On cold, clear
days, rubbing almost any object seems to cause it to be attracted to
or repelled from other objects. After being used, a plastic comb
will pick up bits of paper, hair, and cork, and people wearing
polyester clothing in the winter walk around cursing the phenomenon
dubbed in TV advertisements as "static cling".
We are going to
begin a study of electrical phenomena by exploring the nature of the
forces between objects that have been rubbed or that have come into
contact with objects that have been rubbed. These forces are
attributed to a fundamental property of the constituents of atoms
known as charge. The forces between particles that are not
moving or that are moving relatively slowly are known as
- To discover some of the basic properties of particles which
carry electric charges.
- To develop an appreciation for careful technique. See Caveats.
| Scotch tape strips
|| Silk, polyester, and wool fabric
|Small aluminum balls on strings
||Pivot stand for rods
||Plastic rods (PVC and perspex)
||Metal spheres on insulation stands
Part I: The scotch tape trick...
- You and your partner should each stick a 10 cm or so strip of
scotch tape on the lab table with the end curled over to make a
non-stick handle. Peel the tape off the table, and bring the tip of
the non-sticky side of the tape slowly toward the tip of your
Examine how the distance between the strips affects the
interaction between them. Describe the interaction between the two
strips. Do they attract or repel one another? How does this
interaction seem to depend on the distance between the strips?
Stick two strips of tape
with "handles" on the table, and label them "B"
for bottom. Press another strip of tape with a "handle"
on top of each of the B pieces. Label these strips "T"
for top. Pull each pair of strips off of the table with handle B
and touch them until they have lost any electrostatic attraction to
your hand. Then quickly pull the top and bottom strips apart using
B & T handles. Examine the interaction between the two T strips
when they are brought slowly toward one another.
interaction between the two B strips.
Examine the interaction
between a T and B strip.
Examine the interaction between the
strips and other objects, such as your hand.
- Tabulate your observations.
Part II: Triboelectricity
who has ever felt the "zap" of an electric shock after
walking across a carpet and then touching a metal door knob has
experienced that two objects rubbing together can create
electrostatic charges. Whenever two different materials rub against
each other, it is likely that one will leave with more electrons than
it started with…the other will leave with less. This is
called Triboelectricity (tribo means friction). From the
study of chemistry, we learn that different materials have different
desire for electrons. (This is called electronegativity.) Some
materials are very greedy and will always steal electrons from things
they come in contact with, others are more willing to give upelectrons.
As the neutrally
charged person walks across the wool carpet, his leather soled shoes
have less deisre for electrons than the wool carpet. As a result,
electrons get stolen from the shoe by the carpet. With every step
the person becomes more and more positively charged. That charge
distributes itself over thebody. When the positively charged person
gets near the metal door, he will actually attract charges from the
door which jump in the form of a spark. Notice how only the negative
charges (electrons) are free to move.
It is important to
point out that if he was wearing rubber soled shoes on a wool carpet,
his shoes would steal electrons from the carpet. He would become
more negatively charged with each step. When he gets near the door
the electrons will jump from him to the door. From his point of view
it would look and feel the same as it did in the first example. He
can't tell whether the charges jumped to him or from him.
If we did a study of
many materials and put them in order from those with the least desire
for electrons to those with a very strong desire for electrons, we
would have created a Triboelectric series. If two items from the
list are rubbed together, then the item that is higher on the list
will end up more positive and the lower one will end up more
negatively charged. For example, if leather were rubbed with wool,
the leather becomes positive and the wool negative. Yet if rubber is
rubbed with wool, the rubber becomes negative and the wool positive.
(It is important to note that this series is true only if the samples
are clean and dry. The presense of moisture, dirt, or oils may cause
some of the items to interact differently.)
The goal of Part II
is to study many materials and put them in order from those with the
least desire for electrons to those with a very strong desire for
- Put a mark or piece of tape on
one side of each rod, so that you can identify the hand-held side
from the side which is rubbed by fabric.
- Rub one of the fabrics
vigorously back and forth around one end of the rod, then place the
rod in the frictionless pivot holder.
- Rub one of the fabrics
vigorously back and forth around one end of a second rod and place
this rod near and perpendicular to the first rod. Observe whether
or not the force between the two rods is attractive or repulsive.
Notice how strong the force is and whether or not the hand held side
of either rod shows different behavior than the rubbed side of that
- Do all combinations of fabrics
and rods, carefully tabulating your results, with the goal of
placing all eight things (silk, polyester, rabbit fur, wool, PVC
(white) plastic, perspex, glass, metal rod) in a table (below). The
rod in the holder should be recharged if it loses its charge.
More likely to lose electrons. Becomes more
More likely to steal electrons. Becomes more
- Your table may be in reverse
order since you cannot easily distinguish negative from positive
charge. Reverse order is okay. If one fabric rubbed against one
rod gives an extra large effect, place the fabric and rod at
opposite ends of the table. If one rod is hard to charge, place it
near the middle. Rods that repulse will be on the same side of the
table and the fabrics that charged them will be on the other side of
the table. Rods that have a strong attraction will be on opposite
sides of the table. However, weak attraction can be due to
induction. Induction only requires one of the rods to be charged.
To see if a rod is actually charged, compare its rubbed side with
its hand-held side. If no difference is observable, it is either
uncharged or weakly charged. [Warning: keep hand away from freely
pivoting rod since induction in the hand can induce an attractive
force on the rod.]
You need to
justify your table by explaining how you decided your placement.
Your explanation needs to refer to your tabulations. Therefore,
please number your tabulations in number 4 for easy reference. What
part of your table are you most sure about?
Part III: Measure how
force depends upon distance
A large metal sphere will be charged up with a wimshurst machine, or
some other comparable device. An aluminum ball hanging from a string
will be attached to a transparent ruler on a support stand. Once the
aluminum ball touches the metal sphere, it will take some of the
charge from the sphere and be repulsed. The strength of the
repulsive force, F, can be calculated from the angle, ,
at which the string slants away from the veritcal.
- Measure the length of the
string from the tape to the top of the aluminum ball. Use
centimeters for all length measurements.
- Charge up the metal sphere and
touch the aluminum ball to it. Keep the metal sphere charged.
- Your first measurement should
have the aluminum ball close to the metal sphere. Look down on the
transparent ruler to measure the horizontal displacement, x,
of the aluminum ball from the vertical drop. Discharge the sphere
to avoid getting shocked and measure the horizontal distance of the
vertical drop from the edge of the sphere.
- Uncharge the aluminum ball and
then repeat steps 2 and 3 several more times choosing a larger
distance, d, from the sphere each time. Your last
measurement should havex at least a factor of 5 smaller than
its value in the first measurement.
Analysis for Part III.
- Using the free force diagram
for the ball, calculate how the electrostatic force, F,
depends upon angle Θ. Note: The
tension in the string also depends upon Θ,
so your solution must eliminate T. Your final answer will be
F = constant • trig (Θ)
where trig (Θ) is some
trigonometric function of Θ.
- Calculate Θ and trig(Θ) for each data point.
Calculate r, the
distance from the center of the aluminum ball to the center of the
sphere for each data point.
r = x + d + radius of sphere.
- Use Graphical Analysis to plot trig (Θ) vs. r and use a
power function automatic fit, to find the r - dependence of
the force. Print your plot and fit, and comment on your result. Do
you know what the r - dependence should be?
Part IV: Mr. Coulomb's trick
Coulomb devised a clever trick for determining how much force charged
objects exert on each other without knowing the actual amount of charge
on the objects. He transferred an unknown amount of charge, q,
to a conductor, which meant it spread evenly all over the surface of
the conductor. He then touched the newly charged conductor to an identical
uncharged one. The conducting objects would quickly exchange charge until
both had q/2 on each, since they were identically shaped. After
observing the effects with q/2, Coulomb would discharge one of the
conductors by touching a large piece of metal to it and then repeat the
procedure to get q/4 on each conductor, and so on.
Using this procedure, measure the force between two pith balls or between
a single ball and a larger charged object. Determine the dependence of the
force on the charge present. Plot your results using Graphical Analysis.
Static electricity is notorious for producing inconsistent results.
Make sure every result you write down can be repeated. Document
the steps you took to acquire consistency for any result you report.
Document the problems you encountered in gaining consistent data.