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PPT
Electric Charge
Developer Notes
1. This might be broken into separate activities for magnets and then for static electricity. Or we
could skip the magnets and do them later.
2. Perhaps the balloons should be done as a demo. Sticking to the wall isn’t really opposites
attracting – it results from induction.
3. Need to straighten out Q vs. q. Hewitt and Giancoli use both. I haven’t been able to figure
out their rule. Q also refers to heat, so it makes sense to me to use q.
4. Need to ensure that the students understand charge comes from little bits of stuff – electrons
moving around. Like ants crawling all over stuff. Except there are two kinds that are friends
and enemies. Dress the class in + and – and make them move around. Use boundaries,
uneven numbers, induction.
Version
01
02
03
Date
2004/07/06
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Who
sc
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dk
Revisions
Initial version
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Added information on material construction
Added reading material
Added activity material
Expanded goals
Added exercises
Goals
1. Students should know that there are two types of magnetic charge (pole), north and south.
2. Students should know that in magnets, opposites attract and likes repel.
3. Students should know that magnetic charge can do work.
4. Students should know that there are two types of electric charge, positive and negative.
5. Students should know that for electric charge, opposites attract and likes repel.
6. Students should know that electric charge can do work.
7. Students should understand that most objects are electrically neutral and that electric charge
is caused by an imbalance of protons and electrons.
8. Students should understand that charge is a whole number multiple of the charge on an
electron or proton.
9. Students should understand that the charge on an electron and proton are equal and opposite.
10. Students should know and be able to use the equation E = F/q.
11. Students should know and be able to use the equation V = W/q (V = PE/q?).
12. Students should know and be able to use the equation F = kqq/r2.
13. Students should know that there are parallels between mechanics and electrics.
Concepts & Skills Introduced
Area
Physics
Physics
Physics
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Concept
Magnetic charge (pole)
Electric charge
Static electricity
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Standards Addressed
Time Required
Warm-up Question
Presentation
Static electricity is tricky. The same material from different manufacturers or that has been
handled differently, or on different days, may not produce the same results. Humidity makes it
more difficult.
The primary goals in this activity are to get the students to understand that
 most objects have a neutral charge under normal conditions,
 there are two kinds of charge, - and +,
 opposite charges attract and likes repel,
 the – charge comes from electrons and the + charge from protons,
 electrons move around while protons stay put.
If the students can understand those principles, they should be able to understand what is going
on in the activities. It is impossible for us to show protons and electrons and the fact that the
electrons move while the protons don’t – the students will just have to take that on faith.
These are the steps to show the principles:
 Objects are neutral. Show two identical object that don’t attract each other.
 Objects can take a charge and likes repel. Rub both objects and show that they now repel
each other.
 Opposites attract. Rub a third object and show that it attracts the first two.
 Electrons move around.
Do we have to show that static charge and magnetic charge are different types?
Use magnets as an analogy for charge. There are two types: opposites attract, likes repel. You
need a 3rd object to establish the relationship between the types of magnets/charges. This can be
done as a challenging activity for the students, or it could be done as a demo. The magnets
should NOT have N and S marked on them.
The trouble with the balloon demo or activity is that charged objects stick to neutral objects
because they induce a charge in them.
Electrical force is similar to gravity (Fe  q1q2/r2), except there can be attraction or repulsion
(with gravity there is only attraction).
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Charge - fundamental property, two different types + & - (arbitrary), unit is Coulomb, which is
the number of excess protons or electrons.
Static electricity= when charges don’t move
Recall that atoms are made up of charged particles, protons & electrons- these are the smallest
overall packages of charge. Matter can get a net charge by the gain or loss of electrons.
Here’s a demo to show that there are two types of electrical charge, and that (like magnets),
opposites attract and likes repel. Charge up a balloon and stick it to the wall. Charge up another
balloon and stick it to the wall. Are the charges the same or different? Try moving the balloons
next to each other. They’ll repel, so likes repel, and opposites attract, just like magnets.
Assessment
Writing Prompts
Relevance
Answers to Exercises
1.
Answers to Challenge/ extension
1.
Equipment
Static electricity is tricky! You’ll need to try your equipment carefully before using it in class.
Equipment that worked at one time may stop working on another day or when another person
tries it. You may have to do these activities as demonstrations. If the relative humidity is high,
the materials won’t work as well. Try to keep the materials warm and dry right up until you use
them. Keep oils (from hands) off. You may need to clean them with alcohol.
There are many examples of static electricity setups on the web. Here is what we have found to
work well, tested in Hawaii, where the humidity tends to be high. The real trick is to generate
charge.
An electrophorus with an electroscope attached works well for this activity. An electrophorus
consists of a plate (like styrofoam) that can be charged with static electricity, and a separate
metal plate with an insulating handle. Electrify the charging plate by rubbing it (wool or fur work
well with Styrofoam). Place the metal plate on the charging plate and momentarily ground the
top surface. Remove the metal plate and it holds a charge opposite to the charging plate.
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In our case we attach an electroscope to the metal plate to show the charge. An electroscope is a
device with two hanging leaves. When the leaves are charged with a like charge, they separate
from each other. An electroscope is typically in a bottle to avoid the effects of air movement, but
the bottle itself can take a charge and affect the movement of the leaves. I have left the leaves
exposed for simplicity.
Electrophorus
This design is based on a design by the Exploratrorium.
http://www.exploratorium.edu/snacks/charge_carry.html. There are many other designs available
on the web.
Materials have different affinities for electrons – some like to keep electrons and some like to
give them up. Here is a list of materials listed in approximate order from positive to negative:
rabbit fur, leather, Lucite, Bakelite, acetate, glass, quartz, mica, Nylon, wool, cat fur, silk, paper,
cotton, steel, wood, amber, resin, metal, polystyrene, polyethylene, Teflon. Rub two pieces of
material together to generate a charge. The farther apart on the list the two items are, the stronger
the charging effect. Steel is neutral.
Materials to generate charge
The charging plate needs to be capable of generating charge. It should be flat and large.
 Styrofoam cooler – The cheap type, made of little balls, works well. You can get big flat
pieces by breaking apart the cooler.
 Foam coffee cups – The ones labeled PS (polystyrene, type 6) work well.
1. Foam dinner plate – This didn’t work when I tried it. Apparently, not every piece of white
foam generates charge. Many plastic foam plates, trays, and cups have some anti-static
additive.
 Teflon - Teflon is supposed to work well. I wasn’t able to find any Teflon blocks. Plastic
cutting blocks are not Teflon.
 Plastic cutting blocks – The ones I tried didn’t work at all.
 Plastic freezer bag – It didn’t work at all.
 Overhead transparencies – Didn’t work at all.
 Plastic wrap (Cling) – Didn’t work at all.
 Plastic ruler – Works well. Clear plastic seems to be better than colored. Smooth better than
bumpy.
Materials for the metal plate
The plate should be metal, flat, stiff, and as large as the charging plate.
1. Aluminum pie tin
2. Aluminum foil wrapped around cardboard and taped in place
3. Sheet metal plate
4.
Materials for the metal plate handle
The handle needs to be non-conductive.
 Foam coffee cups – The ones labeled PS (polystyrene, type 6) work well. They can be
attached with tape, but tape doesn’t hold well
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Styrofoam trays or plates – These work pretty well, although they are fragile. Cut from an
edge, they aim up.
Wood dowels – Any kind, but wood tends to hold hand oils and absorb water so that it
becomes conductive.
Plastic rod –
Plastic drinking straws – They work pretty well, but the charge still leaks off through them
slowly.
Foam thread spools – Didn’t try these.
String – OK, but not stiff enough to control the plate.
Tape – Many tapes are conductive, so won’t work.
Cardboard – Too conductive in high humidity, doesn’t work.
Plastic ruler – Works well with a pie plate, but not with a flat plate.
Wax cups – Don’t work, they are too conductive.
Material for the electroscope leaves
The leaves should be light, flat, and straight.
 Paper – I have had the best luck with paper. It is cheap, common, tends to stay straight, and is
light.
 Aluminum foil – Aluminum foil works well, but it tends to bend and can take a curl.
 Overhead transparency – Works OK. It is tough, stays straight and flat, but it is hard to see.
 Video tape - Works OK, but it tends to curl, and is magnetic, so can take a permanent charge.
 Anti-static bags for electronics – The silvery, transparent kind works well, but it tends to hold
a crease. It may be hard for some people to find.
 Anti-static bags for electronics – The black, opaque kind is said to work well. I haven’t tried
it.
 Plastic freezer bags (polyethylene) don’t work. They curl and stick together.
Material to support the electroscope leaves
1. Jumbo paper clip – It’s my favorite for everything, and it works well.
The whole assembly
For storage, it is nice to be able to break the assembly down. Here are some examples of
assemblies.
This assembly uses a circle of cardboard wrapped with aluminum foil taped in place. The handle
is two pieces of Styrofoam produce tray held on by binder clips. The handles are notched at the
end to interlock. The leaf holder is a paper clip held on by a binder clip. The leaves are notebook
paper with reinforcing rings.
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This assembly uses an aluminum pie tin. The handle is a plastic ruler held on with binder clips.
The leaf holder is the same paper clip held on with a binder clip, and the leaves are the same
notebook paper with reinforcing rings.
This one uses a ruler and wool to generate the charge. The leaves are hung on a paper clip that is
stuck through a foam cup. The cup insulates the paper clip from ground. It doesn’t have the area
to generate as much charge as the previous examples, but it works well.
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Background
You know that all objects have mass and that every object attracts every other object
gravitationally. There is just one kind of mass and it always attracts other mass - it never repels
it.
You have probably played with magnets, and you know that magnets attract each other, too, but
they can also repel each other, so the force is different from gravitational force.
Problem
For magnets, figure out how to prove whether likes or opposites attract.
Materials
3
magnets (unmarked)
Procedure
Do not write on the magnets!
1. Play with the magnets. You’ll find that some ends attract each other, and some repel each
other.
2. Take two magnets and determine which ends attract and which repel.
3. Use the third magnet to determine whether the ends that attract are the same or different.
Summary
1. With magnets, do likes or opposites attract?
2. How do you know?
Background
You have probably felt “static” when putting on or taking off clothes, combing your hair,
walking across carpet, or by rubbing a balloon in your hair.
Problem
Static is electric charge. Does electric charge have energy? Is there more than one type of electric
charge? If so, do opposites or likes attract?
Materials
2
balloons
Procedure
Blow up the balloons.
1. Charge up a balloon by rubbing it on your hair. The balloon will get an electric charge. You
can see and feel the effects of the charge by holding the balloon near your hair.
2. Put the balloon against a wall. It should stay there.
3. Use the second balloon to determine if likes or opposites attract.
Summary
1. Does electric charge have energy? How do you know?
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2. Is there more than one type of electric charge? How do you know?
3. Do like or opposite electric charges attract each other? How do you know?
Background
You know that there are two types of static charge and that opposites attract and likes repel.
Think about charge as you go through this activity.
Problem
Learn more about static electricity.
Materials
1
plastic ruler
1
piece of wool (or use your hair if you don’t have wool)
1
insulated foam cup
1
paper clip, bent
2
paper leaves
Procedure
Place one leaf on the table.
1. Rub the ruler briskly back and forth with the wool about ten times (charge it). Hold it near
your hair. You should be able to feel an effect. Move the ruler slowly toward the leaf on the
table.
a. What happens to the leaf? Is it attracted to the ruler?
b. Try to explain what happens based on what you know about charge.
c. Does the charge on the ruler have energy? How do you know?
Set up the material as shown. The leaves should be able to swing freely.
2. Touch the paper clip with your finger (ground it). Charge the ruler. Move the ruler so that the
leaves are touching it. Make sure there is good contact between the ruler and leaves. Move
the ruler away. Do it several times if necessary.
a. Are the leaves attracted to the ruler?
b. What happens to the leaves after you remove the ruler?
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c. Try to explain it based on what you know about charge.
3. Move the ruler slowly toward the leaves from the side.
a. Are they drawn toward the ruler or pushed away from it?
b. Try to explain it based on what you know about charge.
4. Ground the paper clip and charge the ruler. Move the ruler slowly down toward the top of the
leaves, then away.
a. What happens to the leaves?
b. Again, try to explain it.
5. Ground and charge. Move the ruler slowly up toward the bottom of the leaves, then away.
a. What happens to the leaves?
b. Try to explain it.
6. Ground and charge. Move the ruler slowly toward the side of the cup farthest from the
leaves, then move it away.
a. What happens to the leaves?
b. Try to explain it.
Stop here to summarize what you have found.
7. Ground and charge. Move the ruler slowly down toward the top of the leaves until it is near
or touching the paper clip. Ground the paper clip at its end, near the ruler and the top of the
leaves.
a. What happens to the leaves?
8. Move the ruler up.
a. What happens to the leaves?
9. Move the ruler back down.
a. What happens to the leaves?
10. Move the ruler slowly toward the leaves from the side.
a. Are the leaves drawn toward or pushed away from the ruler?
b. Try to explain it.
Summary
1. Draw pictures of the charge on the ruler and the leaves for the situations above.
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Reading
Why can you get electric shocks just by walking across a room? When you rub a balloon in your
hair, why does your hair stick to the balloon? As you have learned, all matter is made of atoms
that contain protons (+ charge), electrons (- charge), and neutrons (neutral charge). Most matter
is electrically neutral because it has equal numbers of protons and electrons. If an object has
more electrons than protons or more protons than electrons, it will have an electric charge.
Benjamin Franklin (1706-1790), the great American statesman, was also a great scientist. He
investigated electricity, and invented the lightning rod, which helps to protect buildings from
lightning strikes. You have probably seen pictures of him in 1752, flying a kite in a
thunderstorm. (He was carefully to avoid getting hit by lightning.) He was the first to call electric
charges plus and minus, and we still use his system. Among many other accomplishments,
Franklin invented bifocal glasses and swim fins.
Opposites attract, likes repel
Just like with magnets, opposite electric charges attract each other, and like charges repel. When
you rub a balloon in your hair, electrons are drawn to the balloon from your hair, so your hair
and the balloon have opposite charges – they attract each other. At the same time, your hair has a
positive charge, so it repels itself – that’s why it stands up. The same kind of thing happens when
you walk across a carpet. An electrical imbalance is created on your body. When you get close to
another body, protons on one side attract the excess electrons on the other side and the electrons
leap across the gap, making a spark. It is the same basic process that causes lightning.
Conservation of charge
Some materials give up their electrons easily while others hold on to them. When two materials
are rubbed together, one piece takes some electrons from the other piece. The one taking
electrons gets a negative (-) charge, while the other gets a positive (+) charge, but the total
amount of charge never changes because electrons and protrons aren’t created or destroyed.
Charge is conserved.
Law of Conservation of Charge
Electric charge is not created or destroyed, but can be transferred.
Here is a list of materials in approximate order from those that give up electrons to those that
take electrons: rabbit fur, leather, Lucite, Bakelite, acetate, glass, quartz, mica, Nylon, wool, cat
fur, silk, paper, cotton, steel, wood, amber, resin, metal, polystyrene, polyethylene, Teflon. Rub
two items together and electrons will transfer from one to the other. The further apart on the list
the two items are, the more electrons are transferred.
The “amount of charge” is due to the number of excess electrons or protons. The smallest
amount of charge is the charge of one electron or proton – they’re equal and opposite. Every
electric charge is a whole number multiple of that charge.
Similarity of gravity and electricity
Electricity is similar to gravity. , Every mass has a gravitational field around it, and every other
mass is pulled toward it. The bigger the mass and the closer the masses, the harder the pull. The
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fundamental unit for gravity is mass, while for electricity the unit is charge (q). q is the number
of excess electrons or protons. A charge has an electric field around it. Another charge will
experience a force that depends on how big and how far apart the charges are.
Look at the following equations for gravity and electricity. (The equations for electricity are only
good if the charges are not moving.)
Newton’s Law of
Gravitation
gravitational field
acceleration
no equivalent quantity
Gravity
F = Gm1m2/r
Electricity
F = kq1q2/r2
a = Gm/r2
a = F/m
? = PE/m
? = Fd/m
? = ad
E = kq/r2
E = F/q
V = PE/q
V = Fd/q
V = Ed
Coulomb’s Law
electric field
electric field
strength of field, volts
The first two equations are very similar. The second equation is named after the French scientist
Charles Coulomb (1736-1806). He found the equation by experimenting and noticed that the
form was very similar to Newton’s. If the amount of charge on either object doubles, the force
doubles, too. If the objects are moved farther apart, the force decreases by the square of the
distance.
The quantity of charge is measured in Coulombs (C). In Coulomb’s Law, the constant k makes
the quantities work so that for two objects, each with 1 C of charge, the force between them is
1N when they are 1 m apart. It turns out that one Coulomb is the total charge of 6.25e18
(6,250,000,000,000,000,000) electrons (or protons). Therefore, one electron has a charge of
1.6e-19 C. (1 C/6.25e18 electrons = 1.6e-19 C/electron.)
Divide the first equations by mass (for gravity) or charge (for electricity) and you get the next
pair. An electric field (E) is similar to a gravitational field. Acceleration in a gravitational field
increases with mass but decreases with distance. For example, if Earth was the same size but
more massive, the acceleration at its surface would be greater – you’d weigh more. If you move
farther from the earth, acceleration will be less – you’d weigh less. An electric field is similar to
a gravitational field except it increases with charge, not mass – a bigger charge creates a stronger
field. The field decreases with distance like gravity does.
You already know a = F/m. At every point in the gravitational field, there’s an acceleration. At
each point, more mass will create more force – a more massive object weighs more. In the same
way, E = F/q. At every point in the electric field, there’s an acceleration. At each point, more
charge will create more force – an object with more charge will be pushed or pulled harder. The
electric field gives the amount of force a charged object would experience at that location.
Pushing two like charges together takes work, as does pulling opposite charges apart. How much
work depends on the electric field and the amount of charge. The stronger the field, the harder it
is, and the more charge, the harder it is, so dividing by charge gives the strength of the electric
field.
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The fourth set of equations is based on the volt, an electrical quantity. Alessandro Volta (17451827) investigated electricity, especially static electricity. The volt (V) is named after him. There
is no corresponding mechanical quantity, but they are still similar. A volt is a measure of the
electric potential energy per charge.
V = W/q
or V = PE/q
Remember that work is force  distance,
W = Fd.
So we can re-write
V = W/q as V = Fd/q.
Since F/q = E, the electric field,
V = Ed
At the same distance, with a stronger field, the voltage is greater. Or, with a weaker field, you
have to go a greater distance to get the same voltage. There is no named similar quantity in
mechanics, but it would be like potential energy/mass, PE/m. PE is Fd, or mad. The mass
cancels out, and you’re left with
? = ad
To increase this quantity, you can increase the acceleration at the same distance, for example, by
going to Jupiter, where an object 1 m off the ground would have a greater acceleration due to
gravity. You could also raise the object to give it more. So, volts measure the strength of the
electric field. If the field is strong, you don’t have to go as far. If the field is weak, you need to
increase the distance.
Electricity and gravity are very similar in some ways, but there are two big differences. First,
there is only one type of gravity, but there are two types of electric charge, positive and negative.
All mass attracts all other mass, but opposite electric charges attract and likes repel. Second,
charge is very strong compared to gravity. If you stand one meter away from another person, you
can’t feel the gravitational force. But just a 1% difference in the number of protons and electrons
in each of you would generate enough force to lift the earth! (Feynman, Lectures on Physics).
Induction
When you charged up your ruler, you were able to pick up scraps of paper, even though the
scraps of paper were neutral. How does that happen? Even neutral objects will experience a force
because the charge on them will redistribute due to the electric field. The ruler has a negative
charge. The paper has mix of positive and negative charges. Hold the ruler near the paper. The
negative charges on the ruler pull the positive charges on the paper closer, and push the negative
charges further away. Then the ruler attracts the paper.
Exercises
1. Explain why balloons stick to walls after you rub them in your hair.
2. Earth’s north magnetic pole is in northern Canada. A compass points north, toward the north
magnetic pole. Does the North or South end of a compass needle point north? [Opposites
attract. The south end of the needle is attracted to the north magnetic pole.]
3. If your hair takes a static charge, why does it get dirtier? [Hair with a charge attracts small
pieces of dirt, just like a charged ruler attracts bits of paper.]
4. Powder coat paint is a popular finish on many products. It makes a smooth, tough, corrosionresistant coating that protects metal parts. The powder coat is applied using electrostatics.
The metal is given a negative charge, and the powder has a positive charge, so the powder is
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attracted to the metal. The metal is then heated so that the powder melts into a smooth
coating and covers the product. Considering the method used, why would there be problems
getting powder to stick in corners? [Like charges repel, so they spread out more or less
evenly over the surface of the object. In the corners, the like charges push each other away,
so there is no charge on the metal and the paint is not attracted.]
5. Why would vehicles that work around flammable chemicals need tires that conduct
electricity? [Tires create friction, therefore charge. A large charge can build up on the
vehicle. When it discharges, it creates a spark that can ignite flammable gases.]
6. Explain how an object with a positive charge and an object with a neutral charge can attract
each other. [The positive charge on the first object attracts the negative charges on the second
object and repels the positive charges. As a result, the negative charges move closer and the
positive charges move farther away. Since the negative charges are closer, the attractive force
overcomes the repulsive force.]
7. In you halve the distance between two charged objects, what happens to the force between
them? [The force quadruples. It is an inverse-squared relationship. This is not exactly true in
normal circumstances, but is true in an ideal condition.]
8. force with changed distance
9. force with changed charge
10. charge in a field
11. Expain why paper, with a neutral charge on it, is attracted to a charged ruler.
12. In a dark room, pull off a synthetic sweater and watch the sparks. What are the sparks, and
where do they come from? [The sparks are discharges of energy due to differences in charge.
The difference in charge comes from the friction between the sweater and your body or
another piece of clothing. Electrons are transferred from one object to the other.]
13. Why are anti-static sheets used in dryers? [Dryers cause a lot of friction. Friction leads to
exchange of electrons and static build-up. Anti-static sheets allow the charge to move around
from one piece of clothing to another so that no charge accumulates in any one place.]
14. In flammable chemical storage areas, there is a pipe connected to the ground. All of the
containers are connected to the pipe, as are any people who work in the area. This helps
prevent a spark from igniting the chemicals. Why does it work? [The common ground point
ensures that no charge can build up on any item relative to another item. If there’s no
difference, there’s no energy to create a spark.]
15. Electronics technicians assemble delicate parts. They have a conductive mat that is grounded,
and they have wrist straps connected to the mat. Why? [Electronic parts are susceptible to
static discharge. Connecting the mat and the person to ground leaves at the same potential so
that no static charge can build up.]
Challenge/ extension
1.
Glossary
 Coulomb
 Volt
 Charge?
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