AP Physics
Monday 14.03.03
Standards:
IIIaa4 Calculate the magnitude
and direction of the force on a
positive or negative charge
placed in a specified field.
Warm Up
How strong is an electric field
200 m from a -1x10-3C charge?
What is the Electric force if a 2x10-4C charge is placed at the
above distance?
Objective: SWBAT find the
potential energy of a particle
subject to a uniform electric
field.
Agenda
1. Warm Up
2. Check HW
3. Review HW
4. Potential Energy in Electric
fields.
Homework
E#7
AP Physics
Tuesday 14.03.04
Standards:
IIIa. Electric Potential Energy &
Potential
Objective: SWBAT find the
electric potential due to an
infinitely charged plate
Agenda
1. Warm Up
2. Review HW
3. Electric Potential
4. Electrical Energy Inquiry`
Warm Up
Find the potential
energy of a 5C
charge that is 2m
from a infinitely
long positively
charge plate?
Homework
E#8
AP Physics
Wednesday 14.03.05
Standards:
2b electric potential of point charges
Objective:
SWBAT find the electric potential due to
point charges.
Agenda
1. Warm Up
2. Review HW
3. Electric Potential due to a
charged particle.
4. Electric Potential due to a
charged particle inquiry
Warm Up
Find the electric potential of an
unknown electric charge in a
uniform electric field of 0.25 N/c
if you want to move it from a
position of 5 meters away from
the electric field source, to 2
meters away from the electric
field source.
b. Find the work done if the
charge was -2C.
Homework
E#9
AP Physics
Thursday 14.03.06
Standards: Students relate stored charge
and voltage for a capacitor.
Warm Up
Find the change in potential energy if a
30nC charge moves from 20m to 5m away
from a -400nC charge.
Objective: SWBAT understand the
charge distribution, electric field, and
potential distribution & be able to solve
problems involving capacitors.
Agenda
1. Warm Up
2. Review HW
3. Finish Inquiry
4. Equipotential Lines & Conductors
5. Capacitors
Homework
C#10
AP Physics
Friday 14.03.07
Warm Up
What is the electric potential
difference between D and A?
Standards: Students relate
stored charge and voltage for a
capacitor.
Objective: SWBAT find the
charge on a capacitor and
describe the movements of
charge through the parallel
plates of a capacitor
Agenda
1. Warm Up
2. Review HW
3. Capacitors
Is work done by the electric field or by you?
Homework
C#11
E#2 Coulomb’s Law Practice
b. F=?
q1=8C
q2=4C
r= 2m
b. F=?
q1=3.9x10-6C
q2=2.2x10-7C
r= 2.6x103m
c. F=?
q1=40μC
q2=20μC
r= 1x10-3 m
d. F=200N
q1=1x10-4C
q2=2x10-4C
r= ?
1.(4) In 1990, a French team flew a kite that was 1034 m long.
Imagine two charges 2.0 nC and -2.8 nC, at opposite ends of the
kite. Calculate the magnitude of the electric force between
them. If the separation of charges is doubled, what absolute
value of equal and opposite charges would exert the same
electric force?
2. (2) Kalyan Ramji Sain, of India, had a mustache that
measured 3.39 m from end to end in 1993. Suppose two
charges, q and 3q, are placed 3.39 m apart. If the magnitude of
the eletric force btween the charges is 2.4x10-6N, what is the
value of q?
1μC=1x106C
1nC=1x109C
E#3 Force between multiple charges
200nC
D
-30nC
5 cm
C
-20nC
30cm
A
1. Find the net force on A
2. Find the net force on C.
1. American athlete Jesse Castenada walked 228.930 km in 24 h in 1976, setting a new
record. Consider an equilateral triangle with a perimeter equal to the distance
Castenada walked. Suppose the charges are placed at the following vertices of the
triangle: q1=-2.4 nC at the bottom right vertex, and q3=4.0 nC at the top vertex. Find the
magnitude and direction of the resultant electric force acting on q1.
Practice: Charges in 2 Dimensions
Find the Net Electric Force on the 1C Charge
1C
4m
-2C
2m
3C
Electric Field Guided Practice
Find the electric field at the red dot.
1μC
5m
1μC
10m
Electric Field Practice E#4
a. E=?
q=1.6x10-6C
r=20m
b. Find the
electric field
at the red
point. --->>>
2x103m
1μC
1μC
c. Find the net
electric field
at the red
point.--->>>
5m
1μC
10m
1. The world’s largest tires have a mass of almost 6000 kg and a diameter of 3.72 m
each. Consider an equilateral triangle with sides that are 3.72 m long each. If equal
positive charges are placed at the points on either end of the triangle’s base, what is
the direction of the resultant electric field strength vector at the top vertez? If the
magnitude of the electric field strength at the top vertex equals 0.145 N/C, what are
the two quantities of charge at the base of the triangle?
Electric Field Lines Rules
1. Electric field lines always extend from a positively charged object to
a negatively charged object, from a positively charged object to
infinity, or from infinity to a negatively charged object.
2. Electric field lines never cross each other.
3. Electric field lines are most dense around objects
with the greatest amount of charge.
4. At locations where electric field lines meet the surface of an object,
the lines are perpendicular to the surface.
Electric Field of an electron
Electric Field of a proton
Electric Field Lines Examples
Electric Field Lines Examples
Electric Field Lines Examples
Drawing & Understanding
Electric Field Lines
-- The idea behind drawing Electric field lines is easy. You
calculate the electric field (like we did yesterday) at different
points, with an emphasis on the direction of the electric field. If
you find the direction of the Electric field at enough points, you
will get something that looks very similar to the electric field
diagrams from previous slides.
Guided Practice: Electric Field Diagrams
Estimate the magnitude and direction of the electric field
at each point shown. Use these vectors to draw an
electric field diagram.
+
-
E#5 Electric Field Diagrams.
Make an electric field diagram for the following configurations.
Use the guided practice as your model.
a.
b.
1C
1C
2C
-1C
36. The diagram above shows electric field lines in an
isolated region of space containing two small charged
spheres, Y and Z. Which of the following statements is
true? (A) The charge on Y is negative and the charge on Z
is positive. (B) The strength of the electric field is the
same everywhere. (C) The electric field is strongest
midway between Y and Z. (D) A small negatively charged
object placed at point X would tend to move toward the
right.(E) Both charged spheres Y and Z carry charge of the
same sign.
E#7 Electrical Potential Energy
a. F=?
E= 40N/c
q=-1.6x10-19C
1.
2.
4.
b. U=?
q=2x10-6C
E=0.5 N/c
d=2.5 m
c. We=?
q=4x10-6C
E=20 N/c
d=1.2 m
(2) The potential energy of an electron (q=-1.6x10-19C) increases
by 3.3x10-15 J when it moves 3.5 cm parallel to a uniform electric
field. What is the magnitude of the electric field through which
the electron passes?
(4) A charged particle moves through a distance of 9.35 m parallel
to a uniform electric field. The electrical potential energy of the
particle increases by 3.17x10-10 J as it moves. the electric field has
a magnitude of 1.25x105N/C. a) What is the charge on the
particle?
(8) A negative ion (q=-4.8x10-19C) moves 0.63cm through a
uniform electric field in a direction opposite to the direction of the
field. The magnitude of the electric field is 279 volts per meter.
What is the change in electrical potential energy of the ion?
E#8 Electrical Potential
a. q1=20μC
q2=40μC
r=400 m
U=?
b. q=12nC
r=3x10-2m
V=?
c. E=20 N/C
d=5m
V=?
1 (5) There is an electric field close to the surface of the earth. This field points towards
the surface and has a magnitude of about 1.5x102 N/C. A charge moves perpendicularly
toward the surface of Earth through a distance of 439 m, the height of the Sears Tower in
Chicago, Ilinois. During this trip, the electric potential energy of the charge decreases by
3.7x10-8 J.
a) What is the charge on the moving particle?
b) What is the potential difference between the top of the Sears Tower and the ground?
c) What is the electric potential of the charge at its final position?
2 (8) A negative ion (q=-4.8x10-19 C) moves 0.63 cm through a uniform electric field in a
direction opposite to the direction of the field. The magnitude of the electric field is 279
volts per meter.
A) What is the change in the electrical potential energy of the ion?
B) What is the electric potential at the final position of the ion?
Electric Fields, Forces & Energy Inquiry
1. Find the electric force between the charged balloon nearest
to you and the infinitely long charged plate. ( You will need
to take measurements.)
2. Find the electrical potential energy between the infinitely
charged plate and the nearest balloon.
3. Find out how much work needs to be done to move your
balloon to 1 meter away from the infinitely charged plate.
4. Find the electrical potential at the point of the nearest golf
ball.
5. Find the electrical potential 1 m away from the infinitely
charged plate.
Electric Fields, Forces & Energy Inquiry #2
1. Find the electric force between the two nearest charges.
2. Find the magnitude of the electric field of the nearest charge to a
point 2 meters away.
3. Find the electrical potential energy between the nearest charge to
you and the furthest charge from you.
4. How much work would it take to move the furthest charge from #3
to a distance of 1 m from the closest charge
5. How much potential is there between the nearest charge and the
projector?
6. How much will the potential change if some unknown charge is
moved from the projector to 1 m away from your nearest charge?
E#9 Electrical Potential For Point Charges
a.
q1=2μC
q2=6μC
r=1x10-3m
U=?
b. q=6μC
r=1x10-3m
V=?
c.
q=6μC
r2=1x10-3m
r1=6x10-4
ΔV=?
1. Find the electrical potential due to a 4nC charge at a point
25m away.?
2. How much would the electrical potential change if wanted to
move an unspecified charge to a distance of 2 m from the 4nC
charge?
2. Find the electric field 20 m away from a charge, if it has a
voltage of 40 J/C.
3. The electrical potential of an object 1Gm away from a 1x10-9
charge is approximately?
Eqipotential Diagrams
The diagram above shows equipotential lines
produced by an unknown charge distribution.
A, B, C, D, and E
are points in the plane.
88. Which vector below best describes the direction of the electric field at point A ?
(A)
(B)
(C)
(D)
(E) None of these; the field is zero.
89. At which point does the electric field have the greatest magnitude?
(A) A (B) B (C) C (D) D (E) E
90. How much net work must be done by an external force to move a –1 μC point charge
from rest at point C to rest at point E ? (A) –20 μJ (B) –10 μJ (C) 10 μJ (D) 20 μJ (E) 30 μJ
E#10 – Conductors
1. What is the electric potential inside of a conductor?
2. What does an equipotential line tell us?
3. How do the electric charges configure themselves in a
conducting sphere?
4. What is the magnitude of the electric field inside of a
conductor?
Electrostatics Guided Practice
1974B5. The diagram above shows some of the equipotentials in a plane perpendicular
to two parallel charged metal cylinders. The potential of each line is labeled.
a. The left cylinder is charged positively. What is the sign of the charge on the other
cylinder?
b. On the diagram above, sketch lines to describe the electric field produced by the
charged cylinders.
c. Determine the potential difference, VA – VB , between points A and B.
d. How much work is done by the field if a charge of 0.50 coulomb is moved along a path
from point A to point E and then to point D?
1985B3. An electron initially moves in a horizontal direction and
has a kinetic energy of 2.0 x 103 electron–volts when it is in the
position shown above. It passes through a uniform electric field
between two oppositely charged horizontal plates (region I) and
a field–free region (region II) before eventually striking a screen
at a distance of 0.08 meter from the edge of the plates. The
plates are 0.04 meter long and are separated from each other
by a distance of 0.02 meter. The potential difference
across the plates is 250 volts. Gravity is negligible.
a. Calculate the initial speed of the electron as it enters region I.
b. Calculate the magnitude of the electric field E between the plates, and indicate its
direction on the diagram above.
c. Calculate the magnitude of the electric force F acting on the electron while it is in
region I.
d. On the diagram below, sketch the path of the electron in regions I and II. For each
region describe the shape of the path.