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Lab 2 – Electrostatics: Detection of Charge
Name _________________________________ Date ____________________________
Lab 2
Electrostatics: Detection of Charge
Overview
An electroscope is an instrument that detects the presence of charge on an object,
either through actual contact (conduction) or through induction. When the electroscope
itself is charged, its two conductive components (which vary from electroscope to
electroscope) will acquire like charge and deflect from the vertical position of
gravitational equilibrium. See Fig. 2.0.1 and Fig. 2.0.2 for an illustration of the standard
metal leaf electroscope and the UVa electroscope that we will use.
Brass
Fig. 2.0.1 Standard Electroscope
Pin
Aluminized
Cardboard Tube
Acrylic
Block
Fig. 2.0.2 UVa Electroscope
The base of the UVa electroscope in Fig. 2.0.2 is constructed out of acrylic, an
insulator, to eliminate charge leakage to the table. The electroscope is quite portable and
very robust. You can also easily assemble it and move it around. Electrical charge can
easily move along the tube and brass support causing a repulsive force to separate the
tube and brass. The angle is a measure of how much charge is present.
The metallic looking conducting tube is aluminized Mylar which is conductive
on the inside and non-conductive on the outside. The important thing about the tube is
that it is conducting. A metal conducting or steel pin is inserted through the tube 2 mm
from the midpoint of the tube seen in Fig. 2.0.3 and glued with conducting glue so that
the inside of the tub is electrically connected to the brass support through the conducting
glue and pin. The tube is now suspended by the pin which is resting on the flat groove in
the curved piece of conducting brass metal. The longer end as measured from the center
of mass is pointing down so that the slightly more mass below the center allows the tube
to be vertical when no charge is present. The brass, pin, and tube are all excellent
University of Virginia Physics Department
Lab 2 – Electrostatics: Detection of Charge
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Name _________________________________ Date ____________________________
conductors so that electrical charge, namely electrons, can move freely from the brass
structure to the tube and cause a deflection of the tube due to repulsive forces between
the charges in the tube and like charges in the brass support plate.
If a charged object is brought into contact and/or rubbed on the upper lip of the
support structure, charge will distribute across the brass and the tube, causing the tube to
deflect from the brass structure. Sometimes the tube will deflect even when you do not
touch it due to the presence of a strong electric field. We will explore more on this later.
Fig. 2.0.3 The UVa Electroscope consists of an insulating base and a metal tube that can
rotate around a steel pin on a brass support.
Fig 2.0.4 The hole on the tube is 0.075 inch (about 2mm) off center so that the weight of
the tube will pull it upright when there is no charge on the electroscope.
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Lab 2 – Electrostatics: Detection of Charge
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Name _________________________________ Date ____________________________
Activity 2 - 1: Using the UVa electroscope to detect the
presence of charge
Objective: Use the electroscope to detect the presence of charge.
Materials:




Electroscope
Acrylic Rod
Teflon Rod
Silk
Procedure:
1. Assemble the electroscope according to Fig. 2.0.2 and Fig. 2.0.3. Set the tube in
the neutral position by touching the top of the brass support with your finger.
Your finger is a conductor and therefore will drain any electric charge on the
electroscope so it becomes neutral after you touch it.
2. Rub the Teflon rod on silk. Then slide the rod on the top of the brass support. See
Fig. 2.1.1. You may repeat the process several times to accumulate more charge.
Describe how the angle of the tube changes as you add charge. Is there a
maximum angle? Why? _____________________________________________
Fig. 2.1.1
3. Touch the top of the brass support with your finger to make it become neutral.
Now the rub the acrylic rod with silk and repeat step 2. You may have to repeat
the process a few times for the effect to be evident. Now what is the polarity of
University of Virginia Physics Department
Lab 2 – Electrostatics: Detection of Charge
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Name _________________________________ Date ____________________________
the charge on the electroscope and how is it the same or different than in step 2?
_________________________________________________________________
4. When the polarity of the charge on the electroscope was negative what real
particle was being transferred to or from the electroscope? When the polarity of
the charge on the electroscope is positive what real particle was being transferred
to or from the electroscope?
5. Touch the upper lip of the brass support to make it neutral. Then using the Teflon
rod, charge up the electroscope until the tube goes out as far as it can. Substitute
the Teflon rod with the acrylic rod. Rub the acrylic rod on silk. Then slowly slide
the rod on the upper lip of the brass support while watching the movement of the
tube very closely. Describe the behavior of the tube as more charge gets rubbed
off the acrylic tube. What is the polarity of the initial charge on the electroscope?
What about the charge added to the electroscope? How does that explain the
behavior of the tube? _______________________________________________
6. What happens when an equal amount of positive charge meets an equal amount
of negative charge? _________________________________________________
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Lab 2 – Electrostatics: Detection of Charge
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Name _________________________________ Date ____________________________
Activity 2- 2: Conductor or Insulator?
Introduction:
The main difference between a conductor and an insulator is that a conductor
allows charge to move through it while an insulator doesn’t. While the atoms in an
object are stationary, the electrons can sometimes escape the grasp of the atomic nuclei
and drift through the material. A material whose electrons can easily move through is
said to conduct electricity. Conversely, a material whose atomic nuclei are strongly
attached to all the electrons is an insulator since they are not allowed to move.
Objective: Determine whether an object is a conductor or insulator.
Materials:







Teflon Rod
Silk
Electroscope
100% Metal Object such as Knife, Fork, or Spoon (from home)
100% Plastic Object such as a Plastic Knife, Fork, or Spoon (from home)
Wooden Rod
Acrylic Rod
Procedure:
1. Rub Teflon rod on silk. Use the rod to charge up the electroscope until a large
angle appears. Now touch the top lip of the electroscope with your finger. As
shown in Fig. 2.2.1. Describe the behavior of the tube and whether or not your
finger is a conductor or insulator. ______________________________________
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Lab 2 – Electrostatics: Detection of Charge
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Name _________________________________ Date ____________________________
Fig. 2.2.1
2. Charge up the electroscope again using the Teflon rod. Hold one end of the
wooden rod in your hand. Touch the top lip of the electroscope with the other
end of the wooden rod, as shown in Fig. 2.2.2. Describe the behavior of the tube
and whether or not the wood is a conductor or insulator.
_________________________________________________________________
Fig. 2.2.2
3. Substitute the acrylic rod for the wooden rod. . Do not forget to neutralize the
acrylic rod. Repeat step 2. Describe the behavior of the tube.
_________________________________________________________________
4. Substitute a Teflon rod for the acrylic rod. Charge the electroscope up with the
Teflon rod. Now neutralize the Teflon rod thoroughly by rubbing your finger up
and down it. Then repeat step 2. Describe the behavior of the tube.
_________________________________________________________________
5. Substitute a metal object for the Teflon rod. Repeat steps 1 and 2. Describe the
behavior of the tube. ________________________________________________
6. Now try your plastic object. Repeat step 2. Describe the behavior of the tube.
_________________________________________________________________
7. How do you decide whether a material is a conductor or insulator based on the
behavior of the tube in the experiments above? State your line of reasoning.
_________________________________________________________________
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Lab 2 – Electrostatics: Detection of Charge
Name _________________________________ Date ____________________________
8. Fill in Table 2.2.1 with your conclusions.
Material
Your Finger
Wood
Acrylic
Teflon
Metal object
Plastic object
Conductor/Insulator?
Table 2.2.1
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Lab 2 – Electrostatics: Detection of Charge
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Name _________________________________ Date ____________________________
Activity 2 – 3: Movement of Charges in a Conductor
Introduction:
We already know that the fundamental characteristic of a conductor is that
charges can move freely through it. If a charged object is placed close to a conductor, it
will affect the spatial distribution of the charges in the conductor by attracting opposite
charge while pushing similar charge away. See Fig. 2.3.1, Fig. 2.3.2, and Fig. 2.3.3.
Fig. 2.3.1
Fig. 2.3.2
University of Virginia Physics Department
Lab 2 – Electrostatics: Detection of Charge
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Name _________________________________ Date ____________________________
Fig. 2.3.3
Objective: Show that the spatial distribution of charges in a conductor can change
easily.
Materials:



Electroscope
Teflon Rod
Silk
Procedure:
1. Rub Teflon rod on silk. Rub Teflon rod across the lip of the electroscope to
charge it up.
2. Rub Teflon rod on silk again. Starting from 10 cm away, slowly move the Teflon
rod close to the top of the tube. Describe the behavior of the tube in the entire
process. __________________________________________________________
3. Pull the rod away. Then slowly move the Teflon rod close to the bottom of the
tube, also starting from 10 cm away. Describe the behavior of the tube in the
entire process. _____________________________________________________
4. How can the tube be repelled and then attracted, while having the same amount
of net charge on it? Explain the paradox by drawing reference to movement of
charges in a conductor. ______________________________________________
University of Virginia Physics Department