Physics 1b
Lab 1: Electrostatics in Your Home
Introduction
Most everyone has been “shocked” by the ability of electrons to transfer from one object to another particularly on dry winters' days! In this lab, you will explore how electrons are transferred between
common house hold objects such as pieces of tape, StyrofoamTM, computer monitors, and even your
fingers. Along the way, you will 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 will be asked to speculate on the how's and why's of electrostactics. Your
experiments might not go exactly as your intuition might predict. 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.”1 Set your preconceptions aside and get ready for perceptive and accurate observation.
In the second part of this lab, you quantify the electrostatic properties of charged packing peanuts.
During the winter holiday you may have had one or more encounters with packing peanuts that seemed
determined to stick to you, the floor, and everything else, while at the same time, they seemed equally
determined to stay away from one another. This type of behavior is do to the a build up of electrons on
the peanuts that causes them to repel one another (like charges repel like charges) or cling to objects
with a deficit of electrons (opposites attract). Using just two charged packing peanuts you can measure
how the amount of repulsion relates to the separation between the pieces of Styrofoam.
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 play safely with electrons using things that can be found in your home.
Even with the power off, electricity, in the form of static, is all around us.
For your lab report please complete the attached lab questionnaire. The questionnaire must be
handed in the appropriate locked boxes Science Center 110 before 5 PM on February 18.
Equipment
•
•
•
•
•
•
Scotch Magic TapeT
StyrofoamTM cup
AM/FM radio with external antenna
TV or CRT computer monitor
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
Reading
If you are interested in learning more about Benjamin Franklin and his scientific activities, you may
enjoy reading 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/
1
John Bigelow, The Works of Benjamin Franklin, (Knickerbocker Press, NY, 1904) Vol II, p. 370
Page 1 / 8
Lab 1: Electrostatics in Your Home
I. Sticky Electrostatics
Physics 1b
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?
Using the Triboelectric table at right, why do you think Saran Wrap doesn't
cling to Styrofoam but does cling to glass. You may want to wait to answer
this question until you've completed the next set of procedures.
∗
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
Page 2 / 8
(readily lose electrons)
(readily steal electrons)
So why does scuffing your feet cause buildup of charge? Neither your
socks nor the carpet is a perfectly smooth surface. When you scuff your
feet across the floor you increase the number of atoms in your socks 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 size of the surface area that comes in 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, however because
they are identical materials the atoms in each balloon have an equal hold on
their electrons and no charge is transferred.
Most Positive
When two surfaces touch (like your socks on a 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 are pulled 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 negative charge) 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 your 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
Polypropylene
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 other,
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).
Physics 1b
Lab 1: Electrostatics in Your Home
I. Procedure
1. Stick a piece of plastic adhesive tape (Scotch Magic tape works well) about 40 cm long 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 of 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 strips off of the base tape (imagine you are
removing a band-aid). Letting the strips dangle freely, slowly bring the strips together.
Experiment with bringing the tape together with the like sides facing each other (non-sticky to
non-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
tapes back together again. Is the behavior of the tapes different?
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?
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?
Page 3 / 8
Lab 1: Electrostatics in Your Home
II. Electrostatic Forces
Physics 1b
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:
!QQ $
F = k# 12 2 &
" r %
where k, the Coulomb constant, has a value of about 9 ! 10 9 N m 2 C2 It takes 6.2 ! 1018 electrons to
create a 1 Coulomb of charge! (1 electron has a charge of 1.6 ! 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.
The sum of the vertical forces are zero which gives us
[Eq. 1]
mg=Tcos!
and the sum of the horizontal forces are zero which gives us
[Eq. 2]
FCoulomb = Tsin !
You can solve for the Coulomb force by dividing Eq. 2 by Eq. 1:
FCoulomb Tsin !
=
!!"!!FCoulomb = mg tan !
[Eq. 3]
mg
Tcos!
x
For small angles tan ! " sin ! =
(since for small angles y ! L ).
L
x
Rearranging Eq. 3 we have FCoulomb = mg
L
!Q Q
Now the Coulomb force is given by Fc = k " 1 2 2 #$ where r is the
r
distance to a second puff off to the right. Substituting the above
expression for Fc in Eq. 3 gives us
k
Q1Q 2
x
= mg
2
r
L
[Eq. 4]
2
Eq. 4 shows that the displacement of the puff, x, is inversely proportional to r .
Page 4 / 8
Physics 1b
Lab 1: Electrostatics in Your Home
II. Procedure
1. Cut two 80-cm lengths of thread and one 20-cm length of thread.
2. Using the needle, string an 80-cm piece of thread through each puff as
shown. The puffs should be centered on the thread.
3. Select one puff as your test puff, and the other as 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).
4. Attach both puffs to long rods (chopsticks, kabob sticks, or
mixing spoons are fine) to form bi-fiber suspensions as shown
(Final Setup). 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).
7. Use a heavy book to hold the source rod in place. Initially, separate the
two rods by about 20 cm.
8. Charge the puffs by rubbing them with fur, hair, wool, or some other electron source.
Page 5 / 8
Physics 1b
Lab 1: Electrostatics in Your Home
9. After you have charged both puffs, 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 you recorded to obtain 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 displacement of the test puff:
QQ
x
1
k 1 2 2 = mg !!!!!!! 2 " x [Eq. 4]
r
L
r
13. Plot x versus 1/r2. Include error bars. Note, plotting x versus 1/r2 means that x is plotted on the
vertical axsis and 1/r2 is plotted on the horizontal axsis. Try to draw a straight line that passes
through all of your data points (the line should pass through the error bar associated with each
data point not the point itself).
14. Eq 4. can be rewritten into the standard form for a straight line, y = nx + b with b = 0,
L
1
L
x=
kQ1Q 2 2 therfore the slope of the line n equals
kQ1Q 2
mg
r
mg
Since you charged the two puffs the same way, it is reasonable to assume they have equal
amounts of charge on them, or Q1 = Q2 so Q1Q2 = Q2,
L
slope = n =
kQ 2 [Eq. 5]
mg
15. What is the value of the slope of your line? Be sure to include units.The slope of the line is
related to the amount of charge on the puffs. You can measure L and look up g and k. We have
measured a handful of puffs and a handful of needles and determined the puffs to have an
average weight of 0.05 ± 0.01 grams and the sewing needles to typically weigh 0.12 ± 0.2
grams. 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 next section!
Page 6 / 8
Physics 1b
Lab 1: Electrostatics in Your Home
III. Charging by Induction - The Electrophorus
Alessandro, Count 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 charged 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!
Page 7 / 8
Physics 1b
Lab 1: Electrostatics in Your Home
III Procedure
1. Tape an upside-down Styrofoam plate to a table or counter top. The tape should touch only 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.
3. Charge the Styrofoam plate by rubbing it with fur, or wool.
4. Untape your test puff + rod from the table and charge it negatively as you did earlier. Bring it
close to the Styrofoam plate. Is it attracted or repelled? What type of charge is on the Styrofoam
plate? When you are done, hang your test puff back up on the side of the table.
5. Make sure the aluminum 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 Styrofoam handle, move the neutral aluminum plate as close to the Styrofoam
dielectric as possible without letting them touch! While keeping the plates as close together as
possible, momentarily touch a finger to the top surface of the aluminum pie plate, and then raise
the aluminum plate.
7. Now while touching only the handle bring the aluminum pie plate near the test puff. Is the puff
attracted or repelled by the aluminum plate? What sign is the charge on the aluminum plate? Is
this the same or opposite of the charge on the Styrofoam plate? Was the process used to charge
the aluminum plate induction or conduction?
8. You can recharge the aluminum plate as many times as you want, as long as you don't allow the
two plates to touch. The process of charging the plate requires energy, which is introduced by the
work done when the aluminum plate is separated from the charged Styrofoam. 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 press the plates together so that they are in close contact but
don’t touch the pie plate yourself. How does the test puff react? What do you notice about the
amount of charge transferred this time? Was the process used to charge the aluminum plate
induction or conduction?
10. Which process do you think is more efficient at transferring charge, induction or conduction?
11. Draw figures that illustrate the movement of charge when charging by induction in the above
experiment. Show the following in your drawings: How the Styrofoam plate got its original
charge? How charges moved when you held the aluminum plate near the Styrofoam? What
happened when you momentarily touched the aluminum plate with your finger? What happened
when you moved the aluminum plate away from the Styrofoam?
Page 8 / 8
Name___________________________________ Lab Section______ Date__________
Collaborators: ___________________________________________________________
Lab Questionnaire
Electrostatics In Your Home
Due Feb 18
(Except where noted, please answer these question in the space provided.)
I. Sticky Electrostatics
1) Using the Triboelectric table on page 2 of the lab assignment, explain why Saran Wrap
doesn't cling to Styrofoam but does cling to glass.
2a) After removing your two test strips of tape from your base tape, what happened when
you brought the test strips together?
b) How does the orientation of the tape affect what you see?
c) What do you think is causing this effect?
3) After running the charged tape between your fingers is its behavior the same or
different? Why or why not?
4) After taping the test strips together and pulling them apart, do the strips behave
differently this time? How do you explain this?
5) After playing with the four charged strips, what do you discover?
Page 1 of 3
Name___________________________________ Lab Section______ Date__________
Collaborators: ___________________________________________________________
II. Electrostatic Forces
1) Please attach your table of measured x and r values, and calculated 1 r 2 values.
2) How could you verify that your puffs didn't lose their charge during the experiment?
3) Please attach your graph of x versus 1 r 2 .
4) Please attach your calculations of Q. What is the charge on each puff?
5a) How many electrons did each puff take from the fur or hair you used as an electron
source (please attach calculations)?
b) Does this number seem surprisingly large or small to you considering the effects you
observed due to the electrostatic charge?
III. Charging by Induction — The Electrophorus
1a) Is a test puff repelled by or attracted to the Styrofoam plate?
b) What is the sign of the charge on the Styrofoam plate?
2a) After you bring the plates together without letting them touch, is the test puff
attracted or repelled by the aluminum plate?
b) What is the sign of the charge on the aluminum plate?
c) Is this the same charge or opposite of the charge on the Styrofoam plate?
d) Was the process used to charge the aluminum plate induction or conduction? Explain?
Page 2 of 3
Name___________________________________ Lab Section______ Date__________
Collaborators: ___________________________________________________________
3) What happens when you bring the plates together a second time, this time with the
Styrofoam free to move (be sure to ground the aluminum plate first)?
4a) After charging the aluminum plate by contact, is the test puff attracted to or repelled
by the plate?
b) What is the sign of the charge on the aluminum plate?
c) Is charge the same or opposite to the charge on the Styrofoam foam plate?
d) Was the process used to charge the plate induction or conduction?
5a) Compare the amount of charge transferred by conduction to the amount of charge
transferred by induction. (How much does the test puff react)?
b) Which process do you think is more efficient at transferring charge, induction or
conduction? Why?
6) Draw and attach figures that illustrate the movement of charge when charging by
induction in the above experiment. Show the following in your drawing(s): How the
Styrofoam plate got its original charge? How charges moved when you held the
aluminum plate near the Styrofoam? What happened when you momentarily touched the
aluminum plate with your finger? What happened when you moved the aluminum plate
away from the Styrofoam?
Page 3 of 3