
This image shows another way the magneticflux can change:
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21.3 EMF Induced in a Moving Conductor
The induced current is in a direction that tendsto slow the moving bar it will take an external
force to keep it moving.
v +12
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21.3 EMF Induced in a Moving Conductor
The induced emf has magnitude
(21-3)
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Lenzs Law
Current induced by changing flux flows insuch a direction tooppose the change thatcaused it
Faradays Law states that
changing flux induces current
Lenzs Law states the
principle that allows you to
determine the induced
current direction
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Lenzs Law
1) emf in coilinducedt
(*
(
2) induced emf induced current
3) induced current induced B field
4) induced B field opposes
the incoming North pole
2
3
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16demo
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Applying Lenzs Law
1. Determinedirection of changing B field
and whether it is increasing or
decreasing
2. Draw direction of induced B field to
oppose this change
3. Use RHR topredict the induced current
direction that will yield this induced B
field
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Lenzs Law
X X X X
X X X X
X X X X
B increasing
intopage
X X
X X
B decreasing
intopage
y y y y y
y y y y y
y y y y
B increasing
out ofpage
y y
y y
y y
B decreasing
out ofpage
identify direction of changing B, Binduced points opposite
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Lenzs Law
flux is decreasing into
thepage
induced current flows
clockwise to create induced
B field intopage tooffset that
change
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Lenzs Law Practice Example 4
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Lenzs Law Practice
Predict direction of induced current in the loop page 589
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Lenzs Law Practice
Predict direction of induced current in loop and through the
resistor R