Electrostatics
I. Definition:
The word electrostatics comes from the Greek word “elektron" which means amber.
The Greeks found that when an amber rock was rubbed, it was capable of picking up
small particles or fibers. The word static, of course, means at rest.
II. Charges:
A positive charge means that the object has lost electrons and is no longer
electrically neutral. Each electron lost gives the particle a charge of +1.6 x 10-19
coulombs. Positive, or vitreous, charges are classically created by rubbing a glass rod
with silk. The rod becomes positive (loses electrons); the silk become negative (gains
electrons). Since electric charge is conserved, the system (glass rod and silk)
maintains a net charge of 0.
A negative charge means that the object has gained electrons. Each electron gained
gives the particle a charge of -1.6 x 10-19 coulombs. Negative, or resinous, charges are
classically created by rubbing a rubber rod with fur. The rod becomes negatively
charged; the fur positively charged. By definition, negatively charged objects have
more mass than an identical neutral object since each extra electron has a mass of
9.11 x 10-31 kg.
In mechanics our basic property of matter was MASS.
In electricity, our basic property is CHARGE.
Who named the two kinds of charge? ___ ___________ (Famous American Scientist)
He thought that the positive charges were moving (the structure of the atom wasn’t
really know yet), and designated the direction of the current to be with the flowing
positive charges. Today, we know that current is flowing negative charges (e-). BUT, by
convention, we still call the positive direction of current as with the flow of positive
charges, so in reality, what the direction we designate as current is actually the
opposite of how the charges are flowing. More on this in the next unit – Electricity.
1
Summary of Electromagnetic Charges
1. What are some examples of static charge?
2. Why do these examples occur?
3. What does ‘static’ mean?
4. What does an atom look like?
5. All matter is made of atoms which are composed of negatively charged
electrons (e-) whirling around a nucleus of positively charged protons and
neutrons. What is the only atom that has no neutron?
6. In mechanics the basic property was mass. In electricity, the basic property is
_________________.
7. How do charges behave?
Like charges…
Opposite charges…
2
III. Models: Electroscope
Electrification by friction occurs when two surfaces are rubbed together. Examples of this
were discussed above when a positive charge was created by rubbing glass with silk and a
negative charge was created by rubbing rubber with fur. The following list details a larger
portion of the triboelectric sequence. When any two substances shown in this list are rubbed
together, the top one will become positively charged while the lower one will become negatively
charged. The further apart the two substances are in the list, the greater the electrification.
+
-
Asbestos
Fur (rabbit)
Glass
Mica
Wool
Quartz
Fur (cat)
Lead
Silk
Human skin,
Aluminum
Cotton
Wood
Amber
Copper, Brass
Rubber
Sulfur
Celluloid
India rubber
Charging by conduction means that the charging rod actually touches the electroscope’s knob.
Since there is contact, electrons from the knob would flow onto a positive rod or off of a
negative rod. Charging by conduction leaves the electroscope with a residual charge
IDENTICAL to that of the charging rod.
Charging by induction means that the charging rod is brought close to the electroscope’s knob
but NEVER touches it. If the electroscope is not grounded, it will remain neutral but be
temporarily polarized while the charging rod is in the immediate vicinity. That is, a positive rod
will induce the electrons in the scope to migrate to the knob. This redistribution of charge will
result in the leaves of the scope being positively charged.
http://online.cctt.org/physicslab/content/phyapb/lessonnotes/electrostatics/lessonelectrost
atics.asp
3
IV.
The atom
All matter is made of atoms. These atoms are composed of negatively
charges electrons (e-) that revolve around a positively charged nucleus (p+
and no).
Why don’t protons pull electrons into the nucleus?
Why don’t protons repel each other in the nucleus?
Electrons and protons have equal but opposite charges. Normal atoms have
exactly enough electrons to balance the protons in the nucleus, leaving the
atom with a net charge that is neutral.
Under certain circumstances electrons may be removed from an atom. When
this happens the atom becomes positively charged. A positively charged ION
is the result. A charged atom is called an ion.
V.
Materials
1. Conductors – electric charges move easily through this material, most
commonly a metal.
a. Reason – Metal have valence electrons (e- that are less tightly
bound to the atom). Therefore they are more likely to be
“stolen” by other atoms.
2. Insulators – electric charges are not easily transferred or moved.
a. Reason – Insulators don’t have these free electrons (e- that are
less tightly bound to the atom). Therefore they are not likely to
be “stolen” by other atoms.
4
VI.
How are charges accumulated on an object? (Use the three
models)
1. Friction – As two objects are rubbed against each other, e- ’s are
scraped off one object and deposited on the other.
Examples:
Balloon –
Rods –
Rub glass with a plastic transparency and e- ‘s leave the glass.
What is the net charge on the glass rod?
Rub a plastic ruler with fur and e- ‘s collect on the plastic.
What is the net charge on the ruler?
Other Examples:
When an object is positively charged, it has an excess of p+ ’s. It has
billions of charged particles overall, but an excess of one type, if it is
charged.
5
2. Contact - Electrons can flow from one material to another when they
are in contact (touching). The object you touch obtains the SAME
CHARGE as the object with which it was in contact.
Examples:
Rods and pith balls. Once they touch, they transfer the charge. Draw
it.
3. Induction –A neutral object can become charged just by holding it
close to a charged object.
Examples:
1. Electroscope:
2. Gold Leaf electroscope:
3. Charging Spheres:
4. Pith balls and charge induction by grounding:
5. Balloon and paper:
Pie Tin demonstration:
When you get SHOCKED, the excess charges are DISCHARGING. That
means electrons are leaving the object through the air between you and the
other object.
Allowing charges to move on or off of a conductor by touching it is called
GROUNDING.
6
How can neutral objects become charged?
When the object is near a charged surface, the atoms rearrange themselves
so the charges line up in one particular direction. This is called polarization.
The object is then said to be POLARIZED. Charges are induced temporarily
by aligning p+’s on one side and e-‘s on the other.
Suppose the positive rod is brought near to an insulator (as shown in the
diagram above), for example, a piece of paper or a section of a wall. Since
electrons are not free to move within an insulator, another process takes
place which still results in the paper or wall becoming polarized. The
particles in the insulator realign themselves - presenting an oppositely
charged layer towards the charged rod. This process is illustrated below.
positively charged rod
top surface "-"
polarized molecules
within the insulator
bottom surface "+"
7
Methods of Charging Lab
Part I. Rub the ruler with the fur.
1. In doing this, the ruler becomes ________________ charged. Why?
2. This process is called charging by ____________________.
Part II. Bring the ruler near (but not touching) the electroscope.
3. Draw a picture of what you see happen to the electroscope:
4. Why does the electroscope react in this way?
5. Show the distribution of charges on your drawing by using + and - symbols.
6. This process is called charging by ____________________.
Part III. While still holding the ruler near the electroscope, touch your finger to the top
of the electroscope and hold it there.
7. Describe what you see happen to the electroscope.
8. Why does this happen?
Part IV. Remove your finger and then remove the ruler (in that order).
9. Draw a picture of what you see happen to the electroscope:
10. Why does the electroscope react in this way?
11. Show the distribution of charges on your drawing by using + and - symbols.
12. The electroscope is left with a ______________ charge.
13. This process of charging is called _________________________.
8
Electrostatic Charging
1. Do plastic/rubber objects pick up or give up electrons?
2. How then do plastic/rubber objects get charged?
(positive or negative)
3. Does hair pick up or give up electrons?
4. How then does hair get charged?
(positive or negative)
5. What does GROUNDING mean?
Rules
1.
2.
3.
of electrostatics
Opposite charges attract, likes repel.
Conservation of charge: charge cannot be created or destroyed.
The electrons are the ones doing the moving.
9
WATER
1. Does water detect charge?
How can you tell?
2. Why is static electricity more “active” during dry weather?
When the air is humid, the water molecules discharge the electrostatic
charges so they don’t stay on the object as long as if the air were dry.
That’s why these demos work best this time of year.
Another important factor in electrostatics is humidity. If it is very humid,
the charge imbalance will not remain for a useful amount of time. Remember
that humidity is the measure of moisture in the air. If the humidity is high,
the moisture coats the surface of the material, providing a low-resistance
path for electron flow. This path allows the charges to "recombine" and thus
neutralize (discharge) the charge imbalance. Likewise, if it is very dry, a
charge can build up to extraordinary levels, up to tens of thousands of volts!
Think about the shock you get on a dry winter day. Depending on the type of
sole your shoes have and the material of the floor you walk on, you can build
up enough voltage to cause the charge to jump to the door knob, thus leaving
you neutral.
10
Coulomb’s Law
Coulomb’s Law describes the force between two charged objects. The
magnitude of the force that a tiny sphere with charge q1 and a second
charge q2, separated by a distance is
Each charged object exerts a force that is _____________________
on the other charged object.
The unit of charge is defined in terms of the amount of force it
produces. Charge is measured in Coulomb’s (imagine that), abbreviated C.
One Coulomb = the charge of ____________ electrons.
Charge of a lightning bolt = __________________.
Charge of an individual electron = __________________ and is called
____________________.
Example Problem: Show work below!
What is the electric force between 2 charges that are each 1 C and are
separated by 1 m?
11
Coulomb’s Law Problems
1. A plastic ball has a charge of +10-12 C.
a. Does it have an excess or a deficiency of electrons compared
with its normal state of electrical neutrality?
b. How many such electrons are involved?
2. What is the magnitude and direction of the force on a charge of +4 x
10-9 C that is 5 cm from a charge of +5 x 10-8 C?
3. Two charges, one of +5 x 10-7 C and the other of -2 x 10-7 C, attract
each other with a force of 100N. How far apart are they?
4. A negative charge of -2 x 10-4 C and a positive charge of +8.0 x 10-4 C
are separated by 0.3 m. What is the force between the two charges?
5. A test charge of +1 x 10-6 C is placed half way between a charge of +5
x 10-6 C and a charge of +3 x 10-6 C that are 20 cm apart. Find the
magnitude and direction of the force on the test charge.
Answers:
1) a) deficiency
3) 3 x 10-3 m
b) 6.25 x 106 electrons
4) 1.6 x 104 C
2) F = 7.2 x 10-4 N away from the +5 x 10-8 C
5) F = 1.8 N, acting toward the +3 x 10-6 C charge
12
The Electric Field
Charges alter the space around them. Call this an electric field. The electric
field extends outward from every charge. A second charge in the field will
react because it feels a force due to the field created.
Take a point charge, Q. A field surrounds it. Draw lines, but no arrows to
represent the field. It alters all the space around it; lines are just
representative of the field.
To determine the field exerted by Q  place a test charge in the field.
Test charge is positive, and so small that the force the test charge exerts
does not change the distribution of the charge creating the field (much like
a small mass at the surface of the Earth does not change the gravitational
field of the Earth).
The field lines are always directed to show how the force a positive charge
would feel in the space. Take + Q, and test charges a, b, and c.
Define the field by what force – magnitude and direction - the tiny positive
test charge feels. The further away, the weaker the force (Coulomb’s Law).
13
Call this condition of space created by a charge the electric field.
“E”
“E” at any point in space is defined as the force (magnitude and direction)
exerted on a tiny positive test charge, per the magnitude of the test
charge.
E = F/q units = N/C = Force per charge
“E” is a vector; it has magnitude and direction.
Also, by substitution:
E=
= kQ/d2
(test charge cancels out)
_________________________________________________________
Draw the following electric fields:
1. E near a single negative point charge.
_________________________________________________________
2. E between + and - charges.
14
3. E between to + point charges:
Matching and Definitions:
1. Test Charge
2. Electric Field, “E”
3. Electric Force
4. Electric Field Lines
5. All masses are surrounded by
6. All charges are surrounded by
7. Charge on 1 electron (or proton)
8. 1 Coulomb =
A. Indicate the direction of the field
B. Force produced by charges and
acting of charges.
C. Condition of space created by the
presence of charge.
D. Small positive charges used to
measure the intensity and direction
of an electric field.
E. 1.6 x 10-19 C
F. a gravitational field
G. 6.24 x 1018 charged particles
(electrons or protons)
H. An electric Field.
Problem Solving Examples:
1. Calculate the magnitude and direction of the electric field at point P
which is 30 cm to the right of point charge Q = -3.0 x 10-6 C.
2. Same as number 1 (above) only make Q positive.
15
Practice Problems:
1. A negative charge of 2.0 x 10-8 C experiences a force of 0.060 N to
the right in an electric field. What are the field magnitude and
direction?
2. A positive test chare of 5.0 x 10-4 C is in an electric field that exerts
a force of 2.5 x 10-4 N on it. What is the magnitude of the electric
field at the location of the test charge?
3. Suppose the electric field in problem 2 was caused by a point charge.
The test charge is moved to a distance twice as far from the charge.
What is the magnitude of the force that the field exerts on the test
charge now?
4. You are probing the field of a charge of unknown magnitude and sign.
You first map the field with a 1.0 x 10-6 C test charge, then repeat
your work with a 2.0 x 10-6 C test charge.
a. Would you measure the same forces with the two test charges?
Explain.
b. Would you find the same fields? Explain.
16
Lab: Balloons acting as an Electroscope
Honors Physics
Mr. Kuffer
Prep:
Measure the mass of a balloon. Blow up two balloons and attach then
to two lengths of string. Suspended at the same height so they will hang
slightly apart in still air, refer to figure 1. When suspended, the balloons will
act as a simple electroscope to detect the presence of electrostatic charge.
For best results, perform electrostatic experiments in dry air.
Figure 1: A representation of the suspended balloons


Fg
qA
qB
Activity:
1. Hold the fur near one of the suspended balloons and look for any
interaction between them.
2. Stroke both balloons with the fur. The balloons gain negative charges
from the fur and become negatively charge, while the fur becomes
positively charged.
3. Bring the two negatively charged balloons close together and observe the
interaction. Objects with similar charges repel each other. Hold the fur
near one of the balloons. Objects with opposite charges attract each
other.
17
4. Using a protractor measure the angle between the two strings, once the
balloons settle. Determine the force of balloon A on balloon B.
5. Measure the distance between the two balloons.
6. Use this force in combination with the distance you measured above to
determine the net charge on each of the balloons, assuming that the
balloons carry identical charges. To do this you will have to identify the
relationships that exist between the Force between the balloons, the
Distance that separates the balloons, and Charge of the balloons.
7. List your mathematical relationship below.
8. Share your mathematical relationship with Mr. Kuffer before continuing
the lab.
9. Stroke the glass rod with the piece of silk. Stroke the plastic rod with
the piece of flannel. Determine the charges of each of the materials by
holding them, one at a time near a negatively charged balloon. It may be
necessary to renew the charge on the balloon by rubbing it again with the
fur. The balloon serves as a reference: objects that attract it are
positively charged, while objects that repel it are negatively charged.
18
10. In your lab notebook, using a data table like the one below, determine the
charges of the marked materials(*) after being rubbed with the
materials in the first column. Record your results with a +/- symbol,
indicating its charge.
Glass rod*
Plastic rod*
Rubber rod*
Fur
Silk
Flannel
Critical thinking:
Lightning is an example of static electricity on a larger scale. To the best of
your ability, explain below how lightning works!
19
Static Electricity
Inducing a Positive Charge on a Sphere
http://www.physicsclassroom.com/mmedia/estatics/esn.html
1. What happens to the charges within the sphere as a negatively
charged object is brought near to the neutral sphere?
2. Draw it!
Prediction
Observation
20
3. What happen to the charges on the sphere if you touch the sphere
with your hand (ground)?
4. Draw it!
Prediction
Observation
5. What is the resulting charge on the sphere when you remove your
hand?
6. Draw it!
Prediction
Observation
21
Static Electricity
Charging a Two-Sphere System by Induction Using a Negative Object
http://www.physicsclassroom.com/mmedia/estatics/esn.html
1. What happens to the charges within the spheres as a negatively
charged balloon approaches the left sphere?
2. Draw it!
Prediction
Observation
22
3. What happen to the charge in each sphere if you remove the sphere
on the right (no longer in contact with the sphere on the left)?
4. Draw it!
Prediction
Observation
5. What is the resulting charge on each of the spheres?
6. Draw it!
Prediction
Observation
23
Static Electricity
Charging an Electroscope by Induction Using a Negatively-Charged Balloon
http://www.physicsclassroom.com/mmedia/estatics/esn.html
1. What happens as a negatively charged balloon approaches the
electroscope?
2. Draw it!
Prediction
Observation
24
3. What happens if you touch the needle with your hand (ground)?
4. Draw it!
Prediction
Observation
5. What happens if you now remove your hand?
6. Draw it!
Prediction
Observation
25
Static Electricity
Charging an Electrophorus by Induction Using a Negatively-Charged Object
http://www.physicsclassroom.com/mmedia/estatics/esn.html
1. What happens to the charges within a neutral pie tin as it approaches
the Styrofoam plate?
2. Draw it!
Prediction
Observation
26
3. What happens if you touch the pie tin with your hand (ground)?
4. Draw it!
Prediction
Observation
5. What is the resulting charge on the pie tin?
6. Draw it!
Prediction
Observation
27
Lightning Video Quiz
NOVA – Lightning
1. Every minute _____________ bolts of lightning strike the Earth.
2. __________ _____________ demonstrated that lightning was simply
____________ ______________.
3. The saying goes “lightning never strikes the same place twice”. Is this
true? ________
4. Lightning is ________ times hotter than the surface of the Sun.
5. Complete the illustration of the lightning strike to the right.
Use symbols (+/-) to indicate charge.
Briefly explain how it works!
6. Surprisingly, lightning travels from the ________ up to the _______.
7. If it could be harnessed, lightning could power a 100-Watt bulb for
____ months.
8. Power Technologies induces lightning strikes with the help of _________.
9. A storm cloud has layers of __________________________________.
10. Complete the diagram of the storm cloud below.
28
11. Some believe that a rare form of _______________ is created when
lightning strikes the sand.
12. A team of scientist release a weather balloon with several instruments to
measure cloud activity. Where does the balloon enter the cloud?
________________
13. Where was the team hoping the balloon would enter the cloud?
________________
14. Was the launch a bust? ____ Why / Why not?
____________________________________________________
____________________________________________________
15. What famous golfer was struck by lightning on the golf course in 1975?
__________ ____________
16. Do you think his description of the event had enough sound effects? ___
17. What was your personal favorite? _____________________________
18. Where should you not seek shelter during a storm?
____________________________________________________
19. Although it is not predictable, lightning usually seeks the…
___________ of _________ ____________________.
20. A car’s rubber tires keep you safe from lightning strikes… Right? _____
Explain: ______________________________________________
____________________________________________________
21. A ______________ is made when lightning strikes sand.
22. the average lightning strike is about ___________ in diameter.
23. How does Dan Davis launch the rockets? ________________________
Why? _______________________________________________
____________________________________________________
24. Are underground cables lightning safe? ______
Why / Why not? _______________________________________
____________________________________________________
____________________________________________________
29
30
Exploring the Van de Graff Generator
Use How Stuffworks.com to explore
1. Create a triboelectric series for the following materials.
a) hard rubber
b) steel
c) wool
d) Teflon
e) Rabbit fur
f) Glass
g) Styrofoam
h) Silk
i) Human hair
j) Silicon
31
2. Explain how possible charge imbalances may occur._______________
_______________________________________________________
_______________________________________________________
_______________________________________________________
_______________________________________________________
3. What role does humidity play in electrostatics? ________________
_______________________________________________________
__________________Why?________________________________
_______________________________________________________
4. What is plasma?________________________________________
_______________________________________________________
_______________________________________________________
5. Draw a Van de Graff and label all of the part identified.
6. Explain how the Van de Graff works. Be clear and concise! Be
thorough!_____________________________________________
_______________________________________________________
_______________________________________________________
_______________________________________________________
_______________________________________________________
7. Explore how to make a Van de Graff.
8. What experiments can be done with a Van de Graff?_____________
_______________________________________________________
_______________________________________________________
_______________________________________________________
_______________________________________________________
32
Additional Problems and Review
33
34
35
36
37
38
39
40
41