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Los Altos Physics Honors
Electrostatics Workbook
Los Altos Physics Honors
Electrostatics:
Electric Fields,
Electric Forces,
Electric Potentials and
Electric Potential Energy
Workbook
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www.LAPhysics.com
Spring 2005
dls.mvla.net/los_altos
Los Altos Physics Honors
Assignments
1) Read 18.1 – 18.4 + review example problems
2) Chapter 18 Problems # 1, 2, 3, 5
3) Conceptual Development 32-2
4) Read 18.5 + review example problems
5) Chapter 18 Problems # 7, 9, 11, 13, 16, 17, 23, 61
6) Workbook Problems # 2, 4, 10
7) Read 18.6 + review example problems
8)
Chapter 18 Problems # 27, 29, 31,
9)
Read 18.7 + review example problems
10) Chapter 18 Problems # 37, 39, 46, 66
11) Read 18.8 – 18.9 + review example problems
12) LAB - Electric Field Lines: Prelab Theory
13) Read 19.1 – 19.2 + review example problems
14) Chapter 19 Problems # 1, 3, 5, 10,
15) Conceptual Development 33-2
16) Read 19.3 + review example problems
17) Workbook Problems # 4, 6, 7, 8, 9
18) Read Chapter 19. 5 + review example problems
20) Workbook Problems # 11, 12
Electrostatics Workbook
Due Date
Los Altos Physics Honors
Quiz: Electrostatic Fields & Forces:
Electrostatics Workbook
4/1/05
Quiz: Parallel Plate Capacitors Fields & Forces 4/8/05
Friday Test: Chapter 18 & 19 4/22/05
Study suggestions
 Reread highlights of book and review example problems
 Highly recommend that in small groups, discuss the “Reviewing Concepts” section at
the end of each chapter.
 Look over and even redo your homework, try the non-assigned problems in book.
 Test yourself with Practice Multiple Choice questions.
 Look at the Chapter Objectives and make sure you have met them.
 Come see me for help at lunch or after school.
Los Altos Physics Honors
Electrostatics Workbook
In-Class Problems: Chapter 18 & 19 Electric Fields, Forces and Potentials
1.
How close must two electrons be if the electric force between them is equal to
the weight of either at the Earth's surface?
2.
Suppose that electrical attraction, rather than gravity were responsible for
holding the moon in orbit around the Earth. If equal and opposite charges Q were
placed on the Earth and the Moon, what should be the value of Q to maintain the
present orbit? (Hint: treat the Earth and moon as point particles.)
Los Altos Physics Honors
3.
Electrostatics Workbook
Calculate the electric field at points A and B in figure 1, due to the two charges
shown in figure 1.
Los Altos Physics Honors
Electrostatics Workbook
4.
Two charges are separated by a distance R. If the distance is increased to 3R and
the magnitude of both charges is halved, how many times smaller is the new
electric force between them?
5.
Calculate the electric field at one corner of a square 1.0 m on a side if the other
three corners are occupied by 2.8 x 10-6 C charges.
Los Altos Physics Honors
Electrostatics Workbook
6.
Consider an electron with an initial velocity of 1.4 x 106 m/s entering an electric
field as shown in the figure. Once the
electron enters the field, how far will
it travel before it comes to rest for the
first time?
7.
Consider a parallel plate capacitor separated by d = 0.03 m. The capacitor is
oriented such that the plates are parallel to the Earth's surface. In between the
plates is a charged particle, m = 1.2 x 10-3 kg and q = -1.6 x 10-18 C. The particle is
accelerating at 24 m/s2 towards the bottom plate. What is the electric potential
difference between the two plates?
Los Altos Physics Honors
Electrostatics Workbook
8.
How much work must be done to bring 3 electrons from a great distance away to
the vertices of an equilateral triangle 1.0 x 10-10m on a side?
9.
What minimum work is required by an external force to bring a charge q = 3.0 C
from a great distance away to a point 0.50 m from a charge Q = 20.0 C ?
Los Altos Physics Honors
10.
Electrostatics Workbook
Consider a parallel plate capacitor separated by d = 0.02 m. The capacitor is
oriented such that the plates are parallel to the Earth's surface. In between the
plates is a charged particle, m = 2.0 x 10-4 kg and q = -1.6 x 10-18 C. The particle is
accelerating at 44 m/s2 towards the top plate. Set the electric potential equal to
zero on the bottom plate and find the value of the electric potential on the top
plate.
Los Altos Physics Honors
11.
Electrostatics Workbook
Consider the following:
-2.0 x 105 volts
3.0 x 107 m/s
0.1 m
Proton
0.2 m
+3.0 x 105 volts
A. Draw the path of the proton.
B. Find E midway between the capacitor.
C. Find V midway between the capacitor.
D. Find the vertical displacement of the proton between entering and leaving.
E. Find the angle at which the proton leaves the capacitor.
F. Draw and label the path of a neutron.
Los Altos Physics Honors
Electrostatics Workbook
Gravitational Potential
If we were to lift an object in a gravitational field, the amount of energy used to lift
the object would depend on three things; it's mass, the strength of the gravitational
field, and the height lifted. Near the surface of the Earth the gravitational field
strength is nearly constant, namely g = 9.8 m/s2 or 9.8 N/kg.
Lifting a Big Mass requires a Big Force which results in Big Work done in lifting the
object a height h above an arbitrary zero point. Of course, when the object is allowed
to fall the stored gravitational potential energy is transformed into a Big Kinetic Energy
at the bottom of the fall: Energybefore = Energyafter
We could, if we wanted to, calculate the energy stored by doing work against gravity in
lifting any massive object to some height h. Often times it's more useful to express the
amount of energy that gravity will give any massive object at a particular height h. This
is called the Gravitational Potential. It's not energy, it's just a number assigned to
every point in space the says something about how much energy it would take to place a
mass there or energy you can get out of dropping a mass from there.
What we get is an expression that tells us: Work by Gravity per unit Mass!
GP 
Work gravity
mass

PE gavity
mass

mg (h f  ho )
m
 gh
(J/kg)
Notice that GP does not depend on the mass that is acted upon by gravity!
Think of GP as an Energy Field associated with gravitational potential energy just as
the Gravitational Field is associated with gravitational forces.
(Example)
Q: From the surface of the Earth, what is the gravitational potential at a
height of 10m?
A: GP = gh = (9.8 N/kg) x (10 m) = 98 (N m / kg) = 98 J/kg
Q: How many Joules of energy will a 10 kg mass receive from gravity if released
from that spot?
A: GPE = mgh = (10 kg) x (9.8 N/kg) x (10 m) = 980 (kg N m / kg) = 980 N m = 980 J
Los Altos Physics Honors
Electrostatics Workbook
Electric Potential - Voltage
If we were to move a charged object in an electric field, the amount of energy used to
move the object would depend on three things; it's charge, the strength of the electric
field, and the distance moved.
Moving a Big charge requires a Big Force which results in Big Work done in moving the
charged object a distance d beyond an arbitrary zero point. Of course, when the object
is allowed to move back the stored electric potential energy is transformed into a Big
Kinetic Energy i.e. Energybefore = Energyafter
We could, if we wanted to, calculate the energy stored by doing work against the
electric field in moving any charged object some distance d. Often times it's more
useful to express the amount of energy that the electric field will give any charged
object at a particular distance d beyond the zero point. This is called the Electric
Potential or Voltage! It's not energy, it's just a number assigned to every point in space
the says something about how much energy it would take to place a charge there or
energy you can get out of letting a charge move from there.
What we get is an expression that tells us: Work by Electric Field per unit Charge!
V 
Work Efield
ch arg e

PEEfield
ch arg e

qE (d f  d o )
q
 Ed
(J/C)
Notice that V does not depend on the charge acted upon by the Electric Field!
Think of V as an Energy Field associated with Electric potential energy just as the
Electric Field is associated with Electric forces. (Example)
Q: Between parallel plates, the E field is a constant 200 N/C. What is the Electric
Potential Difference or Voltage V between the two plates spaced a distance
0.01 m apart?
A: V = Ed = (200 N/C) x (0.01 m) = 2 (N m / C) = 2 J/C
Q: How many Joules of energy will a 2.0 C charge receive from the E field if
placed next to the positive of the two plates?
A: EPE = qEd = (2.0C) x (200 N/C) x (0.01 m) = 4 (C N m / C) = 4 N m = 4 J
Los Altos Physics Honors
Electrostatics Workbook
Los Altos Physics Honors
Electrostatics Workbook
Los Altos Physics Honors
Electrostatics Workbook
Los Altos Physics Honors
Electrostatics Workbook
Los Altos Physics Honors
Electrostatics Workbook