ap physics b ch 21 review ppt

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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The induced current is in a direction that tendsto slow the moving bar it will take an external

force to keep it moving.

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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

ap physics b ch 21 review ppt

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