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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:
x components cancel, y components add28
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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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