Electrostatics: Electrons Gone Wild, Home Edition

Electrostatics: Electrons Gone Wild, Home Edition
Introduction
Most everyone has been “shocked” by the ability of electrons to transfer from one object to
another, particularly on dry winter days. In this lab, you will explore how electrons are
transferred between common household objects such as pieces of tape, StyrofoamTM, computer
monitors, and even your fingers. Along the way, you’ll show that the force between charged
surfaces (those with an excess or deficit of electrons) decreases with distance.
This lab is divided into three parts. In the first section, you will be performing investigations
similar to those performed by Benjamin Franklin during the mid-1700's. Like Franklin, you
strive to learn how charge interacts, and you speculate on the how's and why's of electrostatics.
Your experiments may not go as your prejudiced mind may anticipate. Don't worry! As Franklin
wrote in a letter to Cadwallader Colden on April 23, 1752, “Frequently in a variety of
experiments tho' we miss what we expect to find, yet something valuable turns out, something
surprising, and instructing, tho' unthought of.” Set your preconceptions aside and get ready for
perceptive and accurate observation.
In the second part, you will quantify the electrostatic properties of charged packing peanuts. You
may have had one or more encounters with packing peanuts that seemed determined to stick to
you, the floor, and everything else, while simultaneously, they seemed equally determined to stay
away from one another. This behavior is caused by a build up of electrons on the Styrofoam that
causes them to simultaneously repel one another (like repels like) and cling to objects with a
deficit of electrons (opposites attract). Using just two charged packing peanuts you can (and will!)
measure how the amount of repulsion relates to the separation between the peanuts.
In the final section of the lab, you will explore the difference between induction and conduction.
By using these techniques to charge a standard aluminum pie plate, you will determine the sign
of the “charge-transferred,” as well as the efficiency of each method.
This lab offers you a chance to safely play with electrons using things that should be lurking
about your home. Even with the power off, electricity, in the form of static, is all around us.
NOTE: There is no pre-lab write-up associated with this lab.
1
John Bigelow, The Works of Benjamin Franklin, (Knickerbocker Press, NY, 1904) Vol II, p. 370
Inventory
•
•
•
•
•
Scotch Magic TapeTM
StyrofoamTM cup
2 Packing peanuts / puffs
Needle, thread and scissors
Ruler
•
•
•
•
•
Lamp
2 Rods / chopsticks / mixing spoons
1 sheet of graph paper
Aluminum pie-plate
Styrofoam plate
Suggested Readings
If you are interested in learning more about Benjamin Franklin and his scientific activities, you
may enjoy Benjamin Franklin's Science by I. Bernard Cohen (Harvard University Press, 1990).
Online information on Franklin can be found at http://sln.fi.edu/franklin/
Electrostatics: Electrons Gone Wild, Home Edition
Part I: Clingy Electrons
The likelihood of one atom latching onto another atom's
electrons is something you deal with more often than you
may think. Consider the Saran Wrap in your kitchen. The
cheap stuff doesn't work as well as the name brand, and for
some reason none of it sticks to Styrofoam and all of it sticks
to glass. What's going on?
*
Most Positive
(readily lose
electrons)
So why does scuffing your feet cause charge to build up?
Neither your socks nor the carpet are perfectly smooth
surfaces. When you scuff your feet across the floor you
increase the number of atoms in your socks that will come in
contact with the carpet. The more atoms “touch” and pull
apart, the more electrons will get transferred. Similarly,
when you rub a balloon through your hair, you increase the
number of atoms in your hair that touch atoms on the
balloon. The friction between your socks and the carpet or
between your hair and the balloon has no effect on charge
transfer. What matters is the amount of surface area that
comes into contact. If you want to test this idea, try rubbing
two balloons together. You'll find they have more friction
between them than between one balloon and your hair, but
because they are identical materials the atoms in each
balloon have an equal hold on their electrons and no charge
is transferred.
(readily steal
electrons)
When two surfaces touch (like your socks at the carpet)
chemical bonds can temporarily form between surfaces, as
“touching” atoms latch onto one another's electrons.* When
the surfaces are made of two different materials, the atoms in
one surface often exert a stronger pull on the electrons than
the other surface. As a result, when the surfaces pull apart,
electrons are stripped out of the weaker atoms by the
stronger. These stolen electrons create a negative charge on
one material, leaving positive “charge” (actually, a lack of
charge/electrons) on the other surface. It is strictly the act of
one surface touching and then not touching another surface
that causes the charge transfer.
Most Negative
Rubbing materials together can generate static electricity. You can test this by scuffing you feet
across carpeting on a dry day and then touching a doorknob. ZAP! But is it the friction of
scuffing your feet across the floor that causes the charge buildup, or is something else going on?
Rabbit's fur
Lucite
Bakelite
Acetate
Glass
Quartz
Mica
Human hair
Nylon
Rayon
Wool
Cat's fur
Silk
Paper
Cotton
Wood
Sealing wax
Amber
Resins
Hard rubber
Metals
Polyester
Polystyrene (Styrofoam)
Orlon
Saran Wrap
Polyurethane
Polyethylene
Poluropylene
Sulfur
Celluloid
Vinyl (PVC)
Teflon
Triboelectric Series: Experimenters
have
established
lists,
called
triboelectric series, of the relative
affinities materials have for gaining
and losing electrons. By studying
these lists, you can learn that rubbing
wool on Styrofoam leads to negatively
charged Styrofoam (and positively
charged wool). Materials with similar
properties (e.g. hair, wool, fur) clump
together on the list and don’t interact
strongly. In general, objects listed near
one another, like cotton and amber,
interact poorly. This list’s author notes,
the series is reproducible only in rare
circumstances. Cleanliness, humidity,
and manufacturing differences affect
ordering. Adapted from Electrostatics
and its Applications, edited by A.D.
Moore, (Wiley & Sons, NY, 1973).
Neutral atoms will bond together to create complete shells of electrons. For a detailed description, see:
http://hyperphysics.phy-astr.gsu.edu/hbase/chemical/bond.html
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Electrostatics: Electrons Gone Wild, Home Edition
Procedure I: Clingy Electrons
Please complete the following procedure, answering all questions as you go. If your instructor
provided a questionnaire with this lab, answer the questions on the questionnaire.
Activity
1. Stick a piece of ~40 cm long plastic adhesive tape (Scotch Magic tape works well) onto a
table top. This is your base tape.
2. Cut two 12-20 cm long pieces of tape. Create a non-sticky handle on the end of each
piece by folding over a couple cm sections. These are your working strips.
3. Stick your working strips firmly to your base tape. Make sure they are in full contact with
the base tape by pressing them down firmly with your fingers.
4. Grasping their handles, briskly pull your working stripes off the base tape (imagine you
are removing a band-aid). Letting the strips dangle freely, slowly bring them together.
Experiment with bringing the tape together with the like sides facing each other (nonsticky to non-sticky, and sticky to sticky) and the opposite sides facing each (non-sticky
to sticky). What happens? How does the orientation of the tape affect what you see?
What do you think is causing this effect?
5. One at a time, pass each of the working strips lightly between your fingers. Try bringing
the tape back together again. Is the behavior of the tape different? Why or why not?
6. Carefully stick the two strips of tape together (sticky to non-sticky) so that you have a
double thick piece of tape, and run your fingers down the length of the working strips.
7. Grasping one tape tab in each hand, quickly pull the strips of tape apart, repeating step 4
from this new starting configuration. Do the strips behave differently this time? Is the
behavior the same or different from step 4? How do you explain this?
8. Create four new working strips that are all about 10-cm long.
9. Create two double thick pieces of tape using your 4 new working strips. Use a pen to
mark the tabs of the top and bottom stripes in each pair so you can track which strips
started on the top and bottom. (The piece with the non-sticky side exposed is the top.)
10. Quickly pull the two pairs of tape apart and test all possible combinations of bottom and
top strips as you tested the strips in step 4. What do you discover?
11. At this point you do not know which strips are positive and which are negative. Using
two objects from the list on page 2 (like hair and Styrofoam), create a negatively charged
object.
12. Test a top and bottom piece of tape with the negatively charged object. How are the top
and bottom pieces of tape charged?
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Electrostatics: Electrons Gone Wild, Home Edition
Part II: Electrostatic Forces
The electrostatic or Coulomb force between electrically charged objects is one of the four
fundamental interactions of matter. Like the gravitational interaction, it has an infinite range, but
unlike the gravitational interaction (where there is only one kind of mass, and the interaction is
always attractive), there are two kinds of electrical charge and the force can be either attractive
or repulsive. The electrostatic force between point charges is proportional to the product of the
charges and falls off inversely with the square of the separation between the charges. This is true
for spherically symmetric distributions of charge when the separation between their centers is
much larger than the radius of the charge distribution. This relationship was determined
quantitatively by Charles Augustine de Coulomb in 1785 and is known as Coulomb's law:
F=k
Q1Q2
r2
where k, the Coulomb constant, has a value of about 9×109 N m2/C2. It takes 6.25×1018 electrons
to create 1 Coulomb of charge! (1 electron has a charge of 1.6022×10-19 C.)
In the!next activity you will test the Coulomb inverse square law for two charged Styrofoam
“puffs” (sometimes called packing peanuts) and calculate the amount of charge on these puffs.
In your experimental setup, you will suspend a charged packing peanut from thread. Before you
start the procedure, consider what should happen. Initially, the charged packing peanut dangles
straight down due to gravity. If an object with the same charge is brought near the puff, it will
swing around from the charged source until the Coulomb force and gravity are balanced. In this
new configuration, the puff is balanced between tension (T) from the string holding it up, gravity
(mg) pulling it down, and the electrostatic force (FCoulomb) pushing it sideways.
Mathematically, the vertical, y-component forces balance to:
mg = T cos"
and the horizontal, x-component forces balance to:
FCoulomb = T sin "
You can solve for the Coulomb force by dividing these
! together:
equations
FCoulomb T sin "
=
= tan "
!
mg
T cos"
FCoulomb = mgtan "
For small angles tan " # Lx , and we can reduce this to:
x
FCoulomb = mg
L
!
If you know the second charged source is located some distance
! page to the right) from the puff, you can solve for the
r (off the
product of the charges in Fcoulomb:
!
QQ
mg
k 12 2 =
x
r
L
You can (and will) use this to prove that the displacement x is
inversely proportional to the distance r2.
Free-body diagram of charged puff near
a charged object (off page to right).
!
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Electrostatics: Electrons Gone Wild, Home Edition
Procedure II: Electrostatic Forces
Please complete the following procedure, answering all questions as you go.
1. Cut two 80-cm lengths of thread and one 20-cm length
of thread.
Step 2.
2. Using the needle, string an 80-cm piece of thread through
each puff as shown (Step 2). The puffs should be centered
on the thread.
3. Select one puff to be your test puff, and the other to be your
charge source. Pull the thread tight at the base of the test puff,
and stab the needle through the puff so that it creates a straight
pointer at the puff's bottom (Step 3a). Use the 20-cm piece of
thread to tie scissors to the second puff (Step 3b).
on
Step 3a.
Step 3b.
4. Attach both puffs to long rods (chopsticks, kabob sticks, or mixing
spoons are fine) to form bi-fiber suspensions as shown (Final Setup
below). The suspensions should be as identical as possible so that the
puffs hang at the same height.
5. Tape your test puff+rod to the edge of a table or counter several inches
from one edge. Place a bright light straight in front of the puff so that
the puff casts a sharp shadow on the surface behind it. Tape a ruler to that surface such
that the shadow from the needle touches the 0 on the ruler.
6. Tape a second ruler to the surface behind the puff so that its 0 end is lined up with the rod
on the test puff+rod. (see Final Setup below).
7. Use a BIG book to hold the source rod in place. Initially, separate the two rods by 20 cm.
Final Setup
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Electrostatics: Electrons Gone Wild, Home Edition
8. Make sure the shadow of the test puff is lined up with 0 on the corresponding ruler., then
charge the puffs by rubbing them with fur, hair, wool, or some other electron source.
9. You should notice that the test puff's needle's shadow no longer lines up with 0 on the
ruler. Record the shadow's new position as well as the separation between the two rods.
10. Move the charge source puff+rod progressively closer to the test puff+rod, and repeat
step 9 after each move. Your two rods should be about 1-cm apart for your last
measurement. Charge can discharge (evaporate) from your puffs (especially on humid
days), so you will need to work quickly. Working with a partner is encouraged!
11. When you are done with the initial measurements, you may want to verify that your puffs
haven't discharged too much. How can you do this?
12. During the previous several steps you qualitatively measured how one charged puff
moves in response to a charged source. You can use the numbers your recorded to glean a
quantitative understanding of how the two puffs repelled one another. From the earlier
discussion, we know the square of the separation is related to the motion of the test puff:
QQ
mg
k 12 2 =
x
r
L
1
" 2 #x
r
Plot x (proportional to FCoulomb ) versus 1/r2. Include error bars. Fit a straight line to your
data. Does the data actually fit on a straight line? It's okay if it doesn't. If your data
doesn't trace out a straight line, what do you think occurred?
!
13. The slope of the line is related to the amount of charge on the puffs. What is your line's
slope? If your data deviates from a straight line, what is your best fit? What y-intercept
value did you use? Why?
14. The equation relating the forces on the puff can be rewritten into the form y = mx + b,
where b = 0, as follows:
L
1
x=
kQ1Q2 2
mg
r
L
slope =
kQ Q
mg 1 2
Since you charged the two puffs the same way, it is reasonable to assume they have
similar amounts of charge on them. This means Q1 = Q2 and Q1Q2 = Q2, such that:
L
slope =
kQ2
!
mg
You know the slope from your graph. You can measure L and look up g and k. We have
measured a handful of puffs and needles and determined puffs have an average weight of
0.05 ± 0.01 g and sewing needles typically weigh 0.12 ± 0.2 g. Filling these values into
the equation above, solve!for Q2. What is the charge on each puff? How many electrons
did each puff take from the fur or hair you used as an electron source? Does this number
seem surprisingly large or small to you considering the effects you observed due to the
electrostatic charge?
DO NOT disassemble your test puff+rod until completing the final section!
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Electrostatics: Electrons Gone Wild, Home Edition
III. Charging by Induction — The Electrophorus
Alessandro, Conte Volta is credited with inventing the electrophorus perpetuum in 1775. This
practical machine allowed the (apparent) perpetual generation of charge. The principle behind it
is simple. Like charge repels like charge. When a neutral object is brought near a negatively
charged dielectric, the free electrons in the neutral object flow as far from the charged dielectric
as they can get. If the neutral object is than touched with a conductive object connected to
ground, those electrons will actually flee the neutral object, leaving it positively charged. If the
neutral object is actually touched to the charged source, the electrons on the charged object will
flow onto the neutral object, making it negatively charged.
In this final part of the lab, you will create your own electrophorus perpetuum in a manner
similar to that used by Volta. A regular Styrofoam pie plate becomes a dielectric when it is
rubbed against your hair or a wool sweater. Combine this with an aluminum pie plate with
Styrofoam-cup handle, you're ready to “create” charge!
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Electrostatics: Electrons Gone Wild, Home Edition
Procedure III: The Electrophorus
1. Tape an upside-down Styrofoam plate to a table or counter top. The tape should only
touch the edges of the plate. This is your dielectric.
2. Tape a Styrofoam cup to the inside of an aluminum pie plate. The cup will serve as an
insulating handle for moving the charged pie plate. This is your electrophorus.
3. Charge the Styrofoam plate by rubbing it with fur, your hair, or wool.
4. Untape your test puff+rod from the table and negatively charge it as you did earlier.
Bring it close to the Styrofoam plate. Is it attracted or repelled? What type of charge is on
the plate? When you are done, hang your puff back up on the side of the table.
5. Make sure the pie plate is neutral (uncharged). Touching it with your hands should work.
However, you can verify its neutrality by touching it to a water faucet, which serves as an
excellent “ground.”
6. Holding on to its handle, move the neutral electrophorus as close to the dielectric as
possible without letting them touch! While keeping them as close together as possible,
momentarily touch a finger to the top surface of the pie plate. When you are done, raise
the electrophorus.
7. Continuing to touch only its handle, bring the pie plate near the test puff. Is the puff
attracted or repelled by the electrophorus? What sign is the charge on the electrophorus?
Is this the same or opposite of the charge on the foam plate? Was the process used to
charge the plate induction or conduction?
8. You can recharge the plate as many times as you want, as long as you don't allow the two
plates to touch. The process of charging the electrophorus requires energy, which is
primarily introduced by the work done when the electrophorus is separated from the
charged foam. Try untaping the Styrofoam plate from the table and repeating steps 5 and
6. What happens? When you are done, retape the Styrofoam plate to the table.
9. Repeat steps 3, 5-7, but this time firmly touch the plates together and don't touch the pie
plate yourself. What do you notice about the amount of charge transferred in these two
techniques (How much does the puff react?)? Why do you suppose one technique is more
efficient than the other?
10. As a final activity, draw figures to show the motion of charges throughout this
experiment. Where did the charge start? Where did it move when you held the
electrophorus near the dielectric? What happened when you touched the electrophorus?
What happened when you raised the pie plate away from the dielectric?
Submit
◊
Answers to all questions in the text
◊
Data tables and sketches made during lab
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