AP PHYSICS LARGE GROUP

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AP PHYSICS LARGE GROUP

• CYCLE 16
• THURS., FEB 2, 2006
• Electric Fields, Potential

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Website Link
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HW 16 Electrostatics
AP EXAM ELECTRICITY Day 2 Fri, Feb 17
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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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HW 16 16 - 28
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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 of charged object
• 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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-
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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 per unit of charge
DV Energy 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


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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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E map Parallel Plates
EQUIPOTENTIALS
6V

CONNECT POINTS OF EQUAL POTENTIAL
ELECTRICAL ELEVATION
-
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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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Gravitational Force vs Distance
Area F d
work
Work DUg
Fg
INTEGRATION
distance
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Absolute Gravitational Potential Energy Ug
Ug -Gm1m2
d
Ug always negative
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Ug of Earth - mass System
Ug -Gm1m2
d
Ug
d
RE
Ugo - GMem
Re
Ugo
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Ug of system of masses
m1
m3
m2
total Ug SUg
-U12 -U13 -U23
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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