1. Physics 102: Lecture 3 Electric Potential

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

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 = -W
Use book or brick for prop. Ask students if I am doing positive or negative work, what about gravity.

5. Preflight 3.1 !!!!ACT!!!! Uniform EB
Uniform E
In what direction does the force on a negative charge at point A point?
left
right up

6. Preflight 3.3 C
“because the direction of the displacement is 180 degrees from direction of the force ” A B
Uniform E
When a negative charge is moved from A to B the ELECTRIC force does
positive work.
zero work.
negative work.

8. 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
E.P.E. = -WE field
Electric force did negative work so electric potential energy increased.

9. 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
(A) increases
(B) is constant
(C) decreases
Now switch to point charges!

10. 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
Fg=mg
Fg=mg rf
Fg=mg h
Fg=mg
Fg=mg
Fg=mg
Fg=mg
yi
Fg=mg

12. Work done by YOU to assemble 3 charges
Example
W1 = 0
W2 = k q1 q2 /r
=3.6 mJ
=(9109)(110-6)(210-6)/5
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 mJ
Wtotal = mJ
WE = mJ
DUE = mJ
(watch signs!)
Do this live with balls on the table! 3
5 m
5 m 1 2
5 m

13. 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! 3
W = mJ
W = 0 mJ
W = mJ
5 m
5 m 1 2
5 m

14. 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

16. Electric Potential (like height)*
Mount Tabor
Moving to higher potential  moving uphill

17. 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

18. Preflight 3.7
Electric field Points from greater potential to lower potential
The electric potential at point A is _______ at point B
greater than
equal to
less than

20. 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

21. Preflight Summary Path Vfinal - Vinitial Charge WE field D U = q DV +
Negative
A→ B
Positive
Negative
Positive
Negative
A → C + -
Zero
C → B
Negative + -

23. ACT: Electric Potential+ C B A
The electric potential at A is ___________ the electric potential at B.
greater than
equal to
less than

24. Electric Potential due to Proton
Example
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 =
(9109)(1.610-19) /0.5310-10
= 27.2 Volts
rf = 0.510-10 m +

25. 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 = (9109)(1.610-19) /0.5310-10= 27.2 Volts
What is the electric potential energy of an electron a distance r= 0.5310-10 m from a proton?
U = Vq = (27.2)(-1.610-19)
= -4.3510-18J
 eV
Hydrogen Balloon E≈ (4.3510-18)(610+23J )=106 J!
rf = 0.510-10 m + -

27. 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!

28. 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=(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 A
4 m
6 m
Q=+7.0mC
Q=-3.5 mC
W=DU=DVq
=(+6.3103V)(2mC)
=+12.6 mJ
How much work do you have to do to bring a 2 mC charge from far away to point A?

29. 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

30. To Do
Bring “Problem Solver” to discussion section
Complete preflight.