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Laboratory 1
ELECTROSTATICS
Objective:
The objective of this lab is to gain an understanding of how objects become
electrically charged and how electrical charge is transferred from one object to another.
This will enable us to study the response of an electroscope to charged objects in order to
develop a method for using the electroscope to determine a) the relative magnitudes of
charged objects and b) the sign of charge of an object.
Introduction:
All matter contains positive and negative charges. The natural unit of charge is the
magnitude of charge of the electron or proton, and any observable amount of charge
always exists as an integer multiple of this natural unit, i.e. charge is quantized.
Under normal circumstances, an object is “electrically neutral.” This means that
the number of positive charges the object has is equal to the number of negative charges
the object has, giving zero net charge. When an object becomes electrically charged it
means that the object has either a) acquired a greater number of negative charges than
positive charges (has become negatively charged) or b) has lost negative charges,
resulting in it having more positive charges than negative ones (has become positively
charged). It is also possible to add positive charges to an object to make it positively
charged, but in most cases, it is the highly mobile, light, negatively charged electrons that
are responsible for the charge transfer.
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An important principle to understand is that charge is conserved. This means that
charge is not created or destroyed, but rather transferred from one object to another.
The easiest way to charge two materials is to rub them together. Depending on the
relative affinity (or attraction) for electrons of the two materials, one object will end up
gaining electrons (that is, electrons will be transferred to this material from the other and
hence become negatively charged) and the other one, since it loses electrons, will become
positively charged.
The clear strip in this lab (cellulose acetate) becomes positively charged when
rubbed with a cloth, and the white strip in this lab (polythene) becomes negatively
charged when rubbed with a cloth.
Electric charges have the property that like charges repel and opposite charges
attract. Specifically, two positive charges or two negative charges repel, whereas a
positive charge and a negative charge attract.
In terms of electrical conductivity, materials can be categorized into two
categories: conductors and insulators. Conductors are materials which permit electric
charge to flow freely from one part of the material to the other, while insulators do not
allow free flow of charge. In general, metals are conductors and nonmetals are insulators.
To study electric charge in this laboratory, we will be using an electroscope,
which produces a visible response to the presence of an electric charge.
Electroscope: The rod, R, connects the electrode,
C, to the gold leaf, L. The rod is insulated from the
metal casing by the insulator, I.
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Charging the Electroscope by Conduction
When a negatively charged object is brought into contact with the electrode,
negative charge flows from the object to the electrode. This charge then becomes
distributed over the electrode, the rod, and the leaf. Since the charge on the rod and the
leaf is of the same sign, the leaf will be repelled and hence deflected away from the rod.
A larger charge will cause a larger deflection, but the relationship is nonlinear and
difficult to use quantitatively. When a positively charged object is brought into contact
with the electrode, negative charges flows from the electrode to the object, which also
causes the leaf to be deflected away from the rod. To remove the charge on (or “ground”)
the electroscope, touch the electrode with your finger.
Charging the Electroscope by Induction
When a charged object is brought near an uncharged electroscope, the object will
cause a separation of charges between the electrode and the bottom of the rod. For a
positively charged object, electrons in the electroscope will be attracted to the object and
gather on the electrode, leaving a net positive charge at the bottom of the rod and the leaf.
For a negatively charged object, electrons in the electroscope will be repelled from the
object and gather at the bottom of the rod and the leaf, leaving the electrode and the top
of the rod positively charged. In either case: 1) The charged object has “induced” a
charge at the bottom of the electroscope, and 2) the induced charge causes the leaf to
deflect.
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Experimental Apparatus:
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electroscope
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clear strip of cellulose acetate
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white strip of polythene
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cloth
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polythene plate
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metal disks with insulating handles
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metal can
Experimental Procedure:
Part A: Electroscope Response
For this part of the experiment, observe and carefully record how the gold leaf moves
during each of the following steps, and the amount of deflection of the gold leaf at the
end of each step. Show, by the use of supporting diagrams, what you think the charge
on the gold leaf is at the end of each step.
1. Ensure your electroscope is grounded. (The gold leaf should be hanging straight
down.)
2. Charge the white strip rubbing it with the cloth provided.
3. Bring the white strip near the electrode and ground the electroscope. What
happens to the gold leaf?
4. Slowly pull the strip away.
5. Slowly bring the strip near the electrode again.
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6. Slowly pull the white strip away and bring the clear strip near the electroscope.
What happens to the gold leaf?
7. Interchange the strips and repeat the above 6 steps.
Discussion:
Based on your observations,
1) Discuss a procedure for determining the sign of an unknown charge on the gold
leaf using either one of the two strips. Use diagrams to illustrate the imbalance of
charge to support your response.
2) Discuss a procedure for determining the sign of the charge on an object by using
the electroscope and the white strip, using diagrams to support your response.
Part B: Testing the Sign of an Unknown Charge
1. Momentarily ground the electroscope.
2. Touch the flat part of the (negatively-charged) white strip to the electrode and
remove the strip. If a charge remains on the electroscope, determine the sign of
the charge on the electroscope.
3. Charge the polythene plate by rubbing it. Pick up the large metal disk by the
insulating handle and place it on top of the polythene plate. Ground the top of the
metal disk with your finger then pull the disk away. What is the sign of the charge
on the metal disk? Draw diagrams to support your answer.
4. Place the metal disk on top of the electroscope. What happens to the gold leaves?
What is the sign of the charge on the gold leaves?
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5. Remove the metal disk and continue repeating steps 3 and 4 to increase the
deflection of the leaves of the electroscope.
Results and Discussion:
Draw diagrams representing each step of your procedure, indicating the sign of
the charge that each object attains. Discuss a possible mechanism for charging the
electroscope that is consistent with the results of your experiment.
C. Exploring Charge Distributions
1. Charge the polythene plate by rubbing it and then place the large metal disk on
top of the polythene plate.
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Momentarily ground the top of the disk. Pick up the disk with the insulating
handle and place it on its side on an insulating surface so that both the top and
bottom of the disk are accessible.
3. Bring the small disk into contact with the large disk. The small disk will pick up a
charge that is proportional to the surface charge density of the large disk at the
point of contact.
4. Put the small (now charged) disk on top of the electroscope, and note the size of
the deflection of the gold leaf. Ground the electroscope.
5. Explore the distribution of charge on the large disk by repeating steps 3 and 4 for
several different contact spots on the large disk, being sure to include both sides
and points near the rim and near the centre. Where is the charge on the large disk
most concentrated? Where is it least concentrated?
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6. Explore the charge distribution of the metal can by first charging the metal can
with the large disk (as in Part B), and then using the small disk to determine the
regions of high and low charge concentration.
Results and Discussion:
Describe the charge distribution for the large disk and the metal can. Why is the
charge distributed in the way that you observed?
D. Exploring Transfer of Charge
1. Place the metal can on the grounded electroscope. Charge the small disk (as in
Part B) and contact the small disk to the inside of the can. Repeat this many times.
Is there any change in deflection of the electroscope?
2. Repeat step 1, except touch the small disk to the outside of the can.
Results and Discussion:
Note any changes in deflection of the electroscope. Where is the charge going and how
can you tell? Is it possible to transport charge from a weakly charged object to a strongly
charged object using this method?