1. Physics 102: Lecture 3 Electric Potential
Today’s lecture will cover Textbook Sections
Honors Credit info on web under course description.
Physics 102: Lecture 3, Slide 1

2. Overview for Today’s Lecture
Electric Potential Energy/ Work
Uniform fields
Point charges
Electric Potential (like height)
Show large and small battery 9 volt small, 6 volt large
Physics 102: Lecture 3, Slide 2

3. Recall Work from P101 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 = -Wconservative
Use book or brick for prop. Ask students if I am doing positive or negative work, what about gravity.
Physics 102: Lecture 3, Slide 3

4. Preflight 3.1 C A B
Uniform E
In what direction does the force on a negative charge at point A point?
left
right up
Physics 102: Lecture 3, Slide 4

5. Preflight 3.1 C F A B -
Uniform E
In what direction does the force on a negative charge at point A point?
left
right up
Electric field points in the direction a POSITIVE charge would feel force.
Physics 102: Lecture 3, Slide 5

6. Preflight 3.2 C A B
Uniform E
When a negative charge is moved from A to C the ELECTRIC force does
positive work.
zero work.
negative work.
Physics 102: Lecture 3, Slide 6

7. Preflight 3.2
motion - F C - F
The work is zero because the path is perpendicular to the field - F - F - F A B
Uniform E
When a negative charge is moved from A to C the ELECTRIC force does
positive work.
zero work.
negative work.
Physics 102: Lecture 3, Slide 7

8. Preflight 3.3 C A B
Uniform E
When a negative charge is moved from A to B the ELECTRIC force does
positive work.
zero work.
negative work.
Physics 102: Lecture 3, Slide 8

9. Preflight 3.3 C
The work is negative the electric force opposes the direction of motion A B - F - F - F - F - F -
Uniform E
motion
When a negative charge is moved from A to B the ELECTRIC force does
positive work.
zero work.
negative work.
Physics 102: Lecture 3, Slide 9

10. Preflight 3.5 C A B
When a negative charge is moved from A to B, the electric potential energy of the charge
Increases
is constant
decreases
Physics 102: Lecture 3, Slide 10

11. Preflight 3.5 C A B - - - - -
Uniform E
When a negative charge is moved from A to B, the electric potential energy of the charge
Increases
is constant
decreases
UE = -WE field
Like “climbing up hill” – increases potential energy
Physics 102: Lecture 3, Slide 11

12. ACT: Electric Potential Energy+ C B A
When a negative charge is moved from A to B, the electric potential energy of the charge
(1) increases
(2) is constant
(3) decreases
Now switch to point charges!
Physics 102: Lecture 3, Slide 12

13. ACT: Electric Potential Energy
AC: W=0 E + C
CB: W<0 B A - - - - -
When a negative charge is moved from A to B, the electric potential energy of the charge
(1) increases
(2) is constant
(3) decreases
Now switch to point charges!
1) The electric force is directed to bring the electron closer to be proton.
2) Since the electron ends up further from the proton the electric field did negative work.
3) So the electric potential energy increased
Physics 102: Lecture 3, Slide 13

14. Work and D Potential Energy
W = F d cos(q)
Gravity Electric
Brick raised yi yf
Charge moved ∞  rf
FE = kq1q2/r2 (left)
WE = -kq1q2/rf
DUE= +kq1q2/rf
FG = mg (down)
WG = -mgh
DUG= +mgh
yf rf h
yi
Physics 102: Lecture 3, Slide 14

15. Work done by YOU to assemble 3 charges
Example
W1 =
W2 =
W3 =
Wtotal =
WE =
D UE =
Do this live with balls on the table! 3
5 m
5 m 1 2
5 m
Watch signs!
Physics 102: Lecture 3, Slide 15

16. ACT: Work done by YOU to assemble 3 negative charges
How much work would it take YOU to assemble 3 negative charges? 3
W = mJ
W = 0 mJ
W = mJ
5 m
5 m 1 2
5 m
Physics 102: Lecture 3, Slide 16

17. ACT: Electric Potential Energy of 3 negative charges
Likes repel, so potential energy is still positive! 3
UE = mJ
UE = 0 mJ
UE = mJ
5 m
5 m 1 2
5 m
Physics 102: Lecture 3, Slide 17

18. Preflight 3.11 1 +
5 m
5 m + - 2
5 m 3
The total work required by you to assemble this set of charges is:
(1) positive
(2) zero
(3) negative
Physics 102: Lecture 3, Slide 18

19. Preflight 3.11 1 +
5 m
5 m + - 2
5 m 3
The electric potential energy of this set of charges is:
(1) positive
(2) zero
(3) negative
Bring in (1): zero
Bring in (2): positive
Bring in (3): negative x 2
Physics 102: Lecture 3, Slide 19

20. 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
Comment on lab w/ equipotential lines
Physics 102: Lecture 3, Slide 20

21. Preflight 3.7 The electric potential at point A is _______ at point B
greater than
equal to
less than
Physics 102: Lecture 3, Slide 21

22. Preflight 3.7
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
greater than
equal to
less than
Physics 102: Lecture 3, Slide 22

23. Preflight 3.9 The electric potential at point A is _______ at point B
conductor
The electric potential at point A is _______ at point B
greater than
equal to
less than
Physics 102: Lecture 3, Slide 23

24. Preflight 3.9 The electric potential at point A is _______ at point B
conductor
The electric potential at point A is _______ at point B
greater than
equal to
less than
The electric field is zero at any point within a conducting material
Physics 102: Lecture 3, Slide 24

25. ACT: Electric Potential+ C B A
The electric potential at A is ___________ the electric potential at B.
greater than
equal to
less than
Physics 102: Lecture 3, Slide 25

26. ACT: Electric Potential+ C B A
The electric potential at A is ___________ the electric potential at B.
greater than
equal to
less than
1) Electric field lines point “down hill”
2) AC is equipotential path (perpendicular to E)
3) CB is down hill, so B is at a lower potential than (“down hill from”) A
Physics 102: Lecture 3, Slide 26

27. Electric Potential due to Proton
What is the electric potential a distance r = 0.5310-10 m from a proton? (Let V() = 0)
V = U/q = k q/ r =
What is the electric potential energy of an electron a distance r = 0.5310-10 m from a proton?
U = Vq =
rf = 0.510-10 m + -
Physics 102: Lecture 3, Slide 27

28. 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 = qV
Neither has direction, just location. Sign matters!
Physics 102: Lecture 3, Slide 28

29. Example
Two Charges
Calculate electric potential at point A due to charges
Calculate V from +7mC charge
Calculate V from –3.5mC charge
Add (EASY!)
V = kq/r
V7 =
V3 =
Vtotal = A
4 m
6 m
Q=+7.0mC
Q=-3.5 mC
W=DU=DVq =
How much work do you have to do to bring a 2 mC charge from far away to point A?
Physics 102: Lecture 3, Slide 29

30. 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
III
Q=+7.0mC
Q=-3.5 mC
Physics 102: Lecture 3, Slide 30

31. 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
III
Q=+7.0mC
Q=-3.5 mC
Very close to positive charge potential is positive
Very close to negative charge potential is negative
Physics 102: Lecture 3, Slide 31

32. To Do
Read , , 6
Bring “Problem Solver” to discussion section
Complete preflight before 8:00AM on day of next lecture.
Physics 102: Lecture 3, Slide 32