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Florida Institute of Technology
© 2023 by J. Gering
Experiment 01
Electrostatics
Introduction
This lab activity is largely qualitative and offers the student a shallow learning curve to introduce
triboelectric charging and charging by induction. These activities provide numerical results but
no theoretical values for comparisons. For lab activities, students collect, analyze and
summarize the results of their data on sheets of paper during the laboratory period. This activity
summary is then stapled together and submitted before the student leaves the laboratory.
Concepts
Electric charge is a property of objects that was first discovered and recorded by the Greek
philosopher Thales around 600 B.C. He found that by rubbing pieces of amber (petrified pine
tree sap) with a cloth he could make other small objects move or experience a force. The Greek
word for amber is elektron. This is where we get the terms electron and electricity. So objects
can become electrically charged by rubbing them against one another. This phenomenon is
called triboelectrification. The Greek word for rubbing is tribos.
As we saw in Physics 1, rubbing generates a frictional force which usually does negative work
on the object it acts upon. Work is a form of energy. So by rubbing we are transforming kinetic
energy into thermal energy through friction. This thermal energy elevates the temperature of the
object but it can also remove electrons from the molecules making up the object. As you know
from chemistry, some elements, compounds and materials ‘want’ more electrons and others
readily give them up. So by picking the right two materials to rub together, we can move
electrons from the first object to the second. This leaves the first object positively charged
because the rubbing does not move the protons in the atoms’ nuclei. Likewise, rubbing leaves
the second object negatively charged because it now contains more electrons than it did when it
was electrically neutral. Neutral means the number of positively charged particles in an object
equals the number of negatively charged particles.
In 1733 the French scientist DuFay discovered the “likes repel and unlike attract” nature of what
was then thought of as two different types of ‘electric fluid’. In letters written in 1747,
Benjamin Franklin described the results of his own experiments. It was Franklin who first
coined the terms positive and negative and hypothesized that there was only one electric fluid.
Franklin correctly reasoned that objects became oppositely charged by gaining a surplus or
deficit of this one electrical fluid. In Franklin’s day, the existence of atoms, electrons and
protons were unknown. So when Franklin rubbed a glass rod with a silk cloth he arbitrarily
decided to call the charged state of the glass positive. Later, the idea of an electrical fluid that
flowed between objects was discarded.
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Florida Institute of Technology
© 2023 by J. Gering
When two plastic rods are rubbed with a piece of animal fur, the rods become positively charged
and the fur negatively charged. When suspended from threads, the two rods will push against
each other but will be pulled toward the fur. So we say two objects push (repel) each other when
they have the same electric polarity and conversely two objects will pull on each other (attract) if
they have opposite electric polarity.
In writing the previous paragraphs, care was taken to refer to charged objects and not electric
charge as if it was an object itself. Again, charge is a characteristic of something, not a material
thing. Most books and many instructors do not emphasize this distinction. This plants a
misconception that electric charge is like peanut butter: something that can be spread onto other
objects. The better analogy is to think of electric charge like color or taste or odor (properties of
an object). So if a textbook uses red for positive and blue for negative (as ours does) then it is
best to think of more positive charge as losing electrons and becoming redder.
The Earth (both the planet and the ground beneath our feet) is a source of a vast amount of free
charge. Here free means free to move. In reality this means electrons can easily move up or
down a wire that is connected to a metal post drilled into the Earth/ground. By default we define
the electric potential energy of the Earth/ground as zero. This is analogous to the world-wide
average sea-level being defined as zero topographic elevation. In this way, we use the word
ground to mean a value of zero Joules per Coulomb of charge. Any good conductor of electricity
(a piece of metal or a wire) can be made to have an electric potential energy of zero Joules. In
circuits we deal with electric potential energy divided by a unit of charge that flows in the wires.
This is called electric potential and one Joule / Coulomb is called a Volt. So, in circuits, a piece
of metal or wire connected to the Earth as ground provides zero Volts.
If a positively charged object is insulated and isolated from its environment (tables, chairs,
people) the object will remain charged for quite a while. However water is a moderately good
conductor of electricity. And humidity in the air and the movement of the air will provide a way
for charged particles to travel from the object to the Earth/ground. This slow discharge of the
object increases its speed in humid environments like sunny Florida. You will have to take this
into account when doing these demonstrations. Water is good conductor due to the dissolved
ions usually found in it. Purified, de-ionized water is actually a very good insulator. Cold, dry
air has low humidity and doesn’t move much. This is why it is easier to charge yourself and feel
a shock on dry days. We are constantly separating charge by rubbing parts of our clothes
together or against other objects like carpeting or automobile seats.
Teachers want to induce their students to study. Police want to induce people to obey the speed
limit. Telecom companies want to induce their customers to upgrade their service. So the word
induce means “to cause to happen”. Therefore, charging by induction is to cause something to
become charged but not by a direct method. Rubbing is one direct approach to create a charged
object. Another is bombarding the object with a beam of electrons (some may be familiar with
how an old-fashioned CRT television screen became charged during use). Charging by induction
uses the Coulomb force to pull excess charged particles from the ground onto an object.
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Florida Institute of Technology
© 2023 by J. Gering
Humidity and air currents tend to rapidly discharge isolated, insulated, charged objects. On the
other hand, on a dry winter day, just a little rubbing will charge a balloon or soda bottle (for
example) to a potential of several thousand volts. This is much more than the electrometer can
measure so be sure not to “peg” the meter. NEVER allow the needle to quickly snap back and
forth and bang off of the clear plastic pegs seen at the bottom of the electrometer’s window. On
dry days you may have to wait a while or blow moist breath onto an object to lower its potential
before using the electrometer.
Method
!
Figure 1. The Pasco Electrostatics System
In this lab activity, you will use the PASCO Basic Electrostatics System to make
discoveries about non moving (static) electrical charge, electrical charging, grounding, induced
charging and electrical polarization. The apparatus consists of two conducting, metallic spheres
mounted on insulating stands, two wire cages one inside the other, three wands with black plastic
handles, a high voltage / low current power supply and a special meter for measuring voltage
levels (which will give an indication of how charged an object is). Many of these items are made
of plastic but contain other parts whose function involves some very subtle physics. Read the
following descriptions to learn of these subtleties. Applying these details will lead to correct
observations and help dispel incorrect ideas you may have about electric charge.
The Electrometer: At first glance, the electrometer looks like a cheap voltmeter. However
ordinary voltmeters measure the electric potential energy of charged particles by siphoning off a
small number of the charged particles being measured. This distorts the very thing you are trying
to measure. One way to compensate is to siphon off as little as possible. To do this the
instrument must have a very high internal resistance so very little electrical charge gets drained
away from the object under study. Common voltmeters have an internal resistance of 107 Ohms.
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Florida Institute of Technology
© 2023 by J. Gering
This sounds high but it isn’t high enough for electrostatics experimenting. Electrometers have
an internal resistance of 1014 Ohms. So they will siphon off a current of only pico-Amps instead
of the micro-Amps an ordinary voltmeter will require.
Important: Throughout the activity, the relative charged state of any object will be determined
by the electrometer which measures the electric potential (energy per unit charge) of that object.
The units of electric potential are the Joule / Coulomb which is called a Volt. In your writing, try
to distinguish between charge (which is properly measured in Coulombs) and the electrometer’s
reading which will be proportional to the amount of charge. However, please recognize that
electric potential is not the same thing as charge.
Electrostatic Voltage Source (EVS): This is our ‘power supply’ for this activity. But so little
power is being supplied it is better to view it as a charging device. Through the marvels of
modern semiconductor technology, this device can supply 1,000, 2,000 and 3,000 Volts (Joules
of potential energy per Coulomb of charge) while limiting the maximum current to no more than
8.3 micro-Amps. Ordinarily thousands of Volts are extremely hazardous. But with this level of
current limiting, you can touch bare wires energized to these voltages and not feel a tingle. So
this power supply is very safe despite its high voltage rating. The EVS has solid-state circuitry
powered by four AA batteries. Be sure to turn it OFF at the end of the activity. Also do not let
the wires connected to the EVS come into contact as a small spark will result.
Three Magic Wands: These have black plastic handles and a special white insulating neck near
the disk. The white material is a polycarbonate with an electrical resistance of 1014 Ohms.
When you rub the blue and white face of each disk against each other, the white disk becomes
positively charged and the blue disk becomes negatively charged. The third wand has an
aluminum covered disk. The disk below the aluminum is black polycarbonate mixed with
carbon. This provides a moderate conductor (resistance is 1,000 Ohms) capable of storing
charge with a very good conducting surface. The third wand is used for transferring charge. For
good results you must keep all the disks clean. When not in use, place them on a new sheet of
printer paper. This keeps grime and oils from the table top off the disks. You can clean the disks
with alcohol and paper towel. When transferring charge from the spheres, always touch the
conductive wand so the face of the disk is tangent to the sphere. In Franklin’s day, these wands
would have had magical properties.
Two Conducting Spheres: These are plastic spheres plated with layers of copper, non-sulfurous
nickel and lastly chrome as the outer (shiny) layer. They have a metal screw for connecting an
alligator clip to the sphere. The support rod is a good insulator.
The Faraday Ice Pail: This is the two cylindrical wire cages. The outer cage is a shield which
prevents stray charges from affecting the charged inner cylinder. The inner cylinder is called the
pail. It is your bucket for holding electrically charged particles. The pail is insulated from the
base but the shield is in contact with the base. So the shield and base are effectively grounded
due to contact with the table top. Michael Faraday used a metal ice pail for his experiments in
electrostatics. A solid metal bucket would work here but the wire cages let you see inside.
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Florida Institute of Technology
© 2023 by J. Gering
The terminology of electricity is old and varied. Most of these terms will be coined and defined
as we come to them. However, here is a brief glossary to help you get started.
plug – a protruding piece of bare metal used to make an electrical connection
jack – insulated metal hole used to make a connection; plugs are pushed into jacks
banana plug – a metal plug with an outer metal cover, the cover has slits that run the for most of
its length, the slits act as springs and assure a good connection when this plug is inserted into a
jack, banana plugs can be any color but the plastic insulator of ours just happens to be yellow
piggy-back plug – a plug with a jack made into the rear of the plug, also called stacking plugs
alligator clip – a spring loaded, pinching clip used to make temporary electrical connections,
Americans (especially Floridians) also call them gator clips. The British (and British influenced)
often call them crocodile clips
test lead – insulated wire with a plug on one end and a prong on the other end mounted inside a
pencil-like insulator, the pencil shape allows the user to conveniently grasp the working end and
make brief contact to very small spots on a circuit board
hook-up wire – insulated wires with banana plugs on each end which are used to build circuits
terminal – another name for a jack, usually mounted inside an electrical measuring device such
as a voltmeter or oscilloscope
binding post – a special type of jack that often combines a banana jack with an insulating collar,
the collar can be unscrewed to reveal a drilled hole for inserting a hard metal prong or bare wire,
the collar is then screwed back down to squeeze the wire to insure a good connection
stranded wire – hook-up wire that is made from a twisted bundle of very thin wires surrounded
by insulation
solid conductor wire – hook-up wire that is made from a single, thick bare metal wire, stranded
wire is usually much more flexible than solid conductor wire
solder – an alloy of lead and tin that melts at temperatures ranging from 300°F to 700°F. It is
drawn into spools and used to permanently connect a wire (usually stranded) to some other metal
contact in a circuit; it is pronounced sodder with a silent l.
crimp – a method of making an electrical contact usually between a stranded wire and a solid
metal piece, the wire is inserted into a split collar and the collar is squeezed very hard with pliers,
the collar deforms and makes a secure physical and electrical contact with the wire
spade lug – a two-pronged, fork with a collar; wire is placed into the collar which is crimped or
soldered, the fork is then available to fit part-way around a binding post
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Florida Institute of Technology
© 2023 by J. Gering
Procedure
Part 1
Triboelectric Charging
1)
Examine the electrometer, find the GROUND jack and the ON/OFF button. Make sure
the electrometer is OFF.
2)
Plug the Electronic Voltage Source’s 9-Volt AC/DC adapter into an outlet strip and
connect other end of the adapter’s cable to the only jack on the rear face of the EVS.
Make sure the switch next to the power jack is turned OFF.
3)
Using a black wire, connect the jack labeled COM on the EVS to the electrometer’s
GROUND jack. Now, the Earth ground provided by the third prong on the AC/DC
adapter’s plug is COMmon with the electrometer.
4)
Push and twist to connect the coaxial cable to the coaxial jack on the right side of the
electrometer. Connect the cable’s two wires (using alligator clips) to the outer and inner
wire cages. Black connects to the outer cage and red to the inner cage (the pail). The
outer cage is the shield and it is now an easily accessible ground.
5)
Turn on the electrometer and press the zero button. Then, set the electrometer’s range to
100 Volts. This represents the maximum one-way deflection of the electrometer’s
display. Later, if this setting seems too high, you can lower it. It is always safest to start
by setting a meter’s range to its maximum value.
6)
Remove any stray charged particles from the neck and handle of the two charging wands
by touching these parts of the wand to the shield. Repeat this step often.
7)
Gently rub the blue and white disks together for a couple of seconds. Move one of them
away from the Faraday ice pail and place it on the table. Then touch and continue
touching the shield with your newly freed hand. This removes any stray charged particles
from you and any charged particles you generate by moving. Next, lower the other wand
into the pail without touching the inner wire cage. Try doing all this fairly quickly and
repeat as needed (zeroing the electrometer between trials). Have your partner record the
electrometer reading.
8)
Remove the wand and have your partner press the zero button on the electrometer.
9)
Wave the wand in the air for about 5 seconds. Then, reintroduce the wand into the pail.
Are there any charged particles left on the wand? Remove your grounded hand from the
shield and ground the disk on the wand.
10)
Question: Explain the physics of steps (6) and (7) on your data sheet.
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Florida Institute of Technology
© 2023 by J. Gering
11)
Repeat procedures (6) and (7) using the wand with the other colored disk.
12)
After grounding yourself and the wands, repeat procedure (7), but this time put both
wands into the pail at the same time. Do not touch the pail with the wands and do not let
the wands touch each other. Question: What physics does this exercise demonstrate?
Write a full sentences, not just one or two words, on your data sheet.
13)
Repeat the previous procedure but while the disks are separated and inside the pail, allow
them to touch each other without rubbing, then remove one wand at a time. Record the
electrometer’s readings. Explain the results on your data sheet.
Part 2
Charging by Induction
1)
Using a red wire, connect the electrometer, EVS and one sphere as shown in Figure 1.
Use the 2000 Volt jack on the EVS. This charged sphere will be “the first sphere”. Turn
ON the EVS.
2)
Set the electrometer to the 30 Volt range. Place the surface of a second sphere 5 cm from
the surface of the first sphere. Momentarily ground the second sphere by briefly
connecting an extra wire between the sphere and the shield. As you proceed, keep an eye
on the electrometer and do NOT allow its needle to exceed the maximum of the range.
You can change the electrometer’s range as needed for an accurate measurement. The
reading should be in the upper half of the range.
3)
Touch the first sphere with the aluminum covered disk (called the proof plane), keeping
the disk tangent to the sphere. Then, place the proof plane inside the pail. Do not allow
the disk to touch the pail. Record the polarity of the charge on the first sphere and the
polarity and magnitude of the electrometer reading.
4)
After grounding the proof plane, repeat the measurements a few times sampling at points
on the second sphere that are closest to the first sphere. Ground the proof plane between
trials. Repeat for points on the opposite side of the second sphere.
5)
Questions: Is the polarity of the charge on the near and far sides of the second sphere the
same? Is the charge density on the near and far sides of the second sphere constant?
6)
Next, ground the proof plane. With the two spheres 5 cm apart, have your partner take a
spare, black wire and connect one end to the ground (the shield). Then, (handling the
yellow, insulating plastic) touch the metal banana plug to the side of the second sphere
farthest from the first sphere.
7)
Remove the spare wire and immediately use the proof plane to sample the charge on the
second sphere at the point where you touched the spare wire. Place the proof plane in the
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Florida Institute of Technology
© 2023 by J. Gering
Faraday ice pail and record the voltage. Question: Is the charge on the second sphere the
same as it was in procedure (4)? Explain your findings. If results are inconclusive, try
this procedure again but move the first sphere away from the second sphere just after the
momentary ground is disconnected from the second sphere.
8)
Turn off the EVS and allow the first sphere to discharge for 60 seconds. You can speed
up the discharging by blowing on the sphere. Then, use a spare wire and ground the
sphere. Move the spheres aside.
9)
Turn OFF all equipment, disconnect all wires from the spheres and hang your red and
black hook-up wires on the rack in the back of the laboratory. Leave the coaxial wire
coiled up neatly at your station.
Part 3
Visualizing Electric Fields
1)
Start a web browser and enter this search term into Google: PhET Charges and Fields.
Run the Java applet by clicking on the right pointing triangle. Note: Java must be
updated and authorized to run by an administrator on the computer you are using.
Windows based computers may put up roadblocks to running Java so it is best to perform
this part while you are still in the laboratory. For each of the following procedures make
a hand-drawn sketch of the charge distribution you set-up and its electric field vector
diagram. Notice the use of the phrase field vector diagram. You can include a field line
diagram if you wish but you must include several field vectors too.
2)
Play with the software and develop an idea of the two-dimensional shape of the electric
field around a single, positively charged particle.
3)
Then build a dipole on the screen and determine the shape of its electric field. Also,
determine where the electric field is zero and label this point(s) on your sketch.
4)
Next, build a linear quadrupole, determine the shape of its electric field and make the
corresponding sketch. Note: A quick Internet search will describe how to arrange the
charged particles.
5)
Next, build a planar quadrupole, determine the shape of its electric field and make the
corresponding sketch.
6)
Make a long line of positively charged particles. Determine the shape of its electric field
and make the corresponding sketch. Where is the electric field perpendicular to the line
of positively charged particles? Label these points on the sketch.
7)
Finally, make one long line of positively charged particles and a parallel second line of
negatively charged particles. Keep the two lines separated by two centimeters.
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Florida Institute of Technology
© 2023 by J. Gering
Determine the shape of this charge distribution’s electric field and make the
corresponding sketch.
8)
30% of the credit for your lab activity summary will be for successfully completing Part
3 and including all sketches with your data sheet.
9)
As this is a lab activity, no formal lab report is required. Complete, staple and turn-in all
your papers before you leave the laboratory. To be fair to all students, the activity
summary sheets may not be taken home and turned in later.
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Florida Institute of Technology
© 2023 by J. Gering
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