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Physics 1161: Lecture 3, Slide 1
Physics 102: Lecture 3
Electric Potential
- Today’s lecture will cover Textbook
Sections 20.1 - 20.
Exam I
Physics 1161: 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 1161: Lecture 3, Slide 3
Recall Work from P
- Work done by the force given by:
- W = F d cos(θ)
- 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 = -Wconservative
Physics 1161: Lecture 3, Slide 4
A B
C
Uniform E→
In what direction does the force on a negative charge at point A point?
left
right
up
Preflight 3.
Physics 1161: Lecture 3, Slide 5
A B
C
Uniform E→
When a negative charge is moved from A to C the ELECTRIC force does
positive work.
zero work.
negative work.
Preflight 3.
Physics 1161: Lecture 3, Slide 6
A B
C
Uniform E→
When a negative charge is moved from A to B the ELECTRIC force does
positive work.
zero work.
negative work.
Preflight 3.
Physics 1161: Lecture 3, Slide 10
Work done by YOU
to assemble 3 charges
1
3
2 5 m
5 m 5 m
- W 2 =
- W 3 =
- Wtotal =
- WE =
∆ UE =
Watch signs!
Physics 1161: Lecture 3, Slide 11
ACT: Work done by YOU
to assemble 3 negative
charges
1
3
2 5 m
1. W = +19.8 mJ 5 m 5 m
2. W = 0 mJ
3. W = -19.8 mJ
How much work would it take YOU to assemble 3 negative charges?
Physics 1161: Lecture 3, Slide 12
Preflight 3.
The total work required by you to assemble this set of charges is: (1) positive
(2) zero (3) negative
5 m
5 m 5 m
Physics 1161: Lecture 3, Slide 13
Electric Potential (like height)*
- Units Joules/Coulomb ≡Volts
- Really Potential differences
- Equipotential lines at same height
- Field lines point down hill
- V = k q/r (distance r from charge q)
Physics 1161: Lecture 3, Slide 14
Preflight 3.
The electric potential at point A is _______ at point B
greater than
equal to
less than
Physics 1161: Lecture 3, Slide 15
Preflight 3.
The electric potential at point A is _______ at point B
- greater than
- equal to
- less than
conductor
Physics 1161: Lecture 3, Slide 19
Two Charges
Q=+7.0μC Q=-3.5 μC
A
6 m
4 m
How much work do you have to do to bring a 2 μC charge from far away to point A?
W=∆U=∆Vq =
- Calculate electric potential at point A due to charges - Calculate V from +7μC charge - Calculate V from –3.5μC charge - Add (EASY!)
- V = kq/r V 7 = V 3 = Vtotal =
Physics 1161: Lecture 3, Slide 20
ACT: Two Charges
Q=+7.0μC Q=-3.5 μC
- In the region II (between the two charges) the electric potential is
- always positive
- positive at some points, negative at others.
- always negative
Physics 1161: Lecture 3, Slide 21
To Do
- Read 20-1 – 20-
- Remember -- No Class on Friday
- Submit Homework 1 by Monday
morning.
- Complete preflight before Monday
8:00AM.
