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Name_____________________________ Date_________________
Partner(s)____________________________ ______________________________
LAB 1:
ELECTROSTATICS
OBJECTIVES
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To explore the difference between conducting and insulating materials
To observe the effects of charge polarization in conductors
To understand charge mobility in conductors
To understand two methods of charging: charging by induction and charging by conduction
OVERVIEW
In this lab you will study situations where the main feature of the objects under scrutiny is that they
are charged or not. We will be concerned with how charges distribute themselves in/on conducting
materials and insulators, and what the effects of electric fields have on charges. We won’t worry so
much about the rate of flow of charge. That will be explored later when we look at electric currents.
This lab is intended to be very exploratory: feel free to experiment and make up your own activities
as you go along. It’s fascinating to play with charged objects in a controlled setting.
To review: most materials are electrically neutral. The electrostatic phenomena that you will be
playing with here in lab is caused by ionization, where electrons are ripped from their host atoms,
causing a material to attain a net charge.
Conductors have electrons which are easily mobilized and, once freed, can move freely throughout
the material. Insulators bind their electrons much more tightly.
[Do this outside of lab]
An interesting aside: calculate the net force between you and your lab partner if you both have 1%
more electrons than protons in your bodies. A good exercise in estimation – you’ll be surprised at
the answer! Another demonstration of how strong electric forces are. Consider yourselves to be
point charges.
Put your calculation here:
University of Notre Dame
Physics Department
PHYS 152, Spring 2005
Modified from P. Laws, D. Sokoloff, R. Thornton
Supported by National Science Foundation
and the U.S. Dept. of Education (FIPSE), 1993-2000
and the University of Virginia
2
Equipment List
For the Investigations in this lab, you will need the following:
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PasPort interface outfitted with a Charge/Voltage sensor and the alligator leads
long wire connected to building ground (the sink is good enough)
Faraday Pail
Two Conducting Spheres on insulated stands
Proof plane (long wand with metal disk at the end)
Insulator planes with white and blue disks
Aluminum foil
thread
meter stick
INVESTIGATION 1: CHARGES
Activity 1-1: Charging an Object by Conduction
1. Make sure that the Charge/Voltage sensor is plugged into the PasPort interface, and that you
have a graph of “Charge” displayed on the DataStudio screen.
2. The default axis resolution is way too big for you to see small variations in charge. Clip the leads
of the Charge sensor together, then push the “Zero” button on the sensor body and hold it for
about 10 seconds. The sensor should now be zeroed. Click “Start” and collect about 2 seconds
worth of data in this mode. Then, click on the “autoscale” button (first on the left) so that you
can see small variations. It’s very easy to accidentally generate spurious charge in this
experiment, and you will be working with lots of charged objects. Before every experiment, you
should touch the ground wire to the red Charge Sensor lead and zero the sensor. Just for fun,
you may want to take several short runs here to see how stable the zero point really is. Here
“zero” isn’t as important as knowing what the level of charge is before you begin observations,
so always allow some time to establish a “baseline” zero in each run before you begin the
activities.
3. Now, take the sensor leads, and attach the red lead to the inner mesh cylinder of the Faraday
Pail, and attach the black lead to the outer cylinder, as shown in Figure 1-1, below. Zero the
charge again by grounding the red lead with your grounding connection and zeroing the sensor.
Charge
Sensor
Figure 1-1: Attaching Charge Sensor leads to the Faraday Pail
University of Notre Dame
Physics Department
PHYS 152, Spring 2005
Modified from P. Laws, D. Sokoloff, R. Thornton
Supported by National Science Foundation
and the U.S. Dept. of Education (FIPSE), 1993-2000
and the University of Virginia
3
4. Now, take the two insulator planes with the blue and white disks at the end. Touch each of the
disks with your hands to absorb any excess charge. Start taking data and test to see if each of
them is neutral by placing them inside the inner cylinder in the Faraday Pail and watching the
charge indicator. (Feel free to let DataStudio keep taking data for long periods of time in this
lab.) Be careful not to touch the inner or outer cylinder, yet. If the sensor sees a net charge on
them, try to ground them as best you can.
5. When you have them both electrically neutral, place them both in the Faraday Pail while
recording data. Hopefully, since you just made them neutral, you should see only a very small
net charge, if any at all.
For the next part, try to capture a graph where you can clearly identify the sequence of events. You
will be asked to print this out and turn it in with your lab writeup.
6. Now, rub the white and blue surfaces vigorously together. Try to do it while taking data and
keeping the tips inside the Faraday Pail, if you can.
Prediction 1-1: Do you expect to see the total charge change during this activity? Why or why not?
7. Remove one of the insulating wands, then the other. Put the first one back in.
Question 1-1: What do you see? Describe your graph in a few sentences. Which of the insulators
appears to have positive charge?
You have just shown, although in a rather “black-box” way, that charge comes in two types, positive
and negative!
8. Touch the surface of one of the insulators and put it back in the pail.
Question 1-2: What happens to the magnitude of the charge when you do this? Why?
Try rubbing the proofing planes on other surfaces (be careful not to scratch them). Wool sweaters
are very interesting. Make notes here on what you tried and what the effects were.
University of Notre Dame
Physics Department
PHYS 152, Spring 2005
Modified from P. Laws, D. Sokoloff, R. Thornton
Supported by National Science Foundation
and the U.S. Dept. of Education (FIPSE), 1993-2000
and the University of Virginia
4
Activity 1-2: The Faraday Pail: Response of a Conductor to Charges
In this activity, we will explore the response of a conductor to internal and external charges. Our
primary tool this time will be the Faraday Pail itself.
Question 1-3: The Faraday Pail is made out of two conductors. Imagine that you have a negativelycharged insulator inside the inner pail. How does the Faraday Pail know how much charge is on your
wand? Can you put forth some explanation for how it works?
Prediction 1-2: On top of Figure 1-2 below, draw the charges on the Faraday Pail cylinders for the
situation of a negative charge inside the center cylinder. Indicate the charges and the charge
densities (by drawing your charges closer together or farther apart) on the inside and outside of each
cylinder on the “Top View” drawing.
Side view
Top view
Figure 1-2: Side and Top views of the Faraday Pail and the charges induced on it.
In principle, this prediction can be tested using this setup, but the charge densities on the cylinders
are so low that, in practice, it is very difficult to get sensible results.
Instead, consider the following situation:
Prediction 1-3: If negative charge is placed onto the outer cylinder of an uncharged Faraday Pail,
what will the distribution of charges be once it reaches equilibrium? Draw your prediction on the
blank sketch below, assuming the charge transferred to the cylinder was negative to begin with.
Indicate the charges on the inside and outside of each conductor. What sign charge will the Charge
Sensor read in this case?
Top view
University of Notre Dame
Physics Department
PHYS 152, Spring 2005
Modified from P. Laws, D. Sokoloff, R. Thornton
Supported by National Science Foundation
and the U.S. Dept. of Education (FIPSE), 1993-2000
and the University of Virginia
5
1. Ground the Faraday Pail and zero the Charge Sensor. Connect the black lead of the Charge
Sensor to the grounding lead and not to the Faraday Pail (otherwise, you won’t be able to deposit
any charge!).
2. Rub the blue and white disks together again until they are good and charged. Begin taking data.
This time, after measuring the charge of your disk, touch it to the outside of the larger cylinder
and rub it a bit to transfer a good amount of charge.
Question 1-4: What happens to the reading on the Charge Sensor when you do this? Does this
agree with your prediction?
Prediction 1-4: What will the charge distribution on the cylinders look like if you transfer charge to
the inner cylinder? Draw your prediction on the figure below.
Top view
3. Repeat the process of charge transfer, but this time, transfer the charge to the inner cylinder.
Question 1-5: What do you find? Can you use this data to confirm your predictions for the charge
distributions on the conductors? Explain your reasoning.
University of Notre Dame
Physics Department
PHYS 152, Spring 2005
Modified from P. Laws, D. Sokoloff, R. Thornton
Supported by National Science Foundation
and the U.S. Dept. of Education (FIPSE), 1993-2000
and the University of Virginia
6
INVESTIGATION 2: CHARGING BY INDUCTION
Activity 2-1: Response of a Conductor to an Electric Field
In this activity, you will use the metal test spheres to explore the effects of electric fields on
conductors, similar to what you just did with the Faraday Pail. Here, you will get to play with a Van
de Graaff generator to charge up your spheres.
Charging objects with the Van de Graaff Generator
A Van de Graaff generator works by transferring charge from a conductor at the base of the
generator to the sphere at the top by means of a belt. No external electricity is required, as we
demonstrate here by driving the generator with a hand crank. At the end of Lab, we’ll take the Van
de Graaff apart so you can see what’s in it.
IMPORTANT: Think before you do this! If you have health conditions (pacemaker, other neurological
problems) that could be exacerbated by shocking yourself with tens or hundreds of kilovolts, have
one of your lab partner(s) charge the sphere. Safety is more important than Physics.
1. Start by grounding the Generator with the grounding wand.
2. To charge up your sphere, hold it by the plastic support rod and touch the sphere to the
Generator sphere.
3. Crank the Generator handle three or four times. If you can feel a spark go across to your hand,
you probably don’t have much charge on the sphere, since most of it probably went to you
instead (Why does this happen?).
4. (If you have time: conduct a qualitative experiment to see how many cranks of the Generator you
need to get a spark across to the sphere as a function of how far the sphere is away from the
Generator. Ground the charging sphere and the Generator after each trial. Does the number of
required cranks increase, stay the same, or decrease with the distance to the charging sphere?
Can you explain this? Do you notice any difference in the tendency for the spark to jump across
to your hand as you increase the distance?)
5. Once your sphere is charged, carefully carry it back to your lab table, trying not to touch the
sphere or get it too close to other people.
IMPORTANT: DO NOT allow a spark from an object charged by the Van de Graaff to jump to the
sphere connected to the Charge Sensor. This will FRY the Charge Sensor, and they are expensive to
replace. Keep all charged objects at least 10cm away from the test sphere.
Interactions Between Charged and Neutral Conductors
To anchor the discussion, we need to know the sign of the charge on the charged sphere:
1. Put the charged sphere close to the Faraday Pail (you should have no problem seeing it on the
sensor– if you do, go back and charge it more carefully.) What is the sign of the charge?
University of Notre Dame
Physics Department
PHYS 152, Spring 2005
Modified from P. Laws, D. Sokoloff, R. Thornton
Supported by National Science Foundation
and the U.S. Dept. of Education (FIPSE), 1993-2000
and the University of Virginia
7
While one of you is carefully holding the sphere, the remaining lab team should connect the Charge
Sensor to the second test sphere. Put the red lead on the screw that protrudes from the sphere, and
connect the black lead to the grounding wire or to the base of the sphere. Zero the Charge Sensor,
or, alternatively, touch the grounding wire to the red lead. Now you’re ready for action.
2. Begin taking data while the charged sphere is quite a distance away, and watch the data as the
charged sphere is brought closer to the test sphere. Again, DO NOT LET A SPARK JUMP TO THE
TEST SPHERE. Keep the two spheres more than 10cm apart at all times.
Question 2-1: What happens? Given the sign of the charge on the charged sphere, does the sign
your Charge Sensor is reading back make sense? What is the net charge on the test sphere? On the
figure, below, draw the charge distribution on the test sphere in the presence of the electric field of
the charged sphere. Indicate the sign of charge on the charged sphere and the density and sign of
charges on the test sphere.
Test Sphere
Charged Sphere
Confused? You should be! This is one case where you can actually tell what the physics is behind the
readings on the screen. The sensor you are using is really only measuring Voltage, or Potential
Difference, and converting this somehow to a charge measurement. How can you tell? Well, if it
had properly measured the charge on the sphere, it would continuously show zero. However, the
presence of the charged sphere changes the amount of work necessary to move charges around in
this vicinity, and in a measurable and predictable way. As you will see later in class, the potential,
relative to infinity, of a sphere carrying charge Q (sign and magnitude) is given by:
V=
kQ
r
where r is the distance from an (arbitrarily-chosen) origin. What this means physically is that to bring
a positive charge q in from infinity to near the charged sphere you have to do an amount of work
equal to qV. If the charge Q in question is negative, it makes sense that you need to do negative
work to move in a positive charge: if you let the charge go from far away, it will just be attracted to
the negative charge Q all by itself. You would have to hold it back (negative work) to keep it
stationary. If Q is positive, you would have to push on your positive charge q to move it closer, hence
positive work. So, in the neighborhood of our charged sphere, the potential has the same sign as the
charge on the charged sphere, and this is what the Sensor sees. This explains also how the Faraday
Pail works: there is a difference in potential between the central conductor and the earth’s ground,
which is what the black lead connects to.
There are quite a few interesting things you can do with this setup besides measure potential
1. Move the charged sphere back closer to the test sphere and scoop charge off of the charged
sphere with the proof plane. Notice the effect on the sensor readings.
2. Leave the sphere stationary for a minute or two. Stop taking data and zoom in on this time
period. Can you see the charge “bleeding off” into the air?
University of Notre Dame
Physics Department
PHYS 152, Spring 2005
Modified from P. Laws, D. Sokoloff, R. Thornton
Supported by National Science Foundation
and the U.S. Dept. of Education (FIPSE), 1993-2000
and the University of Virginia
8
Charging by Induction
If there isn’t enough charge left on your sphere after you have experimented with it, go back and recharge it at the Van de Graaff.
Prediction 2-1: On the diagram below, draw the charges on the test and charged spheres during the
steps that are indicated.
Test Sphere
Charged Sphere
Test Sphere
1. Original Polarization
Charged Sphere
2. Grounding Test Sphere
Test Sphere
3. Final Charge
Follow the diagram above, making a fresh data run for this activity:
3. First, move the charged sphere close to the test sphere.
4. Hold the ground lead against the side of the test sphere opposite that of the charged sphere,
then move the ground lead away.
5. Move the Charged Sphere away or ground it.
Print out a copy of this graph for your lab report.
Question 2-3: Does the result match your expectations? Explain. What are the relative magnitudes
of the potential (“charge”) before and after the charging? How do you know there is really charge on
the test sphere?
University of Notre Dame
Physics Department
PHYS 152, Spring 2005
Modified from P. Laws, D. Sokoloff, R. Thornton
Supported by National Science Foundation
and the U.S. Dept. of Education (FIPSE), 1993-2000
and the University of Virginia
9
INVESTIGATION 3: PLAYING WITH PITH
Activity 3-1: Motion of a Conductor in an Electric Field
In this activity, you will use small aluminum foil test spheres to explore the effects of electric fields
on conductors, just like the pith balls in lecture.
1. Make one or two small balls about 5-6mm in diameter out of aluminum foil. Use about a 6”
square piece of foil. The balls should be very light, but try to make the surfaces smooth. You
may want to crush the end of one of the pieces of thread into the ball as you make it so you
don’t have to tie thread later.
2. If you didn’t crush thread into the ball as you made it, tie a piece of thread to the ball. The
thread should be about 20-30 cm long.
3. Set up the Faraday Pail and the Charge Sensor just like it was in Investigation 1. Make sure the
charge sensor is zeroed.
4. Make sure your small sphere is electrically neutral by checking it with the Faraday Pail.
5. Charge up one of the test spheres with the Van de Graaff and bring it back to the lab table.
6. Hold the small ball close to the charged sphere, moving it slowly closer until it is yanked towards
the sphere and bounces back.
Question 3-1: What happened here? Sketch the charges on the surface of the large sphere and the
small ball before and after they touch. Explain why the ball moved towards the sphere initially, and
how you can tell charge was transferred to the small ball. (You can verify that the ball has now been
charged by using the Faraday Pail.)
Before
After
End of Lab Checklist:
 Make sure you have your graphs for Activities 1-1 (charge conservation), 2-1 (two graphs:
potential vs. time, induced charge)
 ground the charge sensor by clipping its leads together
 unplug the grounding lead from the electric socket
University of Notre Dame
Physics Department
PHYS 152, Spring 2005
Modified from P. Laws, D. Sokoloff, R. Thornton
Supported by National Science Foundation
and the U.S. Dept. of Education (FIPSE), 1993-2000
and the University of Virginia