AP Physics B Ch 16 Review PPT

8/9/2019 AP Physics B Ch 16 Review PPT

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What are the electrons doing?


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Negatively charged objects

have gained excesselectrons

Positively charged objectshave lost electrons have

excess positive

elementary charge isquantized e = 1.6 x 10 -19 C

proton q= + 1.6 x 10 19 C

electron q= 1.6 x 10 19 C


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16.3 Insulators and Conductors

Conductor:

Charge flows freely

Metals

Insulator:

Almost no charge flows

rubber, plastic, wood,

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T he Electroscope

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G roundingNeutralizing electric charge on an object by providing apath for excess charge to be transferred to EarthElectrical equilibrium reached by: excess electrons leaving to ground lack of electrons being replenished by ground touching charged object with hand touching it to plumbing fixture

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Not the

correct wayto ground


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2. Conduction touching a charged object

to another to transfer electrons betweenobjects

Charge distributes across a symmetrical object uniformly

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3. Inductionno contact between charged object and the objectacquiring chargecharged object repels like charges out of object toground or another objectresults in charged object with opposite sign

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R edistribution of charge

An electrically charged object attracts aneutral object

Neutral object

attraction only

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Coulombs Law of Electrostatic Force

Inverse square law similar to gravitational force lawForce = vector quantityMagnitude of force same on both charges

Direction:

use magnitude of charge NOT sign in your force calculations attraction or repulsion based on signs draw vector diagrams!

k = proportionality constant 9 x 10 9Nm 2/C 2

1 2

2

kq q F

r !

r = separation distance between charges

must be in meters

q1 q2 must be in coulombs not QC

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Draw vector diagrams to determine net force


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Net Force = Vector SumExample 4 draw vector diagrams using force law for vector directions

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In which region can a + 4q charge be in equilibrium?

+3q - 1q

1 2 3


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16.5 Coulombs LawCoulombs law strictly applies only to point charges.

Superposition: for multiple point charges, the forceson each charge from every other charge can becalculated and then added as vectors.

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Electric Field

Fields exert forces on objects put in them

Earth

G ravity Field

Weight

Electric Field

Attraction force only

charged

object

q

Force

Force

Attraction or repulsion force

Fields are a property of the space around the chargedobject that creates them

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Test for presence of electric fieldPlace a small positive test

charge in field

ratio of force on charge toamount of test charge = fieldstrength

direction of field is directionof force on + test charge

using a charge changes

force not field direction

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Electrons move opposite to field

Force on a charge placed in the field

F qE !r r

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Electric Field Strength

Independent of any charge placed in fieldto test for field strengthA larger charge placed in field experiences

a greater forceproperty of the space around the objectthat creates the field

vector quantity points in the directionthat a P OSITIVE test charge moves infieldUnits:

( N )( C )

N e w t o n s

C o u l o m b

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Electric field strength at P depends on

1) amount of chargeon the objectcreating the field

2) distance r that point P isaway from the object creatingthe field


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Field strength due to a point charge

2

2

kQqkQr E

q r

! !Q q

Q is charge creating theelectric field

r

Superposition principle applies when finding resultant electricfield due to several charges

Draw diagram with E vectors based on + test charge

Be careful for +/ signs of charge Q

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Electric Field Lines

Electric field lines point AWAY from a positivelycharged object

Point charges create NON -UNIFOR M electric field

varies in both magnitude and direction

property of pointin space

field existswhether testcharge in in fieldor not

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Electric Field Lines: Imaginary Map Lines

Electric field lines point I N towards a negatively chargedobject

Lines point in direction asmall positive test chargewould move if placed in thefield

Strength of field indicatedby number and spacing of field lines

strong field

weak field+3C -1QC

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Electric Field = R esultant of all field lines present

point away from +and in toward

Electric field is

tangent to field line atany point

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Field Lines

direction + chargemoves if placed infield

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Net electric field at point P

you will need to be able to identify direction of electric field

draw an imaginary + at point P and determine which way itwould move as a result of the field from A or from B

Electric field is NON -uniform surrounding point charges

+r A

Q A

r B

Q B

2

A

A

A

k r

!

2

B B

B

kQ E r

!

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Net field = vector sum4 field vectors present at C. E 3 cancels

E1 E 4 and E 2 sum up to yield E 24

(b) 4 field vectors present

E1 and E 3 sum up to yield E 13E2 and E 4 sum up to yield E 24

E13 and E 24 sum up to yield E pointing straightup:

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Electric Field between oppositelycharged parallel plates

Electric field

points from + plate to plate

is uniform at all pointsbetween the plates

E has same magnitude at

all pointsField lines are parallel

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16.9 Electric Fields and Conductors

T he static electric field inside a conductor iszero if it were not, the charges would move.

T he net charge on a conductor is on itssurface.

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16.9 Electric Fields and Conductors

T he electric field isperpendicular to thesurface of a conductor again, if it were not,

charges would move.

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Faraday cage

charge goes to outside of metal placed in electric field

high power line worker


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Motion in an electric field

When a charged particle enters an electric field, it experiences a force: E q F

TT

!

m

E qaam E q

T

TTT

!!

Does an electron or a proton experience a greater acceleration whenplaced in a uniform electric field

proton is 10,000 times more

massive than electronsame force on both

electron experiences greater acceleration

+/plate?

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AP Physics B Ch 16 Review PPT