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Physics 102: Lecture 3, Slide 1
Physics 102: Lecture 3
Electric Potential
Physics 102: Lecture 3, Slide 2
Overview for Today’s Lecture
• Electric Potential Energy/ Work– Uniform fields– Point charges
• Electric Potential (like height)– Uniform fields– Point charges
Physics 102: Lecture 3, Slide 3
Recall Work from P101
• Work done by the force given by: – W = F d cos()– 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– Potential Energy = W
Physics 102: Lecture 3, Slide 4
Physics 102: Lecture 3, Slide 5
A B
C
Uniform E
In what direction does the force on a negative charge at point A point?
1) left
2) right
3) up
Preflight 3.1
!!!!ACT!!!!
Physics 102: Lecture 3, Slide 7
A B
C
Uniform E
When a negative charge is moved from A to B the ELECTRIC force does
1) positive work.
2) zero work.
3) negative work.
Preflight 3.3
“because the direction of the displacement is 180 degrees from direction of the force”
Physics 102: Lecture 3, Slide 8
Physics 102: Lecture 3, Slide 9
A B
C
Uniform E
Preflight 3.5
When a negative charge is moved from A to B, the electric potential energy of the charge
1) Increases
2) is constant
3) decreases
E.P.E. = WE field
Electric force did negative work so electric potential energy increased.
Physics 102: Lecture 3, Slide 10
AB
+
ACT: Electric Potential Energy
When a negative charge is moved from A to B, the electric potential energy of the charge
(A) increases
(B) is constant
(C) decreases
CAC: W=0
CB: W<0
Physics 102: Lecture 3, Slide 11
Work and Potential Energy
• Brick raised yi yf • Charge moved ∞ rf
• FE = kq1q2/r2 (left)
• WE = -kq1q2/rf
• UE= +kq1q2/rf
W = F d cos()Gravity Electric
Fg=mgFg=mgFg=mgFg=mgFg=mgFg=mgFg=mg
yi
yf
Fg=mg
h
• FG = mg (down)
• WG = -mgh
• UG= +mgh
rf
Physics 102: Lecture 3, Slide 12
Physics 102: Lecture 3, Slide 13
Work done by YOU to assemble 3 charges
• W1 = 0
m
m
m
• W2 = k q1 q2 /r =(9109)(110-6)(210-6)/5 =3.6 mJ• W3 = k q1 q3/r + k q2 q3/r
(9109)(110-6)(310-6)/5 + (9109)(210-6)(310-6)/5
=16.2 mJ
• Wtotal = +19.8 mJ• WE = -19.8 mJ• UE = +19.8 mJ
(watch signs!)
Physics 102: Lecture 3, Slide 14
ACT: Work done by YOU to assemble 3 negative charges
m
m
m
A.W = +19.8 mJB.W = 0 mJC.W = -19.8 mJ
How much work would it take YOU to assemble 3 negative charges?
Likes repel, so YOU will still do positive work!
Physics 102: Lecture 3, Slide 15
Preflight 3.11
The total work required by you to assemble this set of charges is:
(1) positive
(2) zero
(3) negative
+
+
-m
m
m
1
32
Physics 102: Lecture 3, Slide 16
Physics 102: Lecture 3, Slide 17
Electric Potential (like height)*
Mount Tabor
Moving to higher potential moving uphill
Physics 102: Lecture 3, Slide 18
Electric Potential (like height)*
• Units Joules/Coulomb Volts– Batteries– Outlets – EKG
• Really Potential differences• Equipotential lines at same height • Field lines point down hill• V = k q/r (distance r from charge q)
– V(∞) = 0
Physics 102: Lecture 3, Slide 19
Preflight 3.7
The electric potential at point A is _______ at point B
1) greater than
2) equal to
3) less than
Electric field Points from greater potential to lower potential
Physics 102: Lecture 3, Slide 20
Physics 102: Lecture 3, Slide 21
Preflight 3.9
The electric potential at point A is _______ at point B
1) greater than
2) equal to
3) less than
conductor
Physics 102: Lecture 3, Slide 22
Preflight Summary
A→ B Negative
C → B
A → CZero
Path Vfinal - Vinitial U = q V WE field
Negative
Charge
+-
+-
+-
Negative
PositivePositiveNegative
Physics 102: Lecture 3, Slide 24
Physics 102: Lecture 3, Slide 25
AB
+
ACT: Electric Potential
The electric potential at A is ___________ the electric potential at B.
1) greater than
2) equal to
3) less than
C
Physics 102: Lecture 3, Slide 26
Electric Potential due to ProtonWhat is the electric potential a distance r= 0.5310-10 m
from a proton? (Let V()=0)V =U/q= k q/ r =
rf = 0.510-10 m
Volts
Physics 102: Lecture 3, Slide 27
Electric Potential due to Proton
What is the electric potential a distance r= 0.5310-10 m from a proton? (Let V()=0)
rf = 0.510-10 m
What is the electric potential energy of an electron a distance r= 0.5310-10 m from a proton?
Hydrogen Balloon E≈ (4.3510-18)(610+23J )=106 J!
V =U/q= k q/ r = (9109)(1.610-19) /0.5310-10= 27.2 Volts
U = Vq = (27.2)(-1.610-19)
JeV
Physics 102: Lecture 3, Slide 28
Physics 102: Lecture 3, Slide 29
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 3, Slide 30
Two Charges
Q=-3.5 CQ=+7.0C
A
6 m
4 m
How much work do you have to do to bring a 2 C charge from far away to point A?
W=U=Vq =(+6.3103V)(2C) =+12.6 mJ
• Calculate electric potential at point A due to charges– Calculate V from +7C charge
– Calculate V from –3.5C charge
– Add (EASY!)
• V = kq/r
V7=(9109)(710-6)/5 = 12.6103V
V3=(9109)(-3.510-6)/5 = -6.3103V
Vtotal = V7+V3 = +6.3103V
Physics 102: Lecture 3, Slide 31
ACT: Two Charges
Q=-3.5 CQ=+7.0C
• In the region II (between the two charges) the electric potential is
always positive
positive at some points, negative at others.
always negative
Very close to positive charge potential is positiveVery close to negative charge potential is negative
Physics 102: Lecture 3, Slide 32
To Do
• Bring “Problem Solver” to discussion section
• Complete preflight.
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