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Negative
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Voltage
Permission to use is granted for�in-class direct instruction ONLY.
https://www.youtube.com/c/physicsworks
ONE COULOMB of Charge (+ or -)�� 6.24 x 1018 charges�6,240,000,000,000,000,000 charges
ONE JOULE of Energy
�The ability to do ONE JOULE of Work.�Ex: Lifting a 1 N weight up 1 m.
6.24 x 1018 e-/C
1.60 x 10-19 C/e-
Electrical Potential
Electrical Potential�ENERGY
versus
http://www.aviationpics.de/prev/crane%203.jpg
2,000 N
15 meters
Work = Force · Distance
= 2,000 N · 15 m
= 30,000 J
Work = ΔEnergy
= ΔUg= 30,000 J
GRAVITATIONAL�Potential Energy
Separating�Attractive Bodies
2,000 N
(Joules)
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ELECTRICAL�Potential Energy
Separating�Attractive Bodies
(Joules)
ELECTRICAL�Potential Energy
Bringing Together�Repulsive Bodies
(Joules)
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ELECTRICAL�Potential Energy
Bringing Together�Repulsive Bodies
AND
Separating�Attractive Bodies
20 J/kg
10 J/kg
60 Joules total
30 Joules total
Gravitational Potential Energy
Gravitational Potential Energy
Gravitational Potential
Gravitational Potential
1 m
2 m
1 kg (10 N) each
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20 J/C
10 J/C
60 Joules total
30 Joules total
Electrical Potential Energy
Electrical Potential Energy
Electrical Potential
Electrical Potential
1 Coulomb each
Alessandro Volta
Negative
Positive
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9V
1,800 J
200 C
6V
96,000 J
16,000 C
3,600 J
2,400 C
1.5V
AAA
AP Physics 2 Content
SIGN CONVENTIONS
Positive Work done by an external force increases the Potential Energy.
d
F
E
Positive Work done by the field�decreases the Potential Energy.
d
F
E
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Electric Field�weakens with distance.
Electric Field�UNIFORM
Two Unique Situations:
Electric Field�weakens with distance.
Electric Field�UNIFORM
Two unique situations:
Relative Gravitational Potential Energy
http://www.edinburgschools.net/jh/junior%20high%20%20building/jh_building.htm
RELATIVE Gravitational Potential
ABSOLUTE Gravitational Potential
Find the WORK done moving an object from 1.0 m to 4.0 m.
Distance (m)
Force (N)
Distance (m)
Force (N)
Find the WORK done moving an object from 1.0 m to 4.0 m.
Distance (m)
Force (N)
.0625
Find the WORK done moving an object from 1.0 m to 4.0 m.
Absolute Potential Energy
zero
-2,000 Joules
-8,000 Joules
ΔUG = Final - Initial
ΔUG = -8,000 J – (-2,000 J)
ΔUG = -6,000 Joules
Object LOST 6,000 Joules
ΔUG = Final - Initial
ΔUG = -2,000 J – (-8,000 J)
ΔUG = +6,000 Joules
Object GAINED 6,000 Joules
-2,000 Joules
-8,000 Joules
ΔUE = Final - Initial
ΔUE = -8,000 J – (-2,000 J)
ΔUE = -6,000 Joules
Object LOST 6,000 Joules
ΔUE = Final - Initial
ΔUE = -2,000 J – (-8,000 J)
ΔUE = +6,000 Joules
Object GAINED 6,000 Joules
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+2,000 Joules
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ΔUE = Final - Initial
ΔUE = +8,000 J – (+2,000 J)
ΔUE = +6,000 Joules
Object GAINED 6,000 Joules
ΔUE = Final - Initial
ΔUE = +2,000 J – (+8,000 J)
ΔUE = -6,000 Joules
Object LOST 6,000 Joules
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Electric Field�weakens with distance.
Electric Field�UNIFORM
Parallel Plates
VA
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VB
E
+q
F = qE
Consider Two parallel plates of equal and opposite charge, a distance d apart.
Constant E field: F = qE
Work = Fd = (qE)d
Also, Work = q(VA – VB)
So that: qVAB = qEd and
VAB = Ed
The potential difference between two oppositely charged parallel plates is the product of E and d.
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Electric Field�weakens with distance.
Electric Field�UNIFORM
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E =
F�q
V�r
=
q =
F·r�V
nq =
nmg·r�V
Work = Fd = (qE)d
Equal Elevation Lines
Equipotential�Lines
g - Field
(J/kg)
(N/kg)
(m/s2)
Equipotential�Lines
E - Field
(J/C)
(N/C)
(Volts)
Lake Tahoe�1,645 ft deep
Equipotential�Lines
g - Field
(J/kg)
(N/kg)
(m/s2)
Lake Tahoe�1,645 ft deep
Equipotential�Lines
g - Field
(J/kg)
(N/kg)
(m/s2)
9.0 Volts
9.0 Volts
9.0 Volts
0 V
9.0 V
4.5 V
? V
? V
? V
? V
1.5 V
7.5 V
3.0 V
6.0 V
http://son.nasa.gov/tass/images/electric_fields3.jpg
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