Electrostatics - Key
Vocabulary
Term
Definition
Proton
Positively charged particle found with neutrons in
the nucleus of an atom.
Electron
A low mass particle with a negative charge that
occupies the energy levels in an atom outside the
nucleus; electrons are involved in chemical bonds
and reactions.
Neutron
Electrically Neutral
Static Electricity
Law of
Electrostatics
Charging by:
• Conduction
•
Induction
•
Friction
Polarization
Coulomb’s Law
An uncharged/neutral charge particle found in the
nucleus of an atom; one of three particles in
normal atoms, along with protons and electrons.
Where an object has the same number of protons
and electrons thus has a net charge of zero.
A buildup of either positive or negative charge;
consists of isolated motionless charges, like those
produced by friction.
Like charges repel and unlike charges attract.
Charging an object by direct contact.
Charging an object by bringing a charged object
close to a neutral object.
Charging by rubbing two objects together.
The rearranging of charges.
The formula that states the force between two
electric charges is proportional to the product of
the two charges divided by the square of the
distance between them.
The Atom
The atom is made of three basic subatomic particles.
Proton – has a positive charge. It is in the nucleus of the atom.
Neutron – has a neutral charge. It is in the nucleus of the atom.
Electron – has a negative charge. It orbits around the outside of the atom.
Like charges repel --------- so a positive and a positive
charge repel.
Unlike charges attract --------- so a positive and a
negative charge attract.
Charge of Protons and Electrons
The unit of charge is the Coulomb. Charles Augustin de Coulomb (1736-1806)
A French physicist who performed the first accurate measurements of the force between
charges.
A single proton has a charge of: 1.6 x 10-19 C
An electron has a charge of an electron: - 1.6 x 10-19 C
Charge of an Object
The charge of an atom is neutral. There are equal number of protons
& electrons which provides a perfect cancellation between positive
and negative in matter leaving a net charge of zero. It is
then electrically neutral.
Draw another example below of an electrically neutral object.
Charged Object
An object is charged when its net charge is not zero.
An object with more negative than positive charge has a net negative charge overall and
more positive than negative charge; the object has a positive net charge.
Draw another example below of
(A) A negatively charged object.
(B) A positively charged object.
+4
-12
-8
+10
-6
+4
Family Guy Think-Pair-Share
“the results may shock you”
Directions
You will be divided into different groups, A – E, and your group will be responsible for
answering one of the following questions and presenting your answer to the class. Watch
the following video, then answer the questions below:
Questions
A. How does Peter become electrically charged? What evidence suggests he is
picking up charge? Sketch the charges on Peter’s picture below.
B. How does Peter get rid of the charge he picks up? What evidence illustrates that
excess charge leaves his body? Sketch the charge leaving his body below.
C. Why does Peter rub his feet against the carpet? Would he produce the same
results if instead he rubbed his feet against a wood floor? Why or why not?
D. Does Peter actually have to touch another person or object to get “rid” of the
charge on his body? Why or why not? Sketch below.
E. What do you think would happen if Peter were to rub his feet against the carpet
for a longer amount of time? Develop a relationship between “rubbing” and
“amount of time.”
FRICTION: GETTING RUBBED THE RIGHT WAY
The frictional charging process results in a transfer of electrons between the two objects
that are rubbed together. Rubber has a much greater attraction for electrons than animal
fur. As a result, the atoms of rubber pull electrons from the atoms of animal fur, leaving
both objects with an imbalance of charge. The rubber balloon has an excess of electrons
and the animal fur has a shortage of electrons. Having an excess of electrons, the rubber
balloon is charged negatively. Similarly, the shortage of electrons on the animal fur
leaves it with a positive charge. The two objects have become charged with opposite
types of charges as a result of the transfer of electrons from the least electron-loving
material to the most electron-loving material.
Frictional charging is often demonstrated in Physics class. Two rubber balloons can be
suspended from the ceiling and hung at approximately head height. When rubbed upon a
teacher's head, the balloons became charged as electrons are transferred from the
teacher's fur to the balloons. Since the teacher's fur lost electrons, it became positively
charged and the subsequent attraction between the two rubbed objects could be observed.
Of course, when the teacher pulls away from the balloons, the balloons experienced a
repulsive interaction for each other.
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Activity – Charging by Friction
Materials: plastic tube, paper bits, styrofoam coffee cup, wool or synthetic fur
Purpose:
The purpose of this activity is to study how different objects interact with each other. If
an object is charged, will it attract or repel an object?
Part A: Electrical Interactions
1) Tear part of a piece of paper into small bits. Take the plastic tube and bring it
close to the bits of paper. Can you lift the bits of paper by touching them with the
straw?
2) Now rub the plastic tube briskly with fur or wool or against your hair and try to
lift the bits of paper from the table. Can the scraps of paper be lifted even if you
do not allow the rubbed plastic tube to touch them first?
3) Apparently, after the plastic tube has been rubbed there is an interaction between
the tube and paper which is capable of lifting the scraps of paper. DEFINITION:
The force involved in this interaction is called an electrical force, and was first
observed by the Greeks, who found that pieces of amber (in Greek, elektron)
attracted other things after being rubbed with fur. Materials which are capable of
attracting the bits of paper are said to be electrically charged.
4) Are the bits of paper charged according to the definition given in #3?
5) Devise and perform an mini-experiment to determine whether the paper is
charged. Describe the procedure of your mini-experiment.
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CONDUCTION: CHARGE-SHARING
When objects having different charges touch, conduction may occur. Conduction is the
transfer of electric charge by direct contact. As you might suspect, conduction works
much better between conductors than it does between insulators. Consider the following
examples involving conductors that are identical except for their charge.
To explain the process of charging by contact, we will first consider the case of using a
negatively charged metal sphere to charge a neutral needle electroscope. Understanding
the process demands that you understand that like charges repel and have an intense
desire to reduce their repulsions by spreading about as far as possible. A negatively
charged metal sphere has an excess of electrons; those electrons find each other repulsive
and distance themselves from each other as far as possible. The perimeter the sphere is
the extreme to which they can go. If there was ever a conducting pathway to a more
spacious piece of real estate, one could be sure that the electrons would be on that
pathway to the greener grass beyond. In human terms, electrons living in the same home
despise each other and are always seeking a home of their own or at least a home with
more rooms.
Given this understanding of electron-electron repulsions, it is not difficult to predict what
excess electrons on the metal sphere would be inclined to do if the sphere were touched
to the neutral electroscope. Once the contact of the sphere to the electroscope is made, a
countless number of excess electrons from the sphere move onto the electroscope and
spread about the sphere-electroscope system. In general, the object that offers the most
space in which to "hang out" will be the object that houses the greatest number of excess
electrons. When the process of charging by conduction is complete, the electroscope
acquires an excess negative charge due to the movement of electrons onto it from the
metal sphere. The metal sphere is still charged negatively, only it has less excess negative
charge than it had prior to the conduction charging process.
Before
After
Net charge (VDG):
Net charge (VDG):
Net charge (fur):
Net charge (fur):
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Thinking About What You Observed
1. A neutral metal sphere is touched by a negatively charged metal rod. As a result, the
sphere will be negative and the metal rod will be neutral. Select the two answers in their
respective order.
a. positively charged b. negatively charged c. neutral
d. much more massive
2. A neutral metal sphere is touched by a negatively charged metal rod. During the
process, electrons are transferred from the _____ to the _____ and the sphere acquires a
_____ charge.
a. neutral sphere, charged rod, negative
positive
c. charged rod, neutral sphere, negative
positive
b. neutral sphere, charged rod,
d. charged rod, neutral sphere,
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INDUCTION: SAFE CHARGING
It is possible to use one charged object to charge another object without the two ever
coming into direct contact. Charging by induction is a method of using one object to
charge another without changing the net charge on the first. Consider the following
sequence.
•
A negatively charged object (conductor or insulator) is brought near a
neutral object (preferably a conductor).
When a charged object loses its charge, we say it has been neutralized. An object can be
neutralized if it is allowed to conduct its charge to something much larger than itself. This
process is called grounding because the larger object is often the earth. (In Great Britain,
it’s called earthing.)
Before
After
Net charge (VDG):
Net charge (VDG):
Net charge (ball):
Net charge (ball):
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Polarization
In general terms, polarization means to separate into opposites. In the political world, we
often observe that a collection of people becomes polarized over some issue. For
instance, we might say that the United States has become polarized over the issue of the
death penalty. That is, the citizens of the United States have been separated into opposites
- those who are for the death penalty and those who are against the death penalty. In the
context of electricity, polarization is the process of separating opposite charges within an
object. The positive charge becomes separated from the negative charge. By inducing the
movement of electrons within an object, one side of the object is left with an excess of
positive charge and the other side of the object is left with an excess of negative charge.
Charge becomes separated into opposites.
The polarization process always involves the use of a charged object to induce electron
movement or electron rearrangement. In the above diagram and accompanying
discussion, electrons within a conducting object were induced into moving from the left
side of the conducting can to the right side of the can. Being a conductor, electrons were
capable of moving from atom to atom across the entire surface of the conductor.
A common demonstration performed in class involved bringing a
negatively charged balloon near a wooden door or wooden cabinet. The
molecules of wood will reorient themselves in such a way as to place
their positive charges towards the negatively charged balloon. The
distortion of their electron clouds will result in an alignment of the wood
molecules in a manner that makes the wooden cabinet attracted to the
negatively charged balloon. In human terms, one might say that the wood
does some quick grooming and then places its most attractive side
towards the balloon and its most repulsive side away from the balloon. In
the world of static electricity, closeness counts. The negative balloon is
closer to the positive portion of the wood molecules and further from the
more repulsive negative portion. The balloon and the wall attract with sufficient force to
cause the balloon to stick to the wall.
Charged rubber rods are placed near a neutral conducting sphere, causing a redistribution
of charge on the spheres. Which of the diagrams below depict the proper distribution of
charge on the spheres? List all that apply.
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In the above situation, the conducting sphere is ____. List all that apply.
a. charged
b. uncharged (neutral)
c. polarized
Which of the diagrams below best represents the charge distribution on a metal sphere
when a positively charged plastic tube is placed nearby?
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Notes – Coulomb’s Law
Electrical force also has a magnitude or strength. Like most types of forces, there are a
variety of factors which influence the magnitude of the electrical force. Two like-charged
balloons will repel each other and the strength of their repulsive force can be altered by
changing three variables.
1. Charge
2. Charge
3. Distance
First, the quantity of charge on one of the balloons will affect the strength of the repulsive force.
The more charged a balloon is, the greater the repulsive force. Second, the quantity of charge on
the second balloon will affect the strength of the repulsive force. Gently rub two balloons on top
of your head and they repel a little. Rub the two balloons vigorously to impart more charge to
both of them, and they repel a lot.
Finally, the distance between the two balloons will have a significant and noticeable affect upon
the repulsive force. The electrical force is strongest when the balloons are closest together.
Decreasing the separation distance increases the force. The magnitude of the force and the
distance between the two balloons is said to be inversely related.
The quantitative expression for the affect of these three variables on electric force is known as
Coulomb's law. Coulomb's law states that the electrical force between two charged objects is
directly proportional to the product of the quantity of charge on the objects and inversely
proportional to the square of the separation distance between the two objects. In equation form,
Coulomb's law can be stated as
q1= quantity of charge on object 1
q2= quantity of charge on object 2
d= distance between both objects
k=9.0 x 109 N • m2 / C2
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•Force is inversely proportional to the square of the distance between the charges. Therefore, if
the distance increases by a factor of 2, the force decreases by a factor of 4.
• Force is proportional to the strength of each charge.
• When the two charges have the same sign (positive or negative), the force between them is
repulsive because like charges repel.
• When the charges have opposite signs, the force between them is attractive because unlike
charges attract.
Think about it . . .
We know that like charges repel. So if we have two
positive charges or two negative charges they will
repel one another. The product of both values is a
positive number for force. (+) indicates repulsion.
We also know that opposites attract so if we have
two unlike charges they will attract one another. The
product of both values is a negative number for force.
(-) indicates an attractive force.
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Coulomb’s Law Practice Problems
1. What happens to the force between two charges if the distance between them is tripled?
𝐹 = 𝑘
𝑞1 𝑞2
𝑞1 𝑞2
𝑞1 𝑞2
1
=
𝑘
=
𝑘
=
𝐹
𝑟2
(3𝑟)2
9𝑟 2
9
2. What happens to the force between two charges if the distance between them is
quadrupled?
𝐹 = 𝑘
𝑞1 𝑞2
𝑞1 𝑞2
1
𝑞1 𝑞2
= 𝑘
= 𝑘
=
𝐹
2
2
2
𝑟
(4𝑟)
16𝑟
16
𝐹 = 𝑘
𝑞1 𝑞2
𝑞1 𝑞2
𝑞1 𝑞2
=
𝑘
=
𝑘
= 4𝐹
1
1 2
𝑟2
(2 𝑟)2
𝑟
4
3. What happens to the force between two charges if the distance between them is cut in
half?
(𝑁𝑜𝑡𝑒: 4 𝑓𝑙𝑖𝑝𝑠 𝑡𝑜 𝑡𝑜𝑝)
4. What happens to the force between two charges if the magnitude of one charge is
doubled?
𝐹 = 𝑘
𝑞1 𝑞2
(2𝑞1 )𝑞2
2𝑞1 𝑞2
= 𝑘
= 𝑘 2 = 2𝐹
2
2
𝑟
𝑟
𝑟
5. What happens to the force between two charges is the magnitude of both charges is
doubled?
𝐹 = 𝑘
𝑞1 𝑞2
(2𝑞1 )(2𝑞2 )
(2𝑞1 )(2𝑞2 )
= 𝑘
= 𝑘
= 4𝐹
2
2
𝑟
𝑟2
𝑟
6. What happens to the force between two charges if the magnitude of both charges is
doubled and the distance between them is doubled?
𝐹 = 𝑘
𝑞1 𝑞2
(2𝑞1 )(2𝑞2 )
(2𝑞1 )(2𝑞2 )
4
=
𝑘
=
𝑘
=
=1𝐹
𝑟2
(2𝑟)2
4𝑟 2
4
7. What happens to the force between two charges if the magnitude of both charges is
doubled and the distance between them is cut in half?
𝐹 = 𝑘
𝑞1 𝑞2
(2𝑞1 )(2𝑞2 )
(2𝑞1 )(2𝑞2 )
4𝑥4
=
𝑘
=
𝑘
=
= 16 𝐹
1
1 2
𝑟2
1
(2 𝑟)2
𝑟
4
(𝑁𝑜𝑡𝑒: 4 𝑓𝑙𝑖𝑝𝑠 𝑡𝑜 𝑡𝑜𝑝)
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Name ______________________________________________ Date _________ Period ______
Balloons and Static Electricity
Research Question
After reading the lab, create your own research question below then formulate a hypothesis to
your research question:
______________________________________________________________________________
______________________________________________________________________________
Pre-Lab Discussion
As you are most likely aware, charge can be described as either positive or negative. When + or
– charges are present in equal amounts, the object is considered electrically neutral. Large
numbers of negative particles (electrons) can be transferred in certain circumstances from one
neutral surface to another. This generates an excess of + charges on one object and – charges on
the other. The following simulation illustrates the movement of both + and – charges and the
resulting attractive and repulsive forces.
Procedure
Using the Site
• Access the site by either clicking on the
link: http://phet.colorado.edu/en/simulation/balloons or by following:
• Once your application has started, click “Reset All”. Make sure that only the “Show all
charges” and “Wall” buttons are selected.
Part 1 – One Balloon and Sweater
1. Look at the balloon. What can you say about its overall charge? (Hint: count both types
of charges)
2. Click and drag the balloon and rub it against the sweater. Describe and sketch
what happens to the balloon?
3. How did the balloon get charged, with what type of charge? What process gave the
balloon this charge?
4. What happened to the sweater? How did it get charged?
looks like.
Sketch what the sweater
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5. Bring the balloon in the middle, between the sweater and the wall. What happens to the
balloon when you let it go? Explain why. Sketch the sweater, balloon and wall (sketch
the charges on each and the direction the balloon moves)
Part 2 – One Balloon and Wall
6. What is the overall charge of the wall?
7. Predict what you think will happen when the balloon is brought close to the wall?
8. Bring the balloon in contact with the wall. What happens to the charges in the wall?
Explain and sketch this process.
9. Let go of the balloon. What happens? Explain why.
Part 3 – Two Balloons
10. Click the “Reset All” button. Select “Show all charges”, “Wall” and “Two balloons”.
What can you tell about the overall charge of all the objects in your simulation window?
11. Select “Show charge differences”. Rub each balloon against the sweater. What happens
to each one of them? Sketch this.
12. Why are the two balloons stuck on the sweater?
13. Try to get one balloon off the sweater by using the other balloon. Can you do it? If yes,
explain why this is possible.
Questions
Based on your observations, attempt to explain why you sometimes see flashes of light when
removing a fleece jacket in a dark room.
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Rubric – Balloons & Static Electricity
Physics
Heading
(1 pt.)
Research Question
(2 pts.)
Hypothesis
(5 points)
Sketch & Description
(2 pts.)
Procedure Question
(15 pts.)
Total =
Student labels the date in the
upper left hand corner and title of
lab in all CAPS in lab notebook.
Student generates a research
question after reading the pre-lab
discussion.
Student states hypothesis and
explains their prediction.
Student sketches materials used in
lab and a description of what they
did.
Student answers all questions
accurately and in complete
sentences making sketches where
necessary.
/25 points
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John Travoltage
Physics
Purpose
To determine the likelihood of getting a “carpet shock” (based on controllable variables), and to
avoid or produce such shocks under a series of challenging conditions
Apparatus
computer PhET simulation, “John Travoltage” (available at http://phet.colorado.edu)
Discussion
When you walk across a carpet and touch a metal doorknob, you might feel a small shock. The
friction between the different materials in your footware and the carpet gives you a static charge.
(Physicists call it a triboelectric charge, meaning a charge from frictional contact.) The metal
doorknob is a conductor, so the excess charge on your body jumps to the knob rapidly, creating the
spark. Your nerves are sensitive to electrical impulses, so you feel the shock.
In the simulation, John Travoltage’s foot can be rubbed across a carpet so that his body collects
charge. His arm can be rotated so that a spark will jump from his finger to the doorknob.
Procedure
Step 1: Start the computer and allow it to complete its startup process.
Step 2: Launch the PhET simulation, “John Travoltage.” If you need assistance, ask your instructor for
help. Go to http://phet.colorado.edu/en/simulation/travoltage and click “Run Now.”
Step 3: Rub John Travoltage’s foot back and forth against the carpet until a spark jumps from his finger to the
doorknob.
How does John Travoltage react when the spark jumps? (hint: turn the volume up) Is there anything he says?
____________________________________________________________________________________
____________________________________________________________________________________
Step 4: Explore the simulation.
a.
Move John Travoltage’s hand close to the doorknob, then rub his foot on the carpet again. Compared
to the process you observed in Step 3, what happens differently this time?
____________________________________________________________________________________
____________________________________________________________________________________
b.
How can you get the maximum charge built up on John Travoltage without producing a spark?
Describe what you need to do with John’s foot.
____________________________________________________________________________________
____________________________________________________________________________________
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c.
Under what circumstances—if any—will a spark jump from John Travoltage’s foot to the doorknob?
Why do you suppose this is (in terms of the physics involved)? (Hint: think about what John is
wearing on his foot and what would prevent the charge from moving from his foot to the doorknob.)
____________________________________________________________________________________
____________________________________________________________________________________
Step 5: Meet the challenges.
a. Rotate John Travoltage’s arm so that his finger is pointing directly at the doorknob. Carefully rub his
foot against the carpet without producing a spark. What is the greatest number of charges you can
get onto John Travoltage’s body? (Estimate how many small spherical charges can be collected on his
body before a discharge occurs?)
____________________________________________________________________________________
____________________________________________________________________________________
b.
Produce the longest spark possible: the spark is between John’s hand pointing directly away from
the knob and the doorknob, itself. (Hint: The correct procedure requires less than a minute to
complete.) Describe what you did?
____________________________________________________________________________________
____________________________________________________________________________________
Summing Up
1.
Under which conditions is a spark most likely to jump between John Travoltage and the doorknob?
Describe in terms of the amount of charge on his body and the distance between his hand and the
doorknob.
____________________________________________________________________________________
____________________________________________________________________________________
2.
Consider two variable quantities: the amount of charge Q on John Travoltage’s body and the distance
d from his finger to the doorknob.
What can you conclude about the amount of charge and the likelihood of a spark jumping? (ie as you
increase the amount of charge does that increase or decrease the likelihood of a spark jumping?)
____________________________________________________________________________________
____________________________________________________________________________________
What can you conclude about the distance John’s finger is from the doorknob and the likelihood of a
spark jumping? (ie as you increase the distance from the doorknob does this increase or decrease
the likelihood of a spark jumping?)
____________________________________________________________________________________
____________________________________________________________________________________
3.
When John Travoltage’s hand is turned away from the knob, no spark will start. But in Step 5, you
sustained a spark from that configuration. What does this say about the conductivity of air? (in
other words does air conduct electricity?)
____________________________________________________________________________________
____________________________________________________________________________________
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