Transcript Electric Potential

Physics 102: Lecture 3
Electric Potential
• Today’s lecture will cover Textbook
Sections 17.1 - 3
Honors Credit info on web
under course description.
Physics 102: Lecture 3, 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 3, Slide 2
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
Physics 102: Lecture 3, Slide 3
Preflight 3.1
C
A
Uniform E
In what direction does the force on a
negative charge at point A point?
1) left
2) right
3) up
Physics 102: Lecture 3, Slide 4
B
Preflight 3.1
C
F - A
B
Uniform E
In what direction does the force on a
negative charge at point A point?
1) left
2) right
3) up
Physics 102: Lecture 3, Slide 5
Electric field points in the direction a
POSITIVE charge would feel force.
Preflight 3.2
C
A
B
Uniform E
When a negative charge is moved from A to C
the ELECTRIC force does
1) positive work.
2) zero work.
3) negative work.
Physics 102: Lecture 3, Slide 6
Preflight 3.2
The work is zero because
the path is perpendicular
to the field
motion
F
F
F
F
F
- C
- A
B
Uniform E
When a negative charge is moved from A to C
the ELECTRIC force does
1) positive work.
2) zero work.
3) negative work.
Physics 102: Lecture 3, Slide 7
Preflight 3.3
C
A
B
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.
Physics 102: Lecture 3, Slide 8
Preflight 3.3
The work is negative the electric
force opposes the direction of
motion
C
A
B
F
F - F -F -F - motion
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.
Physics 102: Lecture 3, Slide 9
Preflight 3.5
C
A
B
When a negative charge is moved from A to B,
the electric potential energy of the charge
1) Increases
2) is constant
3) decreases
Physics 102: Lecture 3, Slide 10
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
1) Increases
2) is constant
3) decreases
Physics 102: Lecture 3, Slide 11
UE = -WE field
Like “climbing up hill” –
increases potential energy
ACT: Electric Potential Energy
+
C
A
B
When a negative charge is moved from A to B, the electric
potential energy of the charge
(1) increases
(2) is constant
(3) decreases
Physics 102: Lecture 3, Slide 12
ACT: Electric Potential Energy
+
E
A-
AC: W=0
C
B
-
-
-
CB: W<0
-
When a negative charge is moved from A to B, the electric
potential energy of the charge
(1) increases
(2) is constant
(3) decreases
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
Work and D Potential Energy
W = F d cos(q)
Gravity
• Brick raised yi yf
• FG = mg (down)
• WG = -mgh
• DUG= +mgh
yf
h
yi
Physics 102: Lecture 3, Slide 14
Electric
• Charge moved ∞  rf
• FE = kq1q2/r2 (left)
• WE = -kq1q2/rf
• DUE= +kq1q2/rf
rf
Work done by YOU
to assemble 3 charges
• W1 =
• W2 =
• W3 =
• Wtotal =
• WE =
 D UE =
5m
1
Watch signs!
Physics 102: Lecture 3, Slide 15
3
5m
5m
2
ACT: Work done by YOU
to assemble 3 negative charges
How much work would it take YOU to assemble 3 negative
charges?
1. W = +19.8 mJ
2. W = 0 mJ
3. W = -19.8 mJ
Physics 102: Lecture 3, Slide 16
5m
1
3
5m
5m
2
ACT: Electric Potential Energy
of 3 negative charges
Likes repel, so potential energy is still
positive!
1. UE = +19.8 mJ
2. UE = 0 mJ
3. UE = -19.8 mJ
Physics 102: Lecture 3, Slide 17
5m
1
3
5m
5m
2
Preflight 3.11
1
5m
2
+
+
5m
5m
-
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
Preflight 3.11
1
5m
2
+
+
5m
5m
-
3
The electric potential energy of this set of
charges is:
(1)
positive
Bring in (1): zero
(2)
zero
Bring in (2): positive
(3)
negative
Physics 102: Lecture 3, Slide 19
Bring in (3): negative x 2
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 20
Preflight 3.7
The electric potential at point A is _______ at point B
1) greater than
2) equal to
3) less than
Physics 102: Lecture 3, Slide 21
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
1) greater than
2) equal to
3) less than
Physics 102: Lecture 3, Slide 22
Preflight 3.9
conductor
The electric potential at point A is _______ at point B
1) greater than
2) equal to
3) less than
Physics 102: Lecture 3, Slide 23
Preflight 3.9
conductor
The electric potential at point A is _______ at point B
1) greater than
2) equal to
3) less than
Physics 102: Lecture 3, Slide 24
The electric field is zero at any point
within a conducting material
ACT: Electric Potential
+
E
A
C
B
The electric potential at A is ___________ the electric potential
at B.
1) greater than
2) equal to
3) less than
Physics 102: Lecture 3, Slide 25
ACT: Electric Potential
+
E
C
A
B
The electric potential at A is ___________ the electric potential
at B.
1) greater than
1) Electric field lines point “down hill”
2) AC is equipotential path (perpendicular to E)
2) equal to
3) CB is down hill, so B is at a lower potential than
(“down hill from”) A
3) less than
Physics 102: Lecture 3, Slide 26
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
-
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
Two Charges
• Calculate electric potential at point A due to
charges
– Calculate V from +7mC charge
– Calculate V from –3.5mC charge
– Add (EASY!)
4m
• V = kq/r
V7 =
V3 =
Vtotal =
A
Q=+7.0mC
6m
Q=-3.5 mC
How much work do you have to do to bring W=DU=DVq =
a 2 mC charge from far away to point A?
Physics 102: Lecture 3, Slide 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
Q=+7.0mC
Physics 102: Lecture 3, Slide 30
III
Q=-3.5 mC
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
Q=+7.0mC
III
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
To Do
• Read 17.5-6, 18.1-2, 6
• Bring “Problem Solver” to discussion section
• Complete preflight before 8:00AM on day of
next lecture.
Physics 102: Lecture 3, Slide 32