Electrostatics

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Electrostatics

• The study of stationary charges
• elementary charges
• proton ()
• electron (-)
• Units of Charge
• Coulomb (C)
• 1 C 6.25 x1018 elem ch (e)
• 1 elem charge (e) 1.60 x10-19 C

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Charges of Objects
Conservation of Charge

• All objects carry charges
• Balance of e and p determines net charge

Negative
Neutral
Positive
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Electroscope

• Charge Detector

Neutral
Charged
More charge
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Charge Indicator
Even distribution
Uneven distribution
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Charge Indicator
Even distribution charged
Polarized charged
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Charging Methods

• Conduction contact

Charge Transfer
Charged
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Charging Methods

• Induction indirect

ground
Charge Escapes
Charged
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Electrophorous

• Charging Device

insulated handle
metal disk
take ground away
insulator
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Coulombs Law

• Force between Charges

Q2
Q1
Fe1 -Fe2
Fe2
Fe1
Fe a Q1
Fe a Q2
Fe a 1/r 2
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Coulombs Law

• Force between unlike Charges

Fe1 -Fe2
Q2
-Q1
Fe2
Fe1
Fe a Q1
Fe a Q2
Fe a 1/r 2
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Coulombs Law

• Force between Charges

k
Fe Q1 Q2
r 2
k electrostatic constant
k 9.0 x109 Nm2/C2
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Electric vs Gravitational

• Force Comparison

k electrostatic constant
G gravitational constant
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Wymhurst Generator
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Van de Graaff Generator
Filmloop Van de Graaff
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Regents PhysicsHW 22 Electrostatics
REGENTS PHYSICS NAME _____________________ MR.
WHITMAN DATE _____________________

• READ CHAPTER 20
• PROBLEMS
• 20,21,23,24,25,26,29,30,31,34
• Due Friday, March 24

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HW 22 20
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HW 22 21
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HW 22 23
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HW 22 24
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HW 22 25
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HW 22 26
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HW 22 29
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HW 22 30
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HW 22 31
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HW 22 34
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Gravitational Field (g)

• Point Mass

g
P
r
M
g in direction of force on a mass at point P
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Electric Field (E)

• Positive Point Charge

P
E
Q
r
E in direction of force on a positive charge
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Electric vs Gravitational

• Field Comparison

Fe /q
Fg / m
g
g force / mass (N/kg)
E force / charge (N/C)
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Electric Field (E)

• Negative Point Charge

E
P
-Q
r
E in direction of force on a positive charge
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Electric Field LinesRULES

• lines begin at the and end at the - object
• lines perpendicular to surface of charged object
• Direction of lines same as proton would go
• lines never cross
• concentration of lines a field strength
• all charge is on surface
• no field lines inside a conductor

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Electric Field Map

• Positive Point Charge

field lines in direction of force
on positive charge
Q
(See IP sim)
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Electric Field Map

• Negative Sphere

-Q
(See IP sim)
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-
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Electric Field Map
-

(See IP sim)
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Electric Field Map


(See IP sim)
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Electric Field Map

• Parallel Plates

Uniform Field
net force same
- - - - - - - - - - - - -
-
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Electric Field Map

• Lines of Force

Uniform Field
- - - - - - - - - - - - -
-
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Electric vs Gravitational

• Potential Energy Comparison

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Potential Difference

• Voltage (DV)

DV Energy Charge
DV energy per unit of charge
Volt Joule / Coulomb
Energy DV (Charge) DV q
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Electrical Potential Energy
PEe DVq
1 C
(1 J)
PEe (1V)(1C)
1V
e elementary charge charge of proton
1 e
PEe (1V)(1e)
(1 eV)
1V
eV electron volt
1 eV 1.6 x10-19 Joules
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Potential Difference

• Voltage (DV)

DV Energy Charge
q
DV E Dd
E is constant
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A Different Look atElectric Fields
DV
E
Dd
Electric Field Gradient (V/m)
Electrical topography
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Electric Field

• Two ways to think about it

DV /Dd
Fe /q
(V/m)
(N/C)



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Gravitational Field

• Two ways to think about it

accel.
Fe /m
(m/s2)
(N/kg)


kg
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Topography Contour Maps
Contour Line Conects points of equal elevation

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Electric Field Map

• Equipotentials (Contour Lines)

Q
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Topography




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Electric Field Map

• Equipotential Lines

Topographic Lines
Connect points of voltage
10 V


8V
6V
4V
2V
- - - - - - - - - - - - -
-
0 V
(See IP sim)
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C.A. 17

• Electric Field Mapping

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E map Parallel Plates

-
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E map Parallel Plates

-
Calculate E (V/m)
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E map Parallel Plates

-
Calculate impact V of proton electron
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E map Parallel Plates
EQUIPOTENTIALS
6V

CONNECT POINTS OF EQUAL POTENTIAL
ELECTRICAL ELEVATION
-
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E Field Mapping
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E Map 2 pt charges
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Electric Field Map
-

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Van de Graaff Generator
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Electric Field Map


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Millikan Oil Drop Experiment

• Quantify the charge of electron

a 0
v 0
net force 0
F e
F e F g
oil drop
F g
DV
E
Dd
- - - - - - - - - - - - -
-
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Millikan Oil Drop
F e F g
E q mg
q mgd /DV
(unit check)
C kg N m
J

C
kg V
J/C
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Millikan Oil Drop Data
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Millikan Oil Drop Data
electrons
Q min 1.6 x 10-19C
Q
whole
1.6 x 10-19
1 e 1.6 x 10-19C
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Millikan Sample Question

• Given DV, d, Weight of drop
• Find E, Fe, q drop, e on drop

E F/q
E DV/d
F e
-
oil drop

E 3500V/m
F g 2.8x10-14 N
E 3500N/C
- - - - - - - - - - - - -
-
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Millikan Sample Question

• continued

Fe F g 2.8x10-14N
Eq mg
q mg
(2.8x10-14N)(.1m) 350 V

(8x10-19C)

)
(
1 elem ch
e
(8x10-19C)
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(1.6x10-19C)
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Sample Question

• Given DV, d, mass charge of electron
• Find Fe, accel of electron
• Mass 9.1 e-31kg, charge 1.6 e-19C

Fe gtgtgtgtFg
E 2000 V/m
Fe 3.2 e-16N
Fe Eq
-
electron

a F/m
F g 9.1 e-30N
F ma
a 3.5 e 14 m/s
- - - - - - - - - - - - -
-
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Absolute Electrical Potential (V)
r
P
Q
V /- kQ
r
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Absolute Electrical Potential
Q1
Q2
P
r1
r2
r3
V /- kQ
r
Q3
Vp V1 V2 V3
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Electric vs Gravitational

• Absolute Potential Energy Comparison

k
Ug - M1 M2
G
Ue /- Q1 Q2
r
r
Ug always negative
Ue positive or negative
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