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
Work
W = F d cos(q)
Gravity
• Brick raised yi yf
• FG = mg (down)
• WG = –mgh
• Wyou = mgh
Electric
• Charge moved xi xf
• FE = qE
• WE = –qEd
• Wyou = qEd
(left)
yf 
xi

h
F
W=0
yi 
FG=mg
Physics 102: Lecture 2, Slide 3
W=0
-
d
E
xf

CheckPoint 1.1
C
F – A
B
Uniform E
In what direction does the force on a
negative charge at point A point?
65% 1) left
32% 2) right
3% 3) up
Physics 102: Lecture 2, Slide 4
Electric field points in the direction a
POSITIVE charge would feel force.
CheckPoint 1.2
“Force applied
perpendicular to the
direction of motion brings
about no work”
motion
C
F - A
B
Uniform E
When a negative charge is moved from A to C
the ELECTRIC force does
20% 1) positive work.
70% 2) zero work.
10% 3) negative work.
Physics 102: Lecture 2, Slide 5
CheckPoint 1.3
“The work is negative
because the electric
force opposes the
direction of motion”
C
A
F -
Uniform E
B
motion
When a negative charge is moved from A to B
the ELECTRIC force does
48% 1) positive work.
17% 2) zero work.
35% 3) negative work.
Physics 102: Lecture 2, Slide 6
-WE field = +WYou
Electric force did negative work
You did positive work
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
Path does not matter!
Only end points matter
Work and D Potential Energy
WF = F d cos(q)=-DU
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

CheckPoint 1.5
“The movement of an electron from A to B
requires energy from an outside source.
The energy put in will be released when
the electron moves from B to A.”
C
A
F -
Uniform E
B
motion
When a negative charge is moved from A to B
the potential energy of the charge
-WE field = +WYou
Electric force did negative work
23% 2) remains the same. You did positive work
45% 1) increases.
32% 3) decreases.
Physics 102: Lecture 2, Slide 9
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)?
UE = (9 109)(+1.610-19)(-1.610-19)/0.5310-10 = -4.3510-18J
rf = 0.510-10 m
+
Physics 102: Lecture 2, Slide 10
-
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 11
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?
Likes repel, so YOU will still do positive
work!
A) W = +19.8 mJ
B) W = 0 mJ
C) W = -19.8 mJ
Physics 102: Lecture 2, Slide 12
5m
1
3
5m
5m
2
CheckPoint 2.1
1
5m
2
+
+
5m
5m
-
3
The total work required by you to assemble
this set of charges is:
57% (1)
positive
Bring in (1): zero work
14% (2)
zero
Bring in (2): positive work
28% (3)
Physics 102: Lecture 2, Slide 13
negative
Bring in (3): negative work x 2
Electric Potential
Electric potential
energy per charge
𝑉 ≡ 𝑈𝐸 /𝑞
• Units:
Joules/Coulomb  Volts
• Examples:
– Batteries
– EKG
• Only potential
differences matter
Physics 102: Lecture 2, Slide 14
1.5 J/C
more
energy!
+
+
Electric Potential: like height
Devil’s Tower
Topographical map
Moving to higher potential  moving uphill
Physics 102: Lecture 2, Slide 15
Demo: electric potential
Recall electric dipole
150
150
+
Equipotential lines
100
100
+
–
50
100
50
50
–
00
00
150
• + (–) charge has high (low) potential
Electric field
• Equipotential lines at same “height”
• Electric field lines point “downhill”
Physics 102: Lecture 2, Slide 16
CheckPoint 1.7
To go from B to A, a
positive charge must climb
“up hill” – increases
potential energy. Hence A
is at higher potential than
B.
The electric potential at point A is _______ at point B
46% 1) greater than
32% 2) equal to
22% 3) less than
Physics 102: Lecture 2, Slide 17
ACT
E=0
Now points A and B lie
inside a conductor…
conductor
The electric potential at point A is _______ at point B
A) greater than
B) equal to
C) less than
Physics 102: Lecture 2, Slide 18
The electric field points toward lower
potential, but the electric field is zero
inside a conductor…so the potential
is equal everywhere!
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? (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 19
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 20
6m
Q=-3.5 mC
W=DU=Vq
= (+6.3103V)(2mC)
= +12.6 mJ
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 21
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
Ex:
𝑞
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 22
Electron microscope
ΔV=10kV
Vi
-
Electron
gun
Vf
Uniform E
motion
• What is the final velocity of the electron?
• 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 23
Could solve this using F=ma &
kinematic equations (Phys 101)
TRY AT HOME! (HARDER)
See you Monday!
Physics 102: Lecture 2, Slide 24