Physics 102: Lecture 3
Electric Potential Energy
& Electric Potential
Physics 102: Lecture 2, Slide 1
Overview for Today’s Lecture
• Electric Potential Energy & Work
– Uniform fields
– Point charges
• Electric Potential (like height)
– Uniform fields
– Point charges
Physics 102: Lecture 2, Slide 2
Recall Work from Phys 101
• Work done by the force given by:
–
–
–
–
W = F d cos(q)
Positive: Force is in direction moved
Negative: Force is opposite direction moved
Zero: Force is perpendicular to direction moved
• Careful ask WHAT is doing work!
– Opposite sign for work done by you!
• Conservative Forces
– D Potential Energy = -W
Physics 102: Lecture 2, Slide 3
Preflight 3.1
!!!!ACT!!!!
C
– A
Uniform E
In what direction does the force on a
negative charge at point A point?
1) left
2) right
3) up
Physics 102: Lecture 2, Slide 4
B
Preflight 3.2
motion
C
F - A
B
Uniform E
When a negative charge is moved from A to C
the ELECTRIC force does
1) positive work.
2) zero work.
3) negative work.
Physics 102: Lecture 2, Slide 5
Preflight 3.3
C
A
F -
Uniform E
B
motion
When a negative charge is moved from A to B
the ELECTRIC force does
1) positive work.
2) zero work.
3) negative work.
Physics 102: Lecture 2, Slide 6
ACT: Work
WA-B = work done by
FE moving charge from
A to B
C
A
F -
B
Uniform E
The negative charge is moved from A to C to B.
Is the work done by the electric force:
A) Greater than WA-B
B) Same as WA-B
C) Less than WA-B
Physics 102: Lecture 2, Slide 7
Work and D Potential Energy
W = F d cos(q)
Gravity
• Brick raised yi yf
• FG = mg (down)
• WG = –mgh
• DUG= +mgh
Electric
• Charge moved xi xf
• FE = qE
• WE = –qEd
• DUE= +qEd
(left)
yf 
xi

h
F -
yi 
FG=mg
Physics 102: Lecture 2, Slide 8
d
E
xf

E.P.E. for point charges
E.P.E. of two charges q1 and q2 separated a distance r:
𝑞1 𝑞2
𝑈𝐸 = 𝑘
𝑟
What is the electric potential energy of an electron a
distance r = 0.5310-10 m from a proton (H atom)?
rf = 0.510-10 m
+
Physics 102: Lecture 2, Slide 9
-
Work done by YOU
to assemble 3 charges
• W1 = 0
• W2 = k q1 q2 /r
=(9109)(110-6)(210-6)/5
=3.6 mJ
• W3 = k q1 q3/r + k q2 q3/r
(9109)(110-6)(310-6)/5 + (9109)(210-6)(310-6)/5 =16.2
•
•
•
Wtotal = +19.8 mJ
WE = –19.8 mJ
DUE = +19.8 mJ
(watch signs!)
Physics 102: Lecture 2, Slide 10
5m
1
3
5m
5m
2
mJ
ACT: Work done by YOU
to assemble 3 negative charges
How much work would it take YOU to assemble 3 negative
charges?
A) W = +19.8 mJ
B) W = 0 mJ
C) W = -19.8 mJ
Physics 102: Lecture 2, Slide 11
5m
1
3
5m
5m
2
Electric Potential (like height)
𝑉 ≡ 𝑈𝐸 /𝑞
• Units Joules/Coulomb  Volts
• Examples:
– Batteries
– Outlets
– EKG
• Only potential differences matter
Physics 102: Lecture 2, Slide 13
Electric Potential (like height)
Devil’s Tower
Topographical map
Moving to higher potential  moving uphill
Physics 102: Lecture 2, Slide 14
Demo: electric potential
Recall electric dipole
150
+
Equipotential lines
100
+
–
50
100
50
–
0
0
150
• + (–) charge has high (low) potential
Electric field
• Equipotential lines at same “height”
• Electric field lines point “downhill”
Physics 102: Lecture 2, Slide 15
Preflight 3.7
The electric potential at point A is _______ at point B
1) greater than
2) equal to
3) less than
Physics 102: Lecture 2, Slide 16
Preflight 3.9
conductor
The electric potential at point A is _______ at point B
1) greater than
2) equal to
3) less than
Physics 102: Lecture 2, Slide 17
Potential for Point charges
Electric potential a distance r from a charge q:
𝑉 ≡ 𝑈𝐸 /𝑞
𝑞
𝑉=𝑘
𝑟
What is the electric potential a distance r = 0.5310-10 m
from a proton? (Let V()=0)
V =UE/q= k q/ r = (9109)(1.610-19) /0.5310-10 = 27.2 Volts
rf = 0.510-10 m
+
Physics 102: Lecture 2, Slide 18
Two Charges
• Calculate electric potential at point A due to charges
– Calculate V from +7mC charge
– Calculate V from –3.5mC charge
– Add (EASY! NO VECTORS)
A
4m
V = kq/r
V7 = (9109)(710-6)/5 = 12.6103V
V3 = (9109)(-3.510-6)/5 = -6.3103V
Vtotal = V7+V3 = +6.3103V
Q=+7.0mC
How much work do you have to do to bring
a 2 mC charge from far away to point A?
Physics 102: Lecture 2, Slide 19
6m
Q=-3.5 mC
W=DU=Vq
= (+6.3103V)(2mC)
= +12.6 mJ
ACT: Two Charges
• In the region II (between the two charges) the
electric potential is
1) always positive
2) positive at some points, negative at others.
3) always negative
I
II
Q=+7.0mC
Physics 102: Lecture 2, Slide 20
III
Q=-3.5 mC
ACT: Electric Potential
+
E
A
C
B
The electric potential at A is ___________ the electric potential
at B.
1) greater than
2) equal to
3) less than
Physics 102: Lecture 2, Slide 21
Comparison:
Electric Potential Energy vs. Electric Potential
• Electric Potential Energy (U) - the energy of a
charge at some location.
• Electric Potential (V) - found for a location only –
tells what the EPE would be if a charge were
located there (usually talk about potential
differences between two locations):
U = Vq
• Neither has direction, just value.
Sign matters!
Physics 102: Lecture 2, Slide 22
Relationship between F, E, UE, V
Vector
Number (“scalar”)
F [N]
UE [J]
𝑞1 𝑞2
Ex: 𝐹 = 𝑘 2
𝑟
𝑞1 𝑞2
Ex: 𝑈𝐸 = 𝑘
𝑟
E [N/C]=[V/m]
V
𝐸 ≡ 𝐹/𝑞
𝑞
Ex: 𝐸 = 𝑘 2
𝑟
[J/C]=[V]
𝑉 ≡ 𝑈𝐸 /𝑞
𝑞
Ex: 𝑉 = 𝑘
𝑟
Why so many ways to describe electric force?
Physics 102: Lecture 2, Slide 23
Electron microscope
ΔV=10kV
Vi
-
Vf
Electron
gun
motion
Uniform E
• What is the final velocity of the e-?
• Solve by conservation of energy:
K.E.i + P.E.i = K.E.f + P.E.f
0 + –eVi = ½mv2 + –eVf
𝑣=
2𝑒∆𝑉
=
𝑚
2 1.6 × 10−19 (1 × 104 )
9.1 × 10−31
= 5.9 × 107 𝑚/𝑠
Physics 102: Lecture 2, Slide 24
Could solve this using F=ma &
kinematic equations (Phys 101)
TRY AT HOME! (HARDER)